EP0868976A2 - Lapping apparatus and method for high speed lapping with a rotatable abrasive platen - Google Patents
Lapping apparatus and method for high speed lapping with a rotatable abrasive platen Download PDFInfo
- Publication number
- EP0868976A2 EP0868976A2 EP98301690A EP98301690A EP0868976A2 EP 0868976 A2 EP0868976 A2 EP 0868976A2 EP 98301690 A EP98301690 A EP 98301690A EP 98301690 A EP98301690 A EP 98301690A EP 0868976 A2 EP0868976 A2 EP 0868976A2
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- Prior art keywords
- platen
- abrasive
- sheet
- work piece
- lapped
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B24—GRINDING; POLISHING
- B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
- B24B37/00—Lapping machines or devices; Accessories
- B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
Abstract
Lapping or polishing at high speeds with fine abrasive particles offer
significant advantages in the speed of lapping, savings of time in lapping, and
smoothness in the finished articles. An improved lapping system comprises a
lapper platen system comprising:
There optionally may be
The process of the present invention may also be described as:
Optionally, one may provide a first amount of liquid to assist lapping to said
abrasive surface physically in front of an area where work piece contacts said
abrasive face,
then provide a second amount of liquid to assist in washing solid material from said abrasive surface physically after said area, and then direct air against said abrasive surface physically after providing said second amount of liquid to assist in removing said first and second amounts of liquid from said abrasive surface.
then provide a second amount of liquid to assist in washing solid material from said abrasive surface physically after said area, and then direct air against said abrasive surface physically after providing said second amount of liquid to assist in removing said first and second amounts of liquid from said abrasive surface.
Rotating said platen at a high rotational velocity generates a surface speed of
at least 4,000 surface feet per minute (or even more than 20,000 surface feet per
minute). The boundary layer of any liquid (e.g., coolant or lubricant) applied to
the working surface of the abrasive sheet is controlled by the apparatus and
methods of the invention to improve the uniformity of the lapped surface. The
use of an annular sheet of abrasive material on the platen offers unique benefits.
Description
The present invention relates to lapping, polishing, finishing or
smoothing of surfaces with apparatus and processes which use abrasive
sheeting. In particular, the present invention relates to such processes and
apparatus which use removable or replaceable abrasive sheeting which operates
at high surface speeds and secures the abrasive sheeting to a platen on a flexible
shaft which platen moves the sheeting at those high speeds. The lapping system
is capable of extremely smooth surface finishing at high speeds.
The field of lapping or polishing traces it roots far back into time, even
before substantial technical developments. Early jewelry and decorations were
provided by minerals or materials (shells or wood) that had been smoothed by
natural elements. Stones smoothed by water currents or sand storms gave a
much more pleasant look and feel than unpolished stones or stones which had
been roughly smoothed by available means such as rubbing two stones together.
Early efforts at sharpening blades for plows or swords were amongst the
first technical advances in lapping and smoothing of materials, and these
technical means are still used in much the same way today. Swords and plow
shears were sharpened by moving the blade against a stone surface. The abrasive
action of the stone against the blade removed metal and thinned the blade at its
edge. Grinding wheels, kitchen knife sharpeners, and the like are not
significantly different in function than the stone sharpening tools, such as the
grinding wheel which has been used to sharpen blades for thousands of years.
In the 17th and 18th century, the combination of die casting and abrasive
polishing enabled the manufacture of interchangeable generic parts for
equipment (especially the rifle and hand gun) as opposed to the standard method
of fitting individually made parts into a unique piece of equipment with uniquely
fitting parts. Each succeeding advance in the ability of materials and processes
to create smoother and more uniform surfaces advanced the quality and
capability of the resultant articles to perform whatever tasks for which they were
designed. Lenses with greater smoothness and uniformity advanced the degree
to which observation could be extended downward by microscopy and outward
into space by telescopes. Better fitting parts extended the longevity of
equipment and increased efficiency by reducing internal friction. The need for
increasing efficiency, precision, consistency and smoothness in lapping is as
important today as ever. Each incremental increase in the quality of lapping
materials and processes advances many fields of technology and industry, while
at the same time offering the possibility of reducing the cost of manufacture of
goods.
Lapping and polishing are performed in many fields and industries. Metal
and parts polishing is the most obvious field, but smoothing of surfaces is
extensively used in lens manufacture, semiconductive wafer manufacture, gem
polishing, preparation of supports for optical elements, providing surfaces which
can be joined or seamed and the like. The smoothness and reproducibility of the
processes and apparatus used to create the needed levels of smoothness are
critical to the success of products. U.S. Patent No. 5,584,746 (Tanaka) describes
a method of polishing semiconductor wafers and apparatus therefor. The import
of Tanaka is the physical control placed over the wafer as it is being polished.
The wafer is secured by a vacuum system on a wafer mounting plate. The
relative flexibility of the wafer is discussed as a method of controlling uniformity
of the wafer surface as is the uniformity of the vacuum applied through the wafer
support. The polishing of the wafer surface is accomplished by typical means
including a polishing pad which is mounted on a polishing surface (turntable). It
is suggested that the pad should not be subject to plastic deformation and may be
preferably selected from a group comprising close cell foam (e.g., polyurethane),
polyurethane impregnated polyester non-woven fabric and the like, which are
known materials in the art. No specific means of securing the polishing pad to
the support surface is described in Tanaka. No specific speeds of rotation for
the operation of the process are shown in the examples.
U.S. Patent No. 5,317,836 (Hasegawa) describes an apparatus for polishing
chamfers of a wafer. Hasegawa describes that in the manufacture of wafer
materials from single crystal ingots such as silicon, the wafer is produced by a
combination or selection of processes including slicing, chamfering, lapping,
etching, buffing, annealing and polishing. It is noted that chipping and/or
incomplete surface polishing are a problem in such ingot conversion to wafers.
Hasegawa describes the use of a rotary cylindrical buff formed with at least one
annulargroove in its side describing a circle normal to the axis of the cylindrical
buff and a wafer holder capable of holding and turning the wafer about an axis.
The improvement is described as including at least the ability of the cylindrical
buff being adapted to freely shift axially, that the annular groove has a width
substantially greater than the thickness of the wafer, and that the apparatus
further comprises a means for axially biasing the cylindrical buff. No specific
speeds of rotation for the operation of the process are shown in the examples.
U.S. Patent No. 5,007,209 (Saito) describes an optical fiber connector
polishing apparatus and method. Saito describes a method and apparatus for
polishing optical fiber connectors with high accuracy. Saito indicates that the
polishing is accomplished by using an elastic polishing board rotating at high
speed, but no specific speed of rotation or method of attachment of the polishing
board is described. Positioning pins and other controls are provided in the
system to accurately align the swing fulcrum arm carrying the polishing material.
U.S. Patent No. 4,085,549 (Hodges) describes a lens polishing machine
comprising a lap tool holder and lens blank holder including independent means
to provide linear and rotary movement between a lens blank and lap tool. The
machine is described as useful for high speed grinding and polishing. The
polishing element is gimbal mounted on its lower extreme in a spherical bearing
to allow a lens blank holder to follow the contour of the lens during the polishing
process. The movement between the rotary drive and linear drive mechanisms
independent of each other provides a balanced and low vibration operation. No
specific speeds of rotation are recited and the abrasion is provided by a slurry.
U.S. Patent No. 4,612,733 (Lee) describes a very high speed lap with a
positive lift effect. The apparatus and method comprises a rotary lapping system
which uses a liquid slurry of abrasive particles. The diameters of the particles
are shown to be from about 1.5 to 5 micrometers, but may be outside this range.
The system is described as producing positive lift by presenting leading edge
surfaces with a positive angle of attack in the liquid abrasive slurry, the leading
edge surfaces generating a positive lift through hydrodynamic interaction with
the slurry. Each of the positive lift tools presents a grinding surface to said
workpiece when it is rotated in the slurry. There is again no specific rotational
speed provided in the description, and the use of liquid slurries would cause
higher lapping/abrasive areas on the exterior of the grinding/lapping face as the
slurry would be at higher levels at the outside of the rotating grinding area work
surface.
U.S. Patent No. 4,709,508 (Junker) describes a method and apparatus for
high speed profile grinding of rotatably clamped rotation symmetrical
workpieces. Rather than the grinding element contacting the surface to be
ground with a grinding surface which is rotating within a plane, the edge of the
grinding element (e.g., at the circumference of a disk rather than on its face) is
brought against the surface to be ground.
U.S. Patent No. 5,197,228 describes methods and apparatus for grinding
metal parts, especially with devices having a cooperative workpiece holder and a
tool holder which form a grinding station. The grinder table is reciprocally
moveable along an axis which is at right angle to the axis of travel of the
workpiece. The grinder table may also be equipped for controlled simultaneous
movement along two axes. A microprocessor is designed to send and receive
signals to or from all of the moving parts of the grinding machine for moving the
workpiece table towards or away from the grinding bit.
U.S. Patent No. 4,194,324 describes a carrier for semiconductive wafers
during polishing steps in their manufacture. An annular flange is present to
receive pressure loading from the polishing machine during the wafer polishing
operation. The holder of the polishing machine includes the ability to apply a
vacuum to the carrier to maintain the carrier selectively on the polishing
machine. The arrangement on the equipment allows release of the vacuum
during polishing and enables simple intentional removal of the carrier. Cam
follower-slot arrangements permit tilting of the mounting head.
U.S. Patent No. 5,576,754 describes a sheet holding device for an arcuate
surface with vacuum retention. The sheet and device are described as useful for
internal drum plotters in imaging equipment. Vacuum pressure is applied to
imaging film to keep it securely positioned within the arcuate focal plane of the
imaging equipment.
U.S. Patent No. 5,563,683 describes a substrate holder for vacuum
mounting a substrate. The holder is provided with two kinds of grooves or
clearances in the supporting surface. Circular support faces with multiple
grooves and/or a plurality of pins to support the work are shown. The device is
generally described to be useful as a holder, with such particular uses as in the
manufacture of semiconductors and the support of photosensitive substrate being
shown. Similarly, U.S. Patent No. 4,943,148 describes a silicon wafer holder
with at least one access port providing access to the underside of the wafer with
vacuum pressure. U.S. Patent No. 4,707,012 also describes a method of
applying vacuum holding forces to a semiconductor wafer during manufacture in
an improved manner. U.S. Patent No. 4,620,738 shows the use of a vacuum
pickup system for semiconductor wafers. The wafers are placed into or removed
from holders by the vacuum pickup.
Similarly, U.S. Patent No. 5,414,491 describes a vacuum holder for sheet
materials comprising a plurality of arrays of vacuum channels including a
plurality of vacuum plenums. Flow sensors are provided so that the system can
indicate the presence and/or size of the sheets being held. Specifically described
are common types of imaging materials using sheets of plain paper, photographic
paper and photographic film.
U.S. Patent No. 5,374,021 describes a vacuum holding system which is
particularly useful as a vacuum table for holding articles. The holding table is
particularly described with respect to the manufacture of printed circuit boards.
Controlled passageways are provided which are supposed to control the
application of reduced pressure and to reduce the application of the vacuum
when vacuum support is not required.
U.S. Patent No. 5,324,012 describes a holding apparatus for holding an
article such as a semiconductor wafer. At least a portion of the holder contacting
the wafer comprises a sintered ceramic containing certain conductive materials.
The use of conductive materials and fewer pores reduces the occurrence and
deposition of fine particles during use. The benefits of the materials are said to
be in contributions to the cleanability of the surface, insurance of mechanical
strength, reduction of weight and increased dimensional stability.
U.S. Patent No. 5,029,555 describes a holding apparatus and method for
supporting wafers during a vacuum deposition process. The apparatus is an
improved system for the angled exposure of at least one surface portion of a
substrate supported on a surface holder to an emission of a source impinging
obliquely on the surface portion. The device moves the surface holder to
improve the uniformity of the emission received on the surface portion. Wheel
mechanisms are coupled together to provide maintenance capability for
predetermined positions of the surface. The substrate holder is moved while its
orientation to the source is carefully controlled.
U.S. Patent Nos. 4,483,703 and 4,511,387 describe vacuum holders used to
shape glass. Frames are shown with slidable members moving a deformable
vacuum holder between a shaping station and a mold retraction station. Pistons
drive movable elements, such as the vacuum holder, on a supporting frame.
U.S. Patent No. 4,851,749 describes a motor driven mechanical positioner
capable of moving an arm to any one of about 840 discrete angular positions. An
infrared light emitting device acts with a phototransistor to control the
appropriate angular position. Sensing devices also act on interdependent speed
controls so as to increase the accuracy of the positioning of the arm.
U.S. Patent No. 5.180,955 describes a positioning apparatus comprising an
electromechanical system which provides controlled X-Y motion with high
acceleration, high maximum speeds, and high accuracy, particularly for
positioning an end-effector at predetermined locations. A high speed mini-positioner
is provided comprising a positioning linkage having a changeable
parallelogram structure and a base structure. A main benefit of the system is the
fact that the bars and bearings of the positioner are symmetrical about the X-Y
plane passing through the linkage height. The symmetry means that all actuator
forces and all inertial reaction forces act in vectors lying in the plane of
symmetry.
U.S. Patent No. 5,547,330 describes an ergonomic three axis positioner.
The positioner is intended to move an article along three mutually perpendicular
axes through a system of interconnected slides and slide joints. Rack and
pinions are also used to independently move the slides. The device is suggested
for use in the visual inspection of work, particularly in the semiconductor
industry.
U.S. Patent No. 4,219,972 describes a control apparatus for a grinding
machine. A revolution speeds changing means is provided which can selectively
effect changes at high speeds when grinding and changes at low speeds when
dressing the article. The relationship and control of the timing of the speed
changes and the operations detection circuits and timers.
U.S. Patent No. Re. 30,601 describes an apparatus and method particularly
effective in the positioning of a semiconductor wafer in a preferred plane with
respect to a photomask. Sensors regularly monitor the position of the wafer and
a reference plane. A photoalignment system is provided in which a wafer is not
physically touched by any portion of the photoalignment tool, thereby avoiding
any contamination.
These systems have been described as providing benefits to particular
technical and commercial fields, but they have not been shown to provide any
particular benefits to truly high speed lapping/polishing systems and materials.
Lapping or polishing at high speeds with fine abrasive particles offer
significant advantages in the speed of lapping, savings of time in lapping, and
smoothness in the finished articles. Materials, processes, apparatus and specific
features integrated into the lapping processes and apparatus of the present
invention can provide a unique lapping effect with regard to both the quality
(smoothness and uniformity of the produced surface) and efficiency of the
system. The present invention relates to a new field of lapping technology with
its own unique complexities due to the combination of high rotational speeds on
the abrasive platen and the use of sheets of abrasive material rather than slurries.
The combination of these two aspects creates dynamics and forces which have
not been addressed by previous lapping systems and requires an entirely new
background of engineering to address the problems.
One process of the present invention for lapping a surface comprises:
- (1)
- rotating said platen at a rotational speed of at least 500 revolutions per minute, and a surface speed at an outside edge of said sheet of abrasive material of at least 1500 surface feet per minute, and
- (2)
- contacting said abrasive face and said at least one surface of said workpiece to be lapped.
One preferred lapper system for practicing the present invention comprises:
Sequence of steps A comprising:
Sequence of steps B comprising: a) providing a work piece to be
lapped, having at least one surface to be lapped, which can be adjusted to a
position parallel to said at least one surface of b) where
Sequence of steps C comprising:
Sequence of steps D comprising
Sequence of steps E comprising:
Sequence of steps F comprising:
Sequence of steps G comprising:
Sequence of steps H comprising:
Each of the processes described above as including those sequences of steps
within the broader concept of the process invention preferably includes a sheet of
abrasive material comprising a circular sheet of material which is:
Another preferred aspect of the lapper system of the invention comprises:
- f) a shaft which is connected to said rotatable platen, said rotatable platen having a back side to which said shaft is connected and a front side on said rotatable platen to which can be secured an abrasive sheet, said rotatable platen having i) a back surface, ii) a front surface, and iii) a raised edge forming an abrading plateau on said front surface of said rotatable platen, with an abrasive sheet secured to said raised edge.
Another preferred lapper platen system according to the present invention
comprises:
- a) a shaft which is connected to a platen, said platen having a back side to which said shaft is connected and a front side on said platen to which can be secured an abrasive sheet;
- b) a pivoting joint comprising a gimbal joint,
- c) said shaft being able to pivot about said pivoting joint relative to said platen.
Another aspect is a preferred process within the scope of the invention which
comprises:
A very important process aspect of the present invention includes the initial
positioning and contacting of the workpiece and the abrasive sheet material as in
a process for initiating contact between a workpiece to be ground and an abrasive
surface comprising abrasive sheeting on a rotatable platen, the process
comprising:
said process being further characterized by
Another desirable aspect of the process of the present invention is that
pressure is applied between the work piece and the abrasive sheet by a flexible
joint or engagement or gimbal supporting the work piece. The pressure applied
between the workpiece and the rotating platen may be from 0.1 psi to 100 psi,
preferably from 0.1 to 25 psi, more preferably from 0.1 or 0.5 to 5 psi.
Generally a particular improved process of the invention may be
considered to comprise a process for lapping a surface comprising:
This process particularly benefits when the plateau defines an annular
shape on said front face, and more particularly where the sheet of abrasive
material comprises a circular sheet or annular sheet of material. The sheet of
abrasive material most preferably comprises an annular shape in which a central
open portion is at least three times the radial dimension as the width of said
annular sheet. A reduced gas pressure may be applied against said back surface
of said sheet between said sheet and said platen through vents which are present
at least or only on said flat surface of said plateau, the reduced pressure securing
the sheet against rotational movement relative to the rotatable platen. A
preferred abrasive sheet comprises an annular distribution of abrasive material
on a backing material, with a center area of said sheet being a self-supporting
structure which passes across said center area, contacting inner edges of said
annular distribution of abrasive material. That is, the central area may be free of
abrasive material, such as where said abrasive sheet comprises a continuous
substrate with a central area having no abrasive on said backing material, and an
annular zone of said backing material surrounding said central area having
abrasive material on a surface overlaying said plateau and facing away from said
platen, or where said abrasive sheet comprises an annular zone and said central
area, said central area being bonded to said annular zone, having less height than
said annular zone when said sheet is lying flat, and there being a seam or bond
between said annular zone and said central area.
A preferred lapper platen system according to the present invention may
comprise:
Movement and control of movement of the workpiece holder can be
extremely important in the performance of the present invention. The control of
the movement is best effected by the use of support systems for the workpiece
which allow smooth motion of the workpiece, especially by air pressure,
hydraulic pressure, linear electric motors and the like.
Another improved process for lapping a surface according to the present
invention comprises:
using a lapper system comprising:
using a lapper system comprising:
It is preferred that a safety box system is also included within the lapping
system which may include a means for introducing a first amount of liquid onto
said abrasive surface of said platen at a location before contact between a work
piece held on said work piece holder and said abrasive surface on said platen;
The second amount of water is larger than the first amount, the first
amount providing a function as a lubricant, coolant, or the like, and the second
amount assisting in washing away residue from the work piece and/or the
abrasive sheet. The means for directing air against the abrasive surface of the
platen assisting in the rapid removal of the liquid and the solid matter carried
with it.
A work piece holder may be used which has a control element thereon
which allows for independent movement and alignment of said work piece
holder along three perpendicular axes so that a work piece on said work piece
holder can be adjusted and oriented towards parallelity with said platen so that a
work piece can be lapped; and
wherein said lapper system includes a pivoting lapper platen system comprising:
Rotating of said platen is done at a rotational velocity sufficient to generate
a surface speed of at least 4,000 surface feet per minute (or even more than
20,000 surface feet per minute), which, depending upon the diameter of the
rotating abrasive may be at an angular speed of at least 500 revolutions per
minute (which with a 15.2 cm or 6 inch diameter platen and abrasive sheet,
equates to over 700 surface feet per minute at the periphery of the abrasive
surface), or even more than 3,000 revolutions per minute (which with a 15.2 cm
diameter abrasive sheet equates to over 4200 surface feet per minute and with a
30.4 cm or 12 inch abrasive sheet equates to over 8400 surface feet per minute)
and contacting said abrasive material with said work piece. The process of the
present invention allows the boundary layer of any liquid (e.g., coolant or
lubricant) applied to the working surface of the abrasive sheet to be controlled to
improve the uniformity of the lapped surface.
Figure 1 is a perspective view of a lapping apparatus according to the
present invention.
Figure 2 is a perspective view of a lapping platen for supporting abrasive
sheets according to the present invention.
Figure 3 is a cross-section of a lapping system according to the present
invention.
Figure 4 is a perspective view of an apparatus for applying liquid to the
surface of a lapping platen according to the present invention.
Figure 5 is a side view of a platen with raised peripheral edge portions.
Figure 6 is a perspective view of a platen with raised peripheral edge
portions.
Figure 7 is a cutaway view of a platen with raised peripheral edge portions.
Figure 8 is a cutaway view of a different configuration of a platen with
raised peripheral edge portions.
Figure 9 is a cutaway view of a platen with a pivot connection to a rotary
shaft.
Figure 10 is a perspective view of a single Bellview spring washer.
Figure 11 is a cutaway view of a platen with a pivot control mechanism
within a shaft.
Figure 12 is a perspective view of an annular platen with a beveled edge.
Figure 13 is an edge view of a platen with a beveled edge and a workpiece
being lapped in a linear manner by said platen.
Figure 14 is an edge view of a workpiece and a platen.
Figures 15 are overhead views of abrasive platens with segments of
abrasive sheets thereon.
Figure 16 shows a workpiece holder with a vertical vibration damping
element on it.
Figure 17 shows a platen with abrasive sheeting thereon with special
surface features to improve performance.
Figure 18 shows a workpiece holder with various orientations of gimbals
to reduce tilting torque on the workpiece holder under high speed lapping.
Figure 19 shows an overhead view of a platen and multiple part workpiece
holder according to one aspect of the present invention.
Figure 20 shows cross-sections of platens of an earlier but workable form
(a) of the present invention, and two improved configurations (b) and (c)
according to the present invention.
Apparatus and methods are needed for super high speed lapping at greater
than 500 rpm, greater than 1500 rpm, higher than 2000 rpm, and even speeds of
2500, 3000 rpm and greater, equating to surface speeds at the periphery of the
abrasive sheet of from about 500 to more than 25,000 surface feet per minute
(sfpm, or sfm), depending upon the diameter of the platen and sheet as well as
the angular speed. In addition, these higher speeds should be useable with finer
and harder pre-made abrasive materials without the use of liquid abrasive
slurries. Some earlier attempts at using liquid slurries at high rotational speeds
were less effective than desired because of hydroplaning of the liquid slurries,
excessively rapid movement of the slurries out of the work area, channeling of
the slurry liquid and other effects. The different forces at the different distances
from the rotational center contributed to distributional difficulties in the
placement of the liquid. The different amounts of liquid slurry at different radial
positions caused variations in pressures and thickness at different radial points.
These effects in turn caused the lapping to be less even than should be the
capability of such lapping systems and materials.
A lapping apparatus according to the present invention comprises at least
the following elements:
One process practiced in the present invention is a process for lapping a
surface comprising:
Another process practiced in the present invention may be described as
follows:
Still another process according to the present invention includes a
process for initiating contact between a workpiece to be ground and an abrasive
surface comprising abrasive sheeting on a rotatable plate, said process
comprising:
This process may effect mechanical alignment of said workpiece and/or
said workpiece holder to promote parallelity between a surface of said workpiece
to be ground and said abrasive surface after step c) but before step e). The
process may also have said controlling forces providing a contact force between
0.1 and 10 pounds per square inch between a surface of said workpiece to be
ground and said rotating platen during lapping of said workpiece while said
abrasive sheet is moving with at least 1,500 surface feet per minute while in
contact with said workpiece.
The process may also have the workpiece holder supported by a pivot joint
and said workpiece holder pivoting upon contact between said workpiece and
said abrasive surface to hold a surface of said workpiece to be lapped in a more
parallel orientation with said abrasive surface.
It is more preferred with respect to the protective walls that, rather than
merely having four essentially vertical walls intercept material which is expelled
from the work area by the rotational forces from the rotating platen (and often a
rotating workpiece holder in conjunction with a rotating platen), the surfaces (the
walls) which are intersected by the plane formed by the contact points between
the platen and the workpiece are angled (hereinafter referred to as the
intersection plane), sloped or curved so that impacting expelled material is
deflected downward from the point of contact by the angle of impact. This is a
protective measure which can still be improved by the provision of a lip,
movable lip, fixed frame guard or the like which extends from the walls (or
continues from the walls as a continuous extension of the walls) to provide
additional protection from ricocheting materials. For example, the walls may be
curved, and the curve extends from above the intersection plane towards the
shaft supporting the workpiece to form an umbrella-like protective area. The
extension from the walls may be curved, flat, stepped, movable (e.g., on a
rotating hinge so that it may be lifted), slidable (so that it may be moved back
and forth to open up the work area if access to it is needed), and the like.
This guard wall or enclosure is neither a trivial matter nor a system which
is relevant to traditional lapping. In traditional lapping, much lower rotational
speeds, such as 200 revolutions per minute and/or smaller diameters (producing
lower surface speeds, e.g., less than 300 surface feet per minute) allow materials
such as detritus, used slurry, cooling liquid and the like the flow or stream off the
surface at speeds which are comparable to the rotational speeds of the platen.
With the much higher speeds used in the present invention, and the use of
abrasive sheets, the dynamics, problems, and failure of the system are unique and
require differ considerations.
When high speed platen rotation is used with abrasive sheeting failure of
the system can occur for different reasons and with different results than in lower
speed slurry systems or lower speed abrasive sheet systems. For example, it
must be remembered that the clearance between the platen, sheet and workpiece
are essentially non existent. With the extremely high rotational speeds, events
could and do occur as follows. In one circumstance, the workpiece may be
advanced into contact with the rotating platen at less than perfect parallelity. If
that difference from parallelity is too great, the workpiece may grip and lift, fold,
crinkle or crumple the abrasive sheet. Because there is no volume within which
the abrasive sheet may move (being confined by the platen and the workpiece),
the extremely high speeds of rotation cause extraordinarily high forces to be
brought to bear against the platen, the workpiece and the abrasive sheet. The
result of these extraordinary forces is an explosion created by the kinetic energy
from the high mass inertia and momentum of the platen, but usually also the
workpiece, and possibly the broken workpiece holder and the platen become
muzzle velocity shrapnel from the apparatus. These exploded fragments of
materials do not merely fly parallel to the intersection plane, but spray out of the
work area, bounce off each other, ricochet of the walls and floor of the work
area, and can seriously injure persons in the area or even damage the
environment around the apparatus. This event is unique to combination of the
abrasive sheet and the high platen speed of rotation. Neither the abrasive
sheeting alone nor high speed rotation (with slurry or powder) creates the forces
effecting this explosive event. The guard system is therefore uniquely necessary
with the combined system of the present invention.
A process for lapping a surface according to this invention is also
described wherein a back surface of the workpiece is pivotally connected to a
rotating joint which is in turn connected to a shaft which rotates said workpiece,
and said workpiece is allowed to pivot around said pivot joint as contact is made
between said abrasive surface and said work piece so that said surface to be
lapped becomes more parallel towards said platen after said contact as compared
to before said contact.
The process for lapping a surface according to the present invention may
also comprise an underlying process of:
contacting said abrasive face and said at least one surface to be lapped on said work piece, and allowing said workpiece to pivot around said pivot joint so that said abrasive sheet and said at least one surface to be lapped become more parallel towards each other.
One particular advantage of one optional alternative of the present
invention (the vacuum hold-down of the abrasive sheet) is the ability of the
apparatus to use preformed sheets of abrasive materials at high speeds, and to
rapidly and cleanly replace the sheets without significant delays. During lapping
and polishing processes, it is often necessary to change the abrasive medium at
various stages. In prior art usage of sheets of abrasive materials, the individual
sheets were secured to the chuck or rotating face by an adhesive. The adhesive
may have been precoated on the backside of the abrasive sheet or applied as
coating to the rotating support surface or the backside of the sheet immediately
before use. This adhesive coating adds another parameter or variable which
must be controlled in attempts to precisely lap surfaces. Even the best coating
techniques provide layers which have what are presently considered minor
variations in thickness in some fields of use. However, each variation, no matter
how small, is part of an additive effect upon the final article. The adhesive
creates another problem in that adhesives that are strong enough to secure the
abrasive sheet to the platen do not necessarily remove cleanly from the platen
with the removal of the sheet. Some adhesives build up on the platen surface,
requiring washing or stripping to remove them, if increasing variations in non-planarity
of the surface are to be avoided. This is time consuming, labor
intensive, and expensive. Where the objective of the system is to provide
uniform flatness, even this additional minor variable component becomes
undesirable or limiting in the capability of the final article. This is particularly
true where the variations can cause uneven or non-uniform exposure of abrasive
material towards the workpiece, causing uneven grinding, polishing or lapping of
that workpiece surface. The use of rotational abrasive action, particularly at high
speeds for short duration, can quickly cause undesirable effects upon the
workpiece. When sheets are regularly changed with respect to their degree of
coarseness in the abrasive grit, subsequent variations because of the adhesive
layers will not only fail to correct the previous errors, but add further variations
into the workpiece surface which were not intended. Additionally, some
adhesives remain liquid or pliable (e.g., pressure-sensitive adhesives) and the
centrifugal forces produced in high speed rotational abrasion can cause the
adhesive to shift, flow or shear, altering the thickness of the adhesive layer even
while the process is being performed.
One optional, but highly preferred aspect of the present invention therefore
is to support a sheet having at least one abrasive workface and a backside on a
rotatable support by vacuum forces, and to perform the abrading process with the
vacuum forces maintaining at least part, if not all of the contact between the
support and the backside of the sheet. Adhesive supplemental forces may be
particularly used to advantage where the adhesive contacts or adheres the
abrasive sheet and the rotatable platen in a region which will not place the
abrasive sheet into contact with the workpiece. For example, where an annular
distribution of abrasive is present on the abrasive sheet and the central area has
no abrasive and is not brought into contact with the workpiece, the use of
adhesive between the platen and the abrasive sheet in this region is quite
acceptable, though still not preferred. Although vacuum forces have been used
to support or assist in the support of workpieces, there is not known to be any
description of the vacuum support of abrasive sheet materials in a high speed
lapping process, nor is their any indication of the potential problem with abrasive
sheet thickness variations because of the addition of adhesive coatings between
the support and the sheet. The references described above, even though they
may refer to high speed in production of materials, do not describe rotational
speeds in excess of 1500, 2000, 2500 or even 3000 rpm, or expressed in other
units, with surface speeds at the periphery of the rotatable lapping platen of at
least 550, at least 1,000, more preferably at least 1500 or at least 2,000 sfpm, still
more preferably at least 2,500 or 3,000 sfpm, again still more preferably at least
3,500 or 4,000 sfpm, and most preferably at least 8,000 or 10,000 or even 12,000
and more sfpm. Furthermore, it is usually the abrasive segment of the apparatus
and process of that prior art which is being rotated (although as shown in U.S.
5,317,836, both a semiconductor wafer and the buff are rotated), while the
vacuum secured workpiece remains fixed. There is no teaching in the prior art or
consideration of the physical problems which could be encountered in attempting
to use vacuum pressure, and particularly only vacuum pressure to support an
abrasive sheet at high speed rotational lapping. For example, there is no
consideration in the prior art as to whether the vacuum forces could successfully
restrain movement of the abrasive sheet materials when forces (e.g., rotational)
are applied to the abrasive face. The shearing forces, especially if applied
unevenly on the face by non-symmetrical contact with the workpiece, could
easily be envisioned to cause the abrasive sheet to shift. This would be
disastrous in a lapping system and could well destroy all the earlier polishing
steps performed or ruin the workpiece entirely. Although adhesives provide
problems as indicated above, a change from adhesive support to vacuum support
could have been considered to alter the system in unpredictable ways. As
adhesives can elongate with the rotational forces, there may have been some
benefit to the use of a somewhat elastic layer under the abrasive sheet,
particularly in removing any waves or irregularities in the original positioning of
a sheet (although this would not be technically desirable at low speed polishing
or lapping since the forces would be little likely to have a significant effect).
The use of a vacuum would not allow such elastic behavior in an intermediate
layer, as there would be no intermediate layer. This would be another
unpredictable effect in such a change from adhesive to vacuum support of an
abrasive sheet material in high speed rotational lapping.
In the practice of the present invention, the abrasive sheets comprise sheets
of exposed abrasive grit as either a self-supporting sheet or film material or an
adhered layer on a support sheet. The sheets may have any type of abrasive
material or surfacing on the face which is to contact the workpiece. The
preferred sheets are sheet abrasive material manufactured and sold by Minnesota
Mining and Manufacturing Company, St. Paul, Minnesota, and comprises either
a polymeric backing sheet with high Mohs hardness abrasive particulates on a
coated layer or a self supporting sheet of such high Mohs hardness abrasive
particulates. Preferred abrasive material comprises diamond particles or
particles comprising small diamond particles supported in a binding matrix
(other than any adhesive matrix forming the self-supporting layer or adhering the
particles to a support). The sheets may comprise a single layer of material (e.g.,
a binder with abrasive grit therein or sintered abrasive grit without any other
binder) or multiple layers of materials. Such multiple layers could comprise one
or more supporting layers, intermediate layers (e.g., primer layers, vibrational
damping layers, electrically conductive or antistatic layers, magnetic layers,
printed layers, sealer or barrier layers to prevent migration of materials between
other layers), and an abrasive outer layer. The single layer, at least one layer in
the combination of layers, or the interaction of the combination of layers must be
able to support a vacuum against the back surface. Preferably the back surface
(of the abrasive sheet) itself is non-porous or low porosity. This is desirable as
too much porosity would prevent the sheet from being held against the rotatable
support surface. The sheet does not have to be completely non-porous, although
this is the preferred method of making the sheets used in the present invention,
especially when combined with the vacuum draw-down of the abrasive sheets.
In addition to limiting the porosity of the sheets, the back surface should not
have such a degree of topography which would allow free air flow along the
back surface when it is being held against a surface by a pressure of at least 8, 9,
10, 11 or at least 12 lb/in2. If there were raised channels, ridges or the like which
would allow air flow from the center of the sheet to its outer edges, the pressure
would not consistently support the sheet as air would more readily leak out from
the region between the support surface and the backside of the abrasive sheet.
That construction would be useful, but less preferred in the practice of the
present invention.
The abrasive material may be any known abrasive material, depending
upon the ultimate needs in the process for grinding, polishing or lapping a
particular finished article. The abrasive particulate or raised particulate areas
may comprise any solid, hard, material such as silica, titania, alumina,
Carborundum, boron nitride, homogeneous inorganic oxides (such as metal
oxides) or blends of inorganic oxides, diamonds (natural or synthetic), or any
other material which is harder than the solid surface to be polished, ground or
lapped. The abrasive surface may be abrasive particles bound in a binder, either
partially embedded, superficially bound to the surface, or initially embedded so
that the binder must initially wear away to expose the particles. The abrasive
surface may be a replicated surface structure of a pure abrasive material, an
etched abrasive surface, molded surface or the like. The abrasive surface may
also be deposited islands of abrasive material, with either physicalprocesses used
to place the abrasive (e.g., vapor deposition, screened application of powders
which are fused, powder arrays which are electrostatically deposited and bonded
to the surface, impact embedding of the particles) or chemical processes (e.g.,
electrochemical deposition, chemical deposition at seeded sites) to form the
particles in a random or ordered manner. The preferred material is an abrasive
sheeting manufactured by Minnesota Mining and Manufacturing Co., known as
Diamond Abrasive Disks (3M). These sheets are quite effective for the high
speed, fine finish lapping processes and apparatus of the present invention. Also
useful in the practice of the present invention are diamond particles contained in
a metal matrix on a sheet of plastic backing material (e.g., 3M Metal Bond™
Abrasive). The only modification of the sheets which is essential for making
them completely compatible with the present invention is having the sheet
converted (cut) to fit the abrasion platen. The sheets may be cut into, for
example, circular shapes, with or without positioning holes or a centering hole in
the sheet. This abrasive sheet material has been able to provide an improvement
at high speed lapping which was not recognized at lower speed lapping, where
the problem was not notice and/or was not as significant. The 3M Metal Bond™
Abrasive has islands of the abrasive material, as opposed to having a continuous
matrix of binder with the abrasive particles therein. The islands therefore allow
swarf, debris and liquid to pass between the islands (driven by centrifugal forces)
and away from the contact area between the abrasive sheet and the workpiece.
This prevents the moving material from forcing the workpiece out of alignment,
creating different grinding functions locally, or causing other mischief with the
system.
The present invention may be further understood by consideration of the
figures and the following description thereof. Figure 1 shows a perspective of a
basic lapping apparatus 2 according to the present invention. The apparatus 2
usually comprises at least a main support frame 4 with a vibration absorbing
surface 6 which may be a single layer 6 as shown in Figure 1 or multiple layers
(not shown). The composition of the layer may be thick metal, layered metal,
composite, coated metal, and the like. Two thick sheets of metal (not shown) is
preferred, with one sheet fixed to the main frame 4 and the other sheet fixed to
the frame top 8 at the arms 12 or which is removably attached to the first layer
(not shown). There is also conveniently a frame top 8 which may be removably
or permanently attached to the main frame 4. An electrical enclosure 10 is
shown over the vibration absorbing surface 6. A supporting frame 14 is shown
for a workpiece spindle 16. A computer 18 is also shown in the lapping
apparatus 2 to provide controls over the operation. The abrasive sheet (not
shown) support platen 20 is located at a position on the vibration damping
surface 6 over which the workpiece spindle 16 may be positioned. Various
positioning systems (later shown) which operate to keep the alignment of the
workpiece spindle 16 and the abrasion support platen 20 can be preferred part of
the apparatus 2. An abrasion platen drive motor 22 can be seen underneath the
vibration damping surface 6. The size of the apparatus 2 is somewhat dependent
upon the needs for the user. The length 24 of the base of the main frame 24 may
be, for example, between about 3 to 8 feet (0.9 to 2.42m), the width of the main
frame may be, for example, between 1.5 feet and 4 feet (0.45 to 1.22m), and the
height of the main frame may be, for example, between 1.5 feet and 4 feet (0.45
to 1.22m). Greater variations in the dimensions are of course possible, but the
preferred dimensions are within this range, and especially between 4.5 feet and
5.5 feet (1.64 and 2.0m) in length and 2 to 3 feet (0.68 and 0.91m) in width and
height. A heavy construction is preferred, with at least 0.6cm thick steel plate in
the arms 12, 30, 32, 34, 38, 40, etc. (collectively referred to as the arms 12. The
arms 12 may be hollow with sheet metal of that thickness or larger, or may be
solid. The dimensions of the arms 12 may be, for example, from 2 to twelve
inches (5 to 31 cm) a side (assuming a square). This fairly massive composition
will keep vibration to a minimum. A four wall box 19 is shown surrounding the
platen 20 above its flat surface (e.g., the plane of rotation of the surface). A
curved lip 21 is shown at the top of the four wall box 19 to prevent ricochet of
exploded pieces and to deflect them down within the box 19, possibly to a
collection area (not shown).
Figure 2 shows an abrasive platen 50 useful in the practice of the present
invention. In the practice of the present invention, a wide range of diameters is
useful for such abrasive platens 50. Typical diameters are from 7.5 to 50 cm,
more preferably from 7.5 to 40 cm in diameter. The abrasive platens 50 of the
invention are provided with a sufficient number of ports or holes (not numbered)
to enable a vacuum to be distributed against the backside of an abrasive sheet
(not shown). In Figure 2, three circular distributions of such holes 52, 54, 56 are
shown distributed as a series of holes 58. The holes 58 are a convenient,
exemplary distribution, but are not essential to the practice of the present
invention. Vacuum access to the backside of an abrasive sheet may be provided
in many different types of distribution. The distributions do not even have to be
symmetrical, but should be reasonably distributed so that sections of an abrasive
pad will not lift from the platen 50 during high speed rotation while other areas
are secure. There is no need to have an asymmetric distribution of holes 58, but
it is a feasible construction. A circular distribution is convenient as the abrasive
sheets generally used tend to be circular to fit with the circular motion of rotation
and the usually circular shape of the platen 50. Other shapes may be selected,
but they would tend to be prone to greater eccentricities in their motion and
therefore would be less desirable. The circular set 52 of holes 58 nearer the
center of the top surface 66 of the platen 50 help to secure the center portion of
an abrasive pad to the platen 50. Likewise, the circular distributions 54 and 56
tend to secure an abrasive pad to the surface 66 of the platen 50 along a radius
60. The number and spacing of holes on the platen surface 66 are designed to
secure an abrasive sheet without the holes (e.g., 58) being so large as to deform
the sheet into the contours (not shown) of the holes. Holes on the surface are
preferably less than 5 mm in diameter, more preferably less than 4 mm, still
more preferably less than 3.5 or less than 3.0 mm, and most preferably greater
than 0.5 mm and less than 3 mm. The minimum size and number is determined
by that number and size which will support a vacuum against the backside of an
abrasive sheet. A minimum size of about 0.2 mm is a reasonable starting point
for commercial design. Smaller holes would clog too easily from materials
produced during operation of the apparatus. More preferred would be diameters
of at least 0.5 mm, more preferably at least 0.7, still more preferably at least 1.0
mm. These are average diameters, and hole sizes that differ within each circular
distribution or amongst circular distributions are contemplated. Ranges of
between 0.2 and 5 mm may generally be used. The circumferential edge 68 of
the platen 50 may have engaging grooves or cogs 70. These cogs 70 would be
used to engage with driving gears 72 and 74. A motor (not shown) would drive
these driving gears 72 and 74 to rotate the abrasive platen 50. It is also desirable
to have the material around the edges of the holes hard or abrasion resistant to
avoid enlargement of the holes by abrasive grit being drawn into the holes.
Abrasion resistant coatings, sacrificial coatings, hardened metal (e.g., hard
chrome plating (Rc 80) and the like can be used to strengthen and harden the
holes.
Figure 2 shows an approximately 32.9 cm diameter (13 inch) platen 50
with a centering post 62 which may be a removable centering post 62 inserted
into a hole 64 in the surface 66 of the platen 50. In Figure 2, the first circular
distribution of holes 52 at a diameter of about 62.8 mm (2.5 inch) comprises 30
holes having diameters of about 1.5748 mm (0.062 inches). The third circular
distribution of holes 56 at a diameter of about 29.2 cm comprises 180 holes of
about 1.5748 mm (0.062 inches). The second circular distribution of holes 54 is
at a diameter of 22.8 cm (9.0 inches). Radial, rather than circular patterns of
holes may be easily placed on the surface 66 of the platen 50. Designs or other
patterns, or even random distributions of holes may be placed onto the surface as
long as a vacuum can be supported on the backside of an abrasive sheet.
Smoothness and flatness are two characteristics which are used in the art to
measure the quality of lapping and polishing performance. Smoothness can be
measured by profilometers (either, for example, confocal or stylus) and is
measured in linear dimensions and standard deviations or variations from
uniformity. Flatness is conventionally measured in terms of light bands, using
equipment such as LAPMASTER™ Monochromatic Lights (e.g., Models CP-2
and CP-1) in combination with flat glass over the surface to be evaluated for
flatness. The use of light band units (e.g., the number of lightbands per unit of
horizontal dimension on the surface being evaluated, e.g., per inch) can measure
surface flatness within millionths of an inch. Curvature of radiating lines away
from a line of contact between the glass and the surface against which light is
being projected would indicate a degree of convexity to the surface and lines
curving towards the point of contact would indicate a degree of concavity.
Straight, parallel, evenly spaced lines indicate true flatness. Normal lapping
procedures of the prior art are able to achieve 1-2 lightbands of smoothness, but
the process commonly takes hours, depending on the material started with.
Particularly when the material is hard (e.g., tungsten carbide or special alloys),
conventional lapping is performed in hours, not necessarily including the
necessary cleaning time. The use of the apparatus, processes and materials of the
present invention can easily achieve 4-5 lightbands of smoothness in minutes
(e.g., 5 minutes or less), and with apparatus and processes combining all of the
improvements described in the present invention,. 1-2 lightband smoothness has
actually been achieved in less than an hour (e.g., 15 minutes or less, even at 10
minutes), which time included replacement of sheets at the various stages and
time for normal cleaning operations. Other conventional parameters of lapping
have been exceeded by practice of the technology of the present invention.
It is a standard assumption, proven consistently by reported data and
analysis, that lapping with abrasives causes fracturing within the workpiece to a
depth which is equal to the average diameter of the abrasive particles. That is, if
the average size of particles in a slurry or coated on a sheet are 50 micrometers,
the workpiece, from that operation, will show microfracturing on the lapped
surface which is equal to the average diameter of the abrasive particles used to
lap the surface. Each successive lapping operation (e.g., starting with 50 micron,
then 10 micron, then 2 micron particles) will leave successively smaller
microfractures, but each will be approximately equivalent to the average size of
the abrasive particles used in the last lapping step. The amount of material
removed in each lapping step, however, will more nearly approximate the degree
of damage created in the previous step. Therefore, if 50 micron particles are
used in one step and 10 micron particles are used in a second step, the second
step will remove approximately 50 microns (the damaged depth remaining from
the previous step) and itself leave a damaged depth of about 10 microns. By
operating at speeds of at least 500 rpm (that is surface speeds of at least 1000
surface feet per minute), diminished amount of microfracturing (where
individual grains of material are broken loose, resulting in "pick-out") has been
reported in some cases in the practice of the present invention. By using higher
surface speeds, the micro fracturing continues to be reduced until microfracturing
pickout as little as or less than 90%, 80%, 70%, 60%, and even 50% of the actual
average diameter of the abrasive particles occurs in the work piece. This is a
potentially improved characteristic of the lapping effect of the present invention.
No other lapping operation is known to provide this reduction in pick-out. This
is a definable aspect of a process according to the present invention, and may be
seen in many different materials, such as in tungsten carbide, blends or alloys of
metals (e.g., copper and tungsten), plastics, composites, etc. The process also
tends to smooth out non-homogeneous mixtures with less gouging of material,
thus leaving fewer holes or pits in the surface because lapping and polishing,
rather than gouging, is being effected. Even when performing conventional
lapping processes using slurries of individual abrasive particle material in liquid
carrier, low speeds of 5-200 revolutions per minute (rpm) are normally used.
Some processes do use higher speeds with slurries up to 2500 rpm, for example,
and the pressures used to hold the rotating platen face and the work piece face
together are perhaps 200 pounds with a 10 cm by 10 cm work piece face (which
is about 12.9 pounds per square inch contact force). It is considered by abrasive
technology researchers that a primary method of material removal from the work
piece is for the individual abrasive particles to roll along between the piece part
and the platen, rolling off or flattening high spots, or the abrasive particles are
dragged along by the moving platen and shear off high spots. In either case,
because the average normal clamping force is high, very large localized forces
are concentrated against individual grains or areas of the piece part material at its
surface. These localized forces are strong enough to weaken and break the bond
between the grain in the piece part and the main bulk of the piece part at the
grain boundary. Subsequently, the loosened grain will be forced out of its
original position and leave a void, pocket or pit where it was originally located.
These pits are referred to in the art as "pick-outs" and are very undesirable.
With high speed lapping according to the present invention, the normal
(perpendicular) force can be generally much lower than in lower speed lapping
processes, being as low as 10% of the forces normally encountered in lower
speed lapping, such as only 20 pounds (8 kg) of normal force for a 10 cm by 10
cm work piece. As noted above, the contact pressures in the practice of the
present invention may range from 0.1 to 100 psi, but are more preferred between
0.1 and 10 psi, still more preferred between 0.1 and 5 psi, and most preferred
between 0.1 and 3 or 0.5 and 3 psi. Because this normal force is so much less,
the localized forces on individual grains and abrasive particles are reduced and
much less fracturing of the piece part surface and grains on the piece part surface
occur. Pick-outs on the surface have been shown to be reduced by from 10 to
90% as compared to surfaces with the same flatness, so that the smoothness of
the surface is improved even while the good flatness is preserved. This is
particularly important in the lapping of blends or composite materials where the
surface to be lapped is not uniform on a molecular scale (e.g., solid state
solution), but rather provides a surface with regions of different materials (e.g.,
particles in a matrix, dispersed metal in a matrix, etc.), and where different
responses to the action of abrasive grains may be experienced in local areas of
microscopic proportions. For example, where blends of metals are present (e.g.,
tungsten and copper), high speed lapping will tend to cut off both metals by
impact fracture at the same level or height, providing a superior surface finish
(less roughness, more smoothness).
With the very high speeds of the abrasive particles in the practice of the
present invention, particularly at speeds above 7,500 or above 10,000 surface
feet per minute, as compared to 1,000 surface feet per minute, a completely
different mechanism of lapping appears to occur on the smallest levels of the
materials. With the higher speed lapping by particles on the abrasive sheet, the
tops or high spots on the piece part surface appear to be removed by impact
fracturing in addition to involving the normal mechanisms and effects of
shearing and rolling down high spots. Removal of excess tall material by the
mechanism of impact fracturing results in lower levels of disturbance to grain
boundaries between grains in the piece part and reduces the number of individual
grains being broken loose.
Another significant advantage of the use of the abrasive sheets at high
rotational speeds according to the present invention is that wear on the platen
surface itself is greatly reduced. In slurry processes, the abrasive action works
equally forcefully against the platen face and can eventually wear off the surface
of the platen to a degree where the platen would have to be replaced. Even
though the wear would of course tend to be even, there is no functional reason to
continually sacrifice or wear out the platen. Some uneven patterns of wear may
develop in the platen, and these would be translated into uneven lapping of the
piece part.
Other features of the lapping apparatus of the invention, problems
addressed, and solutions to these problems are also described herein. They are
numerically listed below.
Problem: When grinding or lapping single or multiple piece parts held by a
tool holder with a typical diameter of 4 inches held by a center post, the tool
holder is slowly (or quickly) rotated as it is presented downwardly and vertically.
This movement is intended to uniformly contact the work piece and an abrasive
surface rotating at very high speeds of from 2000 to 3,000 rpm (this can
effectively be equivalent to more than 9,000 surface feet per minute (sfpm),
depending upon the diameter of the platen. During this process, it is important
that the piece part holder be "flat" so that the piece parts which contact the
abrasive first are not damaged. This would be the case if the holder had one
edge lower than another in its presentment to the abrasive sheet. Furthermore,
with high speed lapping and grinding, it has been found to be important that the
piece part holder assembly be held by a ball or gimbal pivot type of device
located as low as possible toward the high speed abrasive surface. This is the
best design found to align the total piece part assembly so all the individual parts
(e.g., the platen carrying the abrasive sheet and the work pieces) are floated
equally by the thin boundary layer of coolant fluid on the surface of the disk
which may be less than 0.001 inch (0.0254mm) in depth. Boundary layers do
not normally remain constant as the distance from the leading edge (contact
point or liquid introduction point, or radial distance on the platen or
circumferential distance along the tangential distance on the workpiece). The
changes in the thickness of the boundary layer cause significant variations in
platen separation distances from the work piece and effective variations in
penetration of the workpiece by abrasive particles on the sheet. With this type of
ball or gimbal pivot, the piecepart tends to lay flat with respect to the platen
abrasive and also this boundary layer thickness has a tendency to remain uniform
even with slight out-of-perfect-perpendicular alignment between the vertical
piece part holder shaft and the high speed abrasive platen. Foreign debris can be
accumulated in pivot joints and create unwanted friction.
Solution: A work holder device is created with the use of a special
ball attached to a shaft which ball and shaft combination provides a pivot action
close to the bottom of the work piece holder assembly. A sandwich of washers
acts as a rigid base to transfer downward a polishing normal force on the vertical
shaft to push the piece parts into the abrasive platen. The pivot action is
restrained by encapsulating the whole assembly with room temperature
vulcanizing (RTV) silicone rubber or other elastomeric resin (e.g.,
fluoroelastomers) which seals the unit from debris and also provides the function
on an elastic restraint that self centers the disk type part holder perpendicular to
the axis of the support shaft. Yet the elastic spring which centers the unit is
weak enough to allow conformal pivoting of the assembly during lapping action.
Thus when there is little side load present, as when lowering the piece part
assembly, the unit is flat aligned. But when the assembly is subjected to a
normal force, the unit is free to pivot. A piece part holder with the back stem
and RTV resin was constructed and used in a piece part assembly for
lappingoptical connectors and appeared to function well.
Problem: The surfaces of metal objects are polished for many reasons
including for optical examination of metallurgical characteristics, to create a
smooth, low-wear, tight hydraulic or fluid seal and others. Usually this polishing
is done at low speed (e.g., 5-200 rpm), with rotating flat platen disk wheels of
various types of construction molding aluminum, steel, plastic cloth and others.
The wheel surface is very flat and the workpiece to be polished is held with
controlled pressure by hand or work holder. Water or other fluid, such a
lubricant or wetted abrasive particles are introduced as a slurry, or disks of fine
abrasive sheets are "stuck" or bonded to the rotating wheel. This process is slow
to produce a highly polished surface, and it is labor intensive if not automated.
Inaccurate platen or shaft machining, loose bearings, or weak machine structure
and framework may cause polishing accuracy problems.
Solution: The present invention enables very high quality polishing
which can be achieved in a fraction of the conventional lapping time by using
abrasive sheeting, such as 3M brand of micro abrasive disk sheets, for polishing
at very high speeds of 2,000 rpm and more using disks about 8-10" in diameter.
However, it is critical that the rotating platen disk run very "true" and flat at the
operation, speed range to provide a mechanically stable moving surface against
which the to-be polished workpiece is held stationary with a controlled normal
force or pressure (against the fine particle wetted abrasive). Options also may
change the pressure as a function of process time or the workpiece rotated to
distribute polishing across the surface.
A unique method to provide a very "flat" and accurate stable rotations
platen disk surface would be to mount the platen to a "weak" shaft which allows
the rotating disk mass to seek a true "smooth" center at speeds above its first
rotating natural frequency. The motor drive speed would be increased above its
natural frequency, the workpiece part presented in contact for polishing; then
removed prior to reducing the disk RPM below its critical harmonic speed.
Problem: The presence of liquid on the abrasive surface adjacent the work piece
has combined with higher rotational speeds to generate significant hydroplaning
of the liquid and unequal forces on the face of the abrasive sheet and the work
piece at differing positions along the radial distribution from the center to the
outer edge of the abrasive sheet and also along the tangential contact length of
the piecepart surface. The liquid is often essential to control heat, friction and
cleansing of waste materials, and can not be easily removed.
Solution: The greatest needs for the liquid are 1) to control friction between the
abrasive surface and the work piece, 2) control the temperature of the sheet and
the work piece, and 3) to wash away residue of abrasive and abraded material
from the work piece. These effects do not have to be performed at the same
location between the sheet and the work piece and do not need the same amount
of liquid (e.g., water, lubricant, coolant, etc.) to accomplish the separate tasks.
The inventor has recognized that the amount of water needed to affect friction (a
surface phenomenon, and essentially two-dimensional [very thin] amounts of
liquid may be effective) tends to be much less than the amount needed to control
temperature (a bulk, three-dimensional phenomenon) and waste removal (a
three-dimensional and mass flow process). With this recognition, it has been
found that liquid may be applied to the lapping process of the present invention
with controlled amounts, specified positions, and timed introduction to perform
the process with reduced likelihood of hydroplaning because of reduced amounts
of liquid between the abrasive ( as a sheet or other form) and the work piece.
This is accomplished in the following manner.
The abrasive sheet is of a sufficient size relative to the work piece that less
than fifty percent (50%) of the abrasive surface will be in contact with the work
piece surface during lapping. Preferably less than 40%, more preferably less
than 25%, and most preferably less than 15% of the total surface area of the
abrasive sheet is in contact with the work piece during lapping at any specific
time. The area where the abrasive and work piece are in actual contact is called
the work area. In a zone or area rotationally before the work area, water is
placed on the surface of the abrasive sheet. The amount of liquid (e.g., water)
provided is preferably less than 120% by volume of that amount sufficient to fill
the valleys between the peaks of the raised abrasive particles (100% essentially
forming a smooth, continuous layer of liquid over the abrasive material). More
preferably it is less than 110%, less than 100%, but at least 30% of that filling
volume of liquid. Preferably the amount is between 30% and 120%, more
preferably between 40 and 115%, still more preferably between 50 and 110%,
and most preferably between 90 and 105% of the volume necessary to exactly
fill the valleys on the abrasive sheet so that an essentially flat film of liquid
appears although surface tension between the peaks and the film may distort the
appearance so that slight circular patterns may appear without dry exposure of
more than 20% by number of the particles. This approximately 100% volume
amount is called the "leveling amount of liquid" in the practice of the present
invention.
At a zone which is rotationally before the work area, a first amount of
liquid equal to 30 to 120% of the leveling amount of liquid is placed on said
abrasive surface. The area where this is performed is called the wetting area. On
the surface of the abrasive sheet, rotationally after the work area, a second
amount of liquid is applied to said abrasive surface, said second amount being
both sufficient to have the sum of said first amount and said second amount
equal to at least 120% of said leveling amount of liquid, and equaling at least
30% of the leveling amount of liquid. Preferably the total of said first and
second amount comprises at least 150%, more preferably at least 170% of said
leveling amount. Likewise, it is preferable that the amount of said second
volume is equal to or greater than at least 50% of said leveling amount, and more
preferably at least 75% or at least 100% of said leveling amount. This second
volume will assist in carrying or washing the total residue on the abrasive sheet
(the residue abrasive and the swarf from the piece part). The second volume is
applied in what is referred to as a flood area on the abrasive surface. The high
rotational speeds will remove a significant amount of the liquid and total residue
on the abrasive surface, but because of the high quality sought in the lapping
performance of the present invention, this may not always be relied upon. To
improve the removal of the liquid carrying the total of the residue, air blades
(e.g., hypodermic air knives) can be positioned between the flood area and
before the wetting area. The air blades, in combination with the rotational
forces, will remove a very high percentage of the applied liquid and the total
residue so that an essentially dry surface can be assumed to enter the wetting
area. To whatever degree it is found that not all liquid is removed by the
rotational forces and air knives, the first amount of liquid may be reduced so that
the appropriate percentage of leveling is provided.
The schematics of this apparatus and process are shown in Figure 4. A
water controlled system 340 according to the present invention is shown
comprising a platen 342 having an annular distribution of abrasive sheeting 344.
The annular distribution 344 is preferred, but not required in the practice of the
present invention. A first liquid (e.g., water) supply means 346 lays over said
annular distribution 344. A second liquid supply means 348 is also shown to
overlay the annular distribution 344. An air blowing means 350 is also shown to
overlay the annular distribution 344 on said platen 342. A work piece 360 is
shown over the platen 342. The rotation direction 370 of the platen 342 is such
that liquid 362 deposited from said first liquid supply means 346 is upstream of
the work piece 360. The liquid 364 provided by said second liquid supply means
348 is located downstream of the work piece 360. The air blowing means 350 is
downstream of the second liquid supply means 348. The air blowing means 350
provides sufficient volume and intensity of air movement to assist in removing
liquid 366 which had been on the platen 344.
Problem: After a high speed 3,000 rpm, 12" (30.5 cm) diameter rotating
abrasive platen has been manufactured and used on a lapping machine, it does
not remain perfectly flat as originally machined. A platen which has been
ground or damaged by wear or impact away from a required or desired flatness is
no longer effective for high precision. For example, a platen should have a
deviation in flatness of less than 0.0005 inch (0.0126 mm) at the outer periphery
with a need for the best performance to reach 0.0002 inch (0.00508 mm) or less
than 0.0001 inch (0.00254 mm). The platen should be flatter than the variations
in thickness of the rotating abrasive disk surface. The platens are ground to the
above tolerances (e.g., less than 0.0126 mm variation in thickness along an entire
circle within the disk surface). These measurements can be made, for example,
with a micrometer or other linear measuring device. The flatness is measured by
reading the variations in thickness along such circles within the disk surface.
The abrasive sheet (e.g., the diamond sheeting) lays relatively flat on the surface
of the platen, but is expected to have some variations in thickness of the backing
material (e.g., plastic film, such as polyester) and the abrasive coating.
However, it is desirable to minimize variations and prevent additive deviations from occurring. This measurement can be made by a dial indicator placed at the outside diameter and the disk rotated by hand for one revolution to measure the maximum excursion. Any deviation acts either as a "valley" where the abrasive does not contact the piece part or a "high spot" which is the only area that contacts the piece part. When the disk rotates at its normal high speed, the high spot will have a tendency to hit the piece part and set up a vibration which will reduce the smoothness of the lapping abrasive action. Localized distortions of the platen surface will also have a tendency to penetrate the boundary layer of liquid between the platen (covered with a thin sheet of diamond or other coated abrasive) and the piece part. This can produce a localized scratch or track on the piece part surface. Any surface defect on the platen structure is generally transmitted through the thin abrasive disk and produces a bump or high spot on the disk.
However, it is desirable to minimize variations and prevent additive deviations from occurring. This measurement can be made by a dial indicator placed at the outside diameter and the disk rotated by hand for one revolution to measure the maximum excursion. Any deviation acts either as a "valley" where the abrasive does not contact the piece part or a "high spot" which is the only area that contacts the piece part. When the disk rotates at its normal high speed, the high spot will have a tendency to hit the piece part and set up a vibration which will reduce the smoothness of the lapping abrasive action. Localized distortions of the platen surface will also have a tendency to penetrate the boundary layer of liquid between the platen (covered with a thin sheet of diamond or other coated abrasive) and the piece part. This can produce a localized scratch or track on the piece part surface. Any surface defect on the platen structure is generally transmitted through the thin abrasive disk and produces a bump or high spot on the disk.
Solution: An existing platen can be "dressed" as a machine by bringing it up
to full high speed RPM and lowering a heavy flat abrasive coated piece unit
directly onto the bare rotating platen and grinding or lapping off the bumps.
High spots and even full out-of-flatness surface variations can be removed by
first using a coarse abrasive and progressively using finer abrasive or lapping
abrasive medium. A typical first abrasive may comprise 40 micron metal-bonded
diamond and a final abrasive may comprise 3 micron or less diamond or
ceramic abrasive depending on if the platen surface is chrome plated, stainless or
base steel. The abrasive lapper disk could be oscillated back and forth across the
platen, it could be stationary or it could rotate at either slow speed or rotate at a
very high speed so the tip speed of the grinding disk will provide uniform
removal of platen material at the low surface speed of the inner radius of the
platen. Different geometries of adhesive disks could be used. Also a piece part
holder already in use for normal lapping could be used to perform this function.
Problem: When lapping or grinding at high speeds of 3,000 rpm on a
12" (30.5 cm) diameter platen producing perhaps 8,000 to 12,000 surface feet
per minute (sfpm) of surface lapping speed by use of wetted plastic disks coated
with thin layers of diamond or other abrasive material, it sometimes is a
disadvantage to have a uniform flat disk surface in flat contact with precision
piece parts. This is because the fluid boundary layer of the wetting liquid has a
tendency to draw the piece part down to the flat surface of the rotating platen and
create large fluid adhesion forces. These fluid adhesion forces require more
force to hold piece parts in combination with bigger motors and require the use
of larger and heavier holding devices for piece parts. This may also create a
lower rate of metal removal and the further disadvantage of the grinding debris
being carried along between the abrasive disk and the work piece surface. This
can produce scratching or other disturbances on the work piece surface.
Solution: A precision ground rotating platen can be fabricated with
slightly raised spiral surfaces having different shapes and/or patterns, these
shapes or patterns varying from the inside center of the platen toward the outer
periphery of the platen. The spiral patterns would create land areas at the top
surface of the platen of the various widths, shapes with areas between these land
areas that are somewhat lower, perhaps from 0.002 inch to .010 inch ( 0.051 to
0.254 mm) or more. Then a thin plastic coated abrasive disk that is uniformly
coated with precision fine abrasive (e.g., the 3M diamond abrasive sheet material
cut into disk form) would be mounted onto the round platen and held in place by
vacuum hold-down holes either on a raised land surface or on the lower surface
area or a combination of holes in both areas. The raised land areas could be
produced by manufacturing a precision platen and acid etching or
photolithographically etching land area geometry configurations. When the
abrasive disk is mounted on the platen, only some portions of the disk would be
in contact with the piece part being ground or lapped. The boundary layer of
fluid coolant would be affected by the length of the land area under the piece
part, the direction the spiral, radial or circular annular land shapes or a
combination of the geometries. The effects on the boundary layer thickness
would be the rotation speed of the platen, as related to the vector speed,
including the direction of the surface relative speed between the two, the
viscosity of the fluid, and the normal force pressure of the piece part holding it to
the platen. The boundary layer thickness, which would vary over the surface of
the piece part, would affect how the individual particles of abrasive (normally
protruding about 1/3 of their size above the binding agent) effectively abrades a
workpiece from the surface of the abrasive disk. If more liquid is applied, the
boundary layer would tend to be thicker and less abrasive material removal is
achieved. Thus the local pattern of the surface of the abrasive contact area can
be utilized for the optimum grinding action using only one portion of the
abrasive disk with the non-raised section between the land areas of the abrasive
allowing free passage of grinding debris. When this surface area of the abrasive
is worn, the disk can be unmounted by the vacuum chuck, rotated to a "fresh"
area of the abrasive, and then grinding would be continued. The disk will remain
uniform and strong throughout an extended service.
Problem: Again, the problem to be addressed is hydroplaning, which distorts
positioning of the abrasive surface and the work piece relative to each other.
Especially with relatively thin or flexible work pieces (e.g., work pieces thinner
than 10 cm, especially thinner than 5 , 2, 1, or 0.5 cm), the worst distortion of the
positioning occurs because of bending or flexing of the work piece. This is
because the flexible sheet may be supported on a relatively inflexible support
platen.
Solution: Two rotating platens may be provided, one each on opposite faces of
the piece part or work piece. The work piece is secured against movement
between the two abrasive surfaces (on the two rotating platens). The two
rotating platens are rotated at the same time, in the same or opposite directions,
with similar amounts of liquid applied between each platen and the work piece.
The disks do not have to be rotated at the same speeds, and when this is done,
the volume flow rate of liquids used need not be as similar since the respective
hydroplaning forces are proportional to the speed and the volume flow rate of
liquid. The relative speeds of rotation and the relative volume flow rates of
water are selected so that the hydroplaning forces are fairly similar at the
opposite outer edges of the work piece. With similar forces pushing against
opposite faces or sides of the work piece at similar radial distances, there is no
effective flexing force applied to the work piece. The increasing forces along the
radial directions of each face of the work piece will be nearly equally balanced
by similarly distributed increasing forces on the opposed side of the work piece.
The two forces thus cancel each other out and there would be no flexing from
hydroplaning. The film of liquid between the abrasive surface and the work
piece would then remain essentially the same from where it was introduced to
where it exits at the periphery. The speed and volume flow of the liquid would
actually decrease from the central region to the exterior region at any given point
along a radial line.
Problem: It is difficult to quickly load piece parts onto a piece part
holder for use with a high speed lapping and polishing system. Also, it is
difficult to generate a flat parallel system of polishing parts where .001" to .002"
(approximately 0.025 to 0.051 mm) of material is removed from a surface to
make the surface smooth, perhaps with variations of no more than 4 lightbands
in smoothness, while the surface remains flat and parallel. Hot melt adhesives
are presently used to fix piece parts onto the piece part holder. The use of these
adhesives is slow and cumbersome to apply. The residue of the adhesives are
also difficult to remove, and may contaminate the precision surface of the piece
part for later use. Typically, the piece part holder has a gimbaled spherical ball
end to freely allow the part to move about radially to self align the piece parts
(one or more) with the surface of the rotating abrasive platen.
Solution: A piece part holder can be constructed out of a heavy metal
such as steel which has substantial mass very close to the surface of the abrasive
disk. The piece part holder unit will be allowed to move freely with the surface
by the ball-end holder. A substantial hole can be made within the ball-end device
which would allow vacuum to be coupled to the piece part holder. Individual
part pockets will firmly hold the flat piece parts tightly against the individual
tight fitting part pockets to create and maintain a good vacuum. A thin layer of
oil or grease can be applied to the piece part to seal any leakage paths. By
simply removing the vacuum applied by a rotary union to the drive shaft open
inside diameter, the part is released, it may then be turned over. The opposite
side may then be lapped to produce a high quality surface which does not
damage the already lapped side because intimate part-to-holder contact is not
made, the parts being separated by the film of oil. The part pocket is still stiff
enough for good polishing action.
Problem: When using a diamond (or other fine and hard abrasive
material) abrasive disk rotating at very high surface speeds of 10,000 sfpm, most
of the abrasive cutting action takes place at the outer periphery of the disk. The
inside area of the disk has low surface velocity and low cutting action and also
low wear rates. When a piece part traverses the disk in a sweeping motion, to
prevent wearing of tracks or grooves in the abrasive, there is uneven wear at the
outer and inner surfaces of the disk. There is typically a small ¼, ½, or 5/8"
(0.626, 1.27, or 1.58 cm) diameter hole at the inside of the disk. The hole is
usually centered to act as a positioning means to fix the abrasive disk at the
center of the platen to obtain good balance for the very high speed system. A
larger diameter round section could be removed from a disk to create an annular
ring of acting abrasive material somewhat larger than the piece part. This would
eliminate the inactive (and raised) uneven section but then the centering
registration hole for positioning the disk is lost.
Solution: A disk can be fabricated with abrasive coated or exposed on
the entire surface of the disk. The inside section of the abrasive disk, toward the
center of the disk, could be removed by grinding or peeling off the abrasive,
leaving the backing material intact with a raised section of the abrasive in an
annular outer ring. The raised area is only where the abrasive is raised above the
surface of the carrier (by the coating thickness). The disk backing material is
usually plastic sheet, which may be reinforced. Another way to construct an
annular ring would be to punch out a center disk section (e.g., a disk of 2 to 6
inches, 5.1 to 15.3 cm) of the disk for separate use and then use a centering plug
(e.g., a 5.1 to 15.3 cm thinner disk) with a small locating hole. The plug could
be centered on a platen center post and the annular disk centered on the plug.
When the disk or annular ring plus disk is fixed into place by the vacuum grip
platen, the plug is or may be removed to enable complete freedom of movement
of piece parts over an annular disk. This complete movement can be effected
since the centering post may also be removed after the annular disk has been
positioned and secured by the vacuum.
The process of using an annular disk element can be effected where the
round sheet has an outer edge and an inner edge defining a cut-out portion and
comprises an annular sheet, said inner edge having a diameter which is greater
than one-third the diameter of said outer edge. The process may also be
performed where said sheet is round and said round sheet has an outer edge and
an inner edge defining a cut-out portion and comprises an annular sheet, said
inner edge having a diameter which is greater than one-third the diameter of said
outer edge.
Problem: When lapping or polishing at very high surface speed of about
10,000 surface feet per minute, it is difficult to mount piece parts onto a rotating
holder. The piece part holders are used for contacting an abrasive disk mounted
or constructed on a rotating platen. The parts must be held in a sufficiently rigid
manner that they are not broken loose from their mount. It is also desirable to
avoid a localized vibration of the typically thin flat piece part (which vibration is
induced by the high speed contact with the rotating platen). Vibrations can cause
patterns of uneven polishing on the surface of the precision part. It is desirable
for efficiency that one or more piece parts are processed at the same time and
that both mounting and unloading of these parts can be done quickly and easily
to provide cost effective polishing rates of production. Furthermore, it is
desirable to have a method of changing parts quickly so that one side be lapped,
that part turned over and the second flat side be lapped to be very parallel to the
first side. This must be done when typically .001" to .002" or less is removed
from each side.
Thin piece parts of about 1"x 2"x .080" (2.54 x 5.08 x 0.23 cm) can be mounted
onto an individual piece of pressure sensitive adhesive (PSA) tape and this taped
piece part can then be held by a vacuum to a workpiece holder. The friction
properties of the non-adhesive side of the tape would be controlled by selection
of tape backing material or by surface conditioning of the backside of the tape to
provide a sufficiently high degree of friction which would resist lateral dynamic
forces in a plane along the surface of the thin workpiece as the nominal 14
pounds per square inch (psi's, 25 inches Hg vacuum, 6635 mm Hg) would apply
a normal force holding the work piece. A large section of adhesive tape could be
used to hold a number of workpieces at the same time. This would allow fast
and easy installation of the workpieces by hand or robot. This flexible assembly
of pressure sensitive adhesive (PS) secured workpieces could than be held in
position against a precision flat surface of a workpiece holder having random
vacuum holes over its surface which would all be sealed by the wide and
complete expanse of tape covering the vacuum holes and at the same time firmly
holding the individual workpieces to the holder. To process the other side, the
group of workpieces would be removed, new tape would be applied to the lapped
surface side, and the tape on the unprocessed side would be easily peeled off.
The tape would not only fix the parts to the holder surface, but also would
protect the precision lapped side from any scuffing action or rubbing on the
holder.
Problem: When holding piece parts on a rotating holder in contact with a
rotating abrasive coated platen rotating at a surface speed of 10,000 feet per
minute, it is difficult to create a gimbaled, free wobble motion which allows the
contacting surface to be continuously aligned by itself to the flatness of the
rotating platen, while at the same time the contacting surface of the piece part is
held stiffly enough in a nominally flat position. This is particularly true when
first lowering the workpiece holder to the abrasive surface while rotating the
workpiece so as not to have one corner of a workpiece contact the abrasive
before other corners or surfaces. This would cause the corner to be preferentially
abraded away, thereby producing an uneven workpiece surface. Vacuum piece
part clamping hoses could also create problem forces.
A coiled spring can be used to apply a self correcting force between the work
piece holder plate having a gimbaled spherical bearing and the rotating drive
shaft of the rotating piece part holder. This spring could be made of metal or
plastic material which would allow the straightening action to be applied but also
would introduce vibration damping for excitation vibrations set up by the high
speed, contact abrasive action. One or more solid plastic coupling bars could
provide damped spring action. Also, if a vacuum hose were to be used to
provide vacuum clamping of the piece part to the piece part holder through a
hollow drive shaft, this type of hose could extend from the shaft and be coiled to
provide a spring support action (with perhaps less than one complete turn, one
complete turn or multiple turns which nominally lay flat with the upper surface
of the work piece holder, which would minimize the creation of uneven
"normal" turns).
Problem: Many of the problems herein discussed for lapping with the flat
surface of a platen are also encountered with beveled edge lapping, where the
edges of a platen are beveled, and abrasive is on the face of the bevel. That
abrasive face is then used to lap or grind another surface.
Solution: There are two fundamental ways of addressing this issue. Both
involve the use of an annular abrasive sheet. The sheet has an outer edge and an
inner edge (defining the inner edge of the cut-out portion of the sheet, where it is
cut-out from a circular sheet, forming a central, round hole). The annular sheet
should be placed on a platen, which is either a) flat, with the outer periphery
bent, or beveled, b) or the inner annular section beveled, or both the inner and
outer edge being beveled. The outer edge should not extend significantly
beyond the outer edge of the bevel or platen (e.g., less than 1 mm, more
preferably less than 0.5 mm, still more preferably less than 0.1 mm). The inner
edge should in likewise dimensions likewise not extend beyond the interior edge
of the bevel or the bend. If the annular disk is positioned on a flat platen, the flat
platen may be bent substantially (with the same or like dimension tolerances) at
the interior edge of the annular disk to form the lapping abrasive edge on the
platen. The only caution which must be exercised is to assure that no folds or
wrinkles appear in the annular disk. A preformed annular disk may be shaped to
fit on the angled or beveled element. The element may be molded or formed to
fit the shape of the platen surface (for example, by having a truncated conical
sheet segment with the inner, smaller diameter hole (formed by cutting the cone)
fitting the slope of the beveled edge, with the abrasive on the interior, upward
facing surface of the cone (within the original cone volume as opposed to being
on the external surface of the cone. The annular disk may be secured by
adhesive, but the vacuum securement of the present invention is preferred.
Problem: Operation of the high speed lapping devices envisioned by
the present invention are at revolutionary or rotational speeds of at least 500 rpm,
or at least 1,500 rpm, and preferably at 2,000 to 3,000 RPM with a fine abrasive
sheet, such as the preferred 3M diamond coated abrasive disk of about 12"
(30.5 cm) diameter. These sheets are normally held to a steel rotating platen by
water film surface tension and positioned by a ½" (1.27 cm) diameter hole at the
center of the disks. These positioning holes were used with a ½" (1.27 cm)
diameter post at the center of the platen. When such a rotational speed of
operation was attempted with the disk secured by water film tension, the disk
lost its surface tension adhesion and was thrown off the platen while polishing a
tungsten carbide piece part. The forces on the disk were such as to lift it off the
½" (1.27 cm) centering post and the whole disk was thrown off to the side of the
machine opening cavity at the top of the machine post.
Solution The ½" (1.27 cm) centering post could be made larger in diameter to
perhaps 1" (2.54 cm) diameter or more. Also, the post could have a hexagonal
shape or an oval shape which would prevent the disk from rotating relative to the
tangential surface of the disk by having the apices of the hexagons (or other
polygon) resist rotation against a similar cut hole in the sheet or disk. The post
could also be made higher so the chance of the self-destructing disk climbing up
the height of the post would be diminished during this type of event. Another
technique would be to employ a clamp type of device to any of these round or
non-round posts to clamp/hold the disk firmly to the surface of the platen at the
center areas of the disk which is not used for polishing. This clamping force
would be effective because of the slow lineal velocity in that sector. The clamp
could consist of a spring locked washer pressed on the disk surface with a thread
nut engaged with a top threaded post. Springs could also be used to control the
amount of force and to evenly spread the force uniformly. Ball insert or other
snap latch fixing devices could also be employed.
Problem: Using round disks of minute particle coated sheets (e.g.,
abrasive particle sheets and especially hard abrasive particles such as diamonds)
of plastic film on 1,500, 2,000 or even 3,000 RPM spinning platens provides
significant difficulties. It is particularly difficult to hold the abrasive sheet in
contact with the platen when the lapping apparatus is operating in contact with
stationary or semi-stationary workpieces. When an abrasive disk becomes loose
by breaking the conventional water filter "adhesive" surface tension between the
disk and the platen, the abrasive sheet has a tendency to rip or bunch-up and
wedge between the workpiece holder and the high inertia spinning platen and
can easily damage a workpiece part or can destroy portions of the workpiece
assembly with the possibility of great danger to the operator. This is a unique
problem due to the very high rotational speeds of 1,500, 3,000 or even greater
RPM with a platen of 15"(38.1 cm) diameter or more constructed of heavy steel
which could generate explosive type failures or at least high velocity projectile
failure. As this equipment is operated horizontally for the most part, the whole
surrounding area around the machine is susceptible to this danger. A previous
attempt by applicants to reduce the likelihood of this type of separation problem
was to coat one side of the diamond abrasive disk with a PSA, pressure sensitive
adhesive film to temporarily bond the disk to the platen. This adhesive created a
flatness accuracy problem in that its normal thickness accuracy varied greatly
around the disk which causes high areas of lapping contact for this super
precision abrasive contact. Secondly, when a disk was removed, some sectors or
pieces of transparent PSA adhesive remained in the platen and formed a bump
when the next abrasive disk was installed on the platen. This then destroyed the
smooth vibration free abrasive lappings at high speeds.
Solution: Use a diamond or other abrasive disks without using PSA adhesive
and first position the disk at the true center of the platen by use of a center hole
in the disk positioned over a post positioned at the center of the platen (or by
other centering means) and then by holding the abrasive disk to the platen by use
of vacuum by use of a rotating union on the hollow rotating platen shaft. The
preferred area to apply the vacuum would be at the inner radius of the disk which
would seal out air first as the disk is installed at the platen center. Because this
inner one-fourth or so of radius is not used as much for lapping because of the
slow surface lapping velocity, there would be less direct forces applied at this
portion of the disk. The second most preferred vacuum area (e.g., the outermost
edge region of the disk) would also not be used much and would have large
holding force.
Problem: It is difficult to quickly lap hand metal or ceramic or other
materials with conventional lapping techniques using disk platens which are 12"
(30.5 cm) to 43" (109 cm) in diameter operating at 200 to 300 RPM using loose
abrasive paste media. The amount of time used contributes to cost and time
delays. Larger diameter platens are potentially dangerous at high speeds and
paste could be used in extremely large amounts as it would be difficult to retain
on the platen surface.
Solution: A high speed lapping system can be a sheet of abrasive material
such as fixed diamond abrasive coated or plated on a disk sheet of material.
These sheets or disks may be used on a rotating platen disk with a diameter of,
for example, 12" (30.5 cm). When operating at 500, 1,500, 2,000 or 3,000 RPM,
the apparatus gives a surface speed of about 9,000 to 20,000 feet per minute. If a
larger diameter platen wheel of 15" inches is used, the RPM can be lowered
somewhat to perhaps 2,500 RPM to achieve the same 10,000 (or 9,000) feet per
minute (fpm). Similarly, if the wheel diameter of the platen is 18" diameter, then
the speed can be further reduced to produce 9,000-10,000 fpm at the outer
periphery of the disk. Any reduction of angular or rotational speed created by
larger diameters is desirable because of the particular danger of a high inertia
wheel creating problems if a disk or part is damaged or comes loose. The higher
speeds used in the practice of the present invention, plus the controls shown for
maintaining accurate address between the abrasive surface and the workpiece
allows for much faster and therefore more economic lapping. Work that
previously took hours, including intermediate cleanup steps, can be performed in
minutes using the apparatus and methods of the present invention.
Problem: The surface finish smoothness and flatness of hard parts made of
metal or ceramic or other materials vary as a function of the work force on the
piece part as the workpiece is held against the surface of a high speed 9,000 to
10,000 fpm abrasive lapping action. Unexplained variations in the quality and
accuracy of the lapping action were observed.
Solution: It was found that the amount of coolant, lubricating water or liquid
applied to the surface of the high speed rotating disk affects the quality of the
lapping action. If a reduced flow rate of water is applied, the abrasive cutting
rate is increased as the relative dimensions of the boundary layer and the total
liquid thickness and dimensions between the base of the abrasive disk and the
piece part are increased. This increase in the relative dimensions of the
boundary layer and the decreasing of the separation of the abrasive disk and the
piece part by the liquid allows the exposed diamond particles to be more active
in removing material as they penetrate deeper into the surface of the material.
Also, if the water flow rate is reduced and the piece part is more "flooded", then
a thicker boundary layer of water or liquid builds up between the part and the
surface of the disk and the piece part. This keeps the (e.g., diamond) abrasive
particles away from the piece part and allows some fraction of their normal
penetration which results in a smoother and flatter surface on the part. One
method of utilizing this performance is to have reduced water flow at the first
portion of the lapping period for more aggressive material removal with an
increased roughness of the surface. Subsequently the water flow is increased
somewhat during the middle portion of the abrasive cycle to get better surface
finish and yet have a medium material removal rate and then to substantially
increase the water flow rate at the end of the cycle to produce a very smooth and
flat surface with a low rate of material removal. This could be easily done with
an automatic water flow rate control system. This would change the water flow
rate automatically at various stages in the abrasive cycle.
The liquid (especially water) introduced as a lubricant between the platen
and the work piece is normally filtered to eliminate particles which are 1 micron
or larger in their largest dimension. The use of a positive displacement pump
such as a gear pump or piston pump can be helpful in determining the optimum
quantities of flow and charge during operation of the system, at the beginning,
middle and end of operation of the lapping cycle.
Problem: When performing abrasive lapping at high surface speeds of over
1000 fpm up to about 10,000 fpm on round platens rotating at 3,000 RPM with
diameters of 12", 15" and 18", there is substantial danger when a piece part is
broken off its holder (as it normally is held with a weaker adhesive or mounting
system, and as uniquely effected in the present invention with the use of abrasive
sheeting and high speed platen rotation) and the piece part being thrown off the
platen or getting stuck on the platen and ripping the diamond or other abrasive
disk causing further possibility of fast destruction of parts of the machine with
parts thrown out and endangering an operator or others or equipment due to large
kinetic energy contained in the rotating disk.
Solution: The rotating platen is round in shape with about a 12" or 15" (30.5
cm to 43.5 cm) diameter. A box is constructed which is rectangular in shape
with "square" corners (4 each) and with the walls some distance away from the
round platen, typically 6" or more. Also the box is desirable to be constructed of
a soft plastic (or rubber) such as ½" thick high density polyethylene which would
tend to absorb impact from a heavy metal part free flying, broken loose parts
without ricocheting the part back into contact with the rotating disk which would
reinitiate this impact action. It also prevents this reinitiated contact from
damaging the part. Also, the "square" corners provide a remote area to trap the
part and to contain the part as it stopped moving by being impacted in one or
more rubber or plastic walls or lined metal walls. Having a distance between the
flat walls and the rotating disk which is somewhat larger than the largest size of
the piece part, centrifugal force would tend to drive the part off the disk radially
and allowing it to roll or move tangentially to a neutral corner of the box away
from the disk. At the same way, crumpled abrasive disks are collected by the
neutral open corners. Having a ledge over the inside portion of the box also
helps trap the parts.
The use of a safety box with at least 10% (of the diameter of the platen)
clearance on each side of the platen within the safety box area is quite effective.
It is more preferred to have the safety box with a clearance of 20%, 30% or even
more than 50% of the diameter of the platen (on each side of the platen within
the box or at least from at least one side of the platen) in the practice of this
aspect of the invention. It is particularly desirable to have the workpiece holder
moving assembly lift the workpiece holder out of the safety box so that the box
may be cleaned without contacting the platen. A removable bottom section may
be constructed on the box for bottom cleaning without having to significantly
move the platen, but any openings or movable pieces may add to vibration
potential in the system and is therefore not the most desirable engineering
approach to the construction of the safety box.
The box may have a high center section and be angled or curved in the
outer section so that any loose parts or pieces would tend to drop below the
rotating platen and not be picked up by the platen and projected back toward the
opening in an area above the abrasive surface of the platen (e.g., towards the
operator). As liquids are used in the lapping action, a tapered bottom of the
safety box area toward one or more drain holes allows the expended liquid (and
any carried particulates) to be easily collected for disposal, even without opening
of the safety box area. The angle of the box bottom to obtain the best flow
conditions for the liquid will be selected to provide a washing action on the
surface to minimize buildup of ground particles on the surface of the bottom of
the safety box. Grooves to concentrate water flow or passage may also be
provided.
A temporary cover may be provided over the opening of the platen top
access hole to provide additional safety to the operator from projectiles and also
to contain any mist formed by the high speed shearing and projection of liquids.
Duct work can also be installed in the box to withdraw air born vapor and
particles as well as the liquids, with reduced pressure removing the undesirable
materials at a controlled rate. Filter elements may also be associated with these
removal systems.
Problem: When a heavy workpiece holder is held up by an air cylinder and
controlled to provide normal force on a workpiece against a high speed 10,000
fpm rotating disk by moving vertically up and down to load parts and lap. Then
there is potential great danger if air pressure is lost due to air line leaks or
electrical failure. If this load of the disk rotating motor assembly which may
weigh 60 lbs. (27.2 kg), drops on the 12" (30.5 cm) heavy rotating disk operating
at 3,000 RPM, there is great danger in that the abrasive disk can be torn or cut,
jam up and create danger to the operator or severely damage piece parts which
may have great value.
Solution: The vertically moving piece part assembly can be mounted on
vertical slide and a chain or cable used with a counterweight which is perhaps 10
lbs. (4.54 kg) heavier than the 60 lb. (13.6 kg) assembly. Upon loss of electrical
power which would interrupt power to the normally used suspension air cylinder
or a line leak to the cylinder, the piece part assembly would simply and quickly
retract to the upper position, taking it out of contact with the rotating platen and
thereby reducing the chance of danger. This could also be a more assured event
by using an e-stop (emergency-stop) action switch which would not require
power to obtain safe action.
Problem When lapping parts, it is typically quite difficult to hold the lapped
parts in a fixture so that they are flat, stable and parallel when presented to, in
contact with, and when removed from the lapping platen wheel particularly when
the platen is rotating at high speeds of 3,000 rpm as compared to 200 rpm. Also
a part which is fixed by mechanism clamping is subject to be loose or compliant
(soft), which results in ground surface patterns or a lack of highly accurate
surface finish such as (4) four light bands is not attained. It is also difficult to
quickly and accurately load and unload parts. Also, for parts to be polished on
both sides of the parts, the already polished surface finish adjacent to the part
holder side of the mounting may be disrupted or destroyed when lapping the
other side of the part.
Solution: Functional mechanical parts, which are typically 1 to 2 inches (2.54 to
5.08 cm) in diameter (or shaped other than circular cross-section, such as
rectangular) which may be thin (.010inch, 0.254 mm) or thick (.500 inch, 12.7
mm) can be affixed to a precision flat steel, other metal or other material plate by
use of paraffin wax as a bonding agent. Here the plate or part can be coated with
wax or the wax simply melted on the plate between the part and plate and the
part placed on the plate, heat applied, and the two pieces would have a fully
wetted surface of molten wax. The parts could be positioned by mechanical or
other means of uniform pressure or force so that they lay flat with a uniform and
controlled thickness of molten wax. Upon cooling the part/plate assembly, the
parts would be positioned accurately and firmly for the plate ready for lapping
action. Then the plate could be attached to a piece part holding device by use of
a vacuum chuck or by use of a magnetic chuck if the plate were, for example,
steel. The piece part holder could have a ball type pivot close to the lapping
action surface. Plates could hold one or many individual parts. Upon lapping one
side, the plate/part assembly could be heated , the parts removed and, if desired,
the parts could be reassembled with heated wax on a plate with precise parallel
alignment with no danger of damage to the lapped surface because of separation
from the plate with no wax. And this way many plates could be preassembled for
high production rates with a single lapper.
Problem: It is desirable to have a simple drive mechanism to position a
stationary or rotating workpiece on the outer periphery of a high speed rotating
(3000 rpm) abrasive disk so that for most of the processing time there is a small
portion of the polishing or lapping time spent at the inner radius portion of the
abrasive disk where the surface speed is reduced and the abrasive action is
reduced.
Solution: A simple, eccentric harmonic motion, constant speed rotation can
be provided by a DC or AC gear motor hub used to drive a linkage system. This
system will provide a smooth continuous motion at a workpiece with most of the
time in a given hub rotation cycle being spent with the workpiece operating at
the outer periphery of the abrasive disk which has the highest surface speed and
also grinding action. Only a very small portion of the cycle time would be spent
at the inner radius having a low surface speed and reduced grinding action
portion of the disk.
Problem: It is difficult to hold small hard parts which are thin (typical size:
1" x 1" x 1/8", 2.54 x 2.54 x 0.318 cm) in such a fashion that surfaces (usually
two) with flat features can be polished with a lapping action by a high speed
(e.g., as high as 3000 rpm) rotating disk with a preferably diamond abrasive disk
exerting substantial lateral force by the moving platen powered by a (e.g., 2 HP)
motor for a 12" (30.5 cm) diameter disk when subjected to about 10 (4.55 kg)
pounds of normal clamping force when subjected to surface water spray. This
lateral force can separate the part from the part holder.
Solution: These small parts can be affixed to a flat surfaced piece part holder
or a holder which has small shallow pocket areas just larger than the length and
width of the flat part so that an exposed surface of the part protrudes away from
the holder. This will allow the abrasive disk polishing action lateral force to be
applied to the piece part and not separate the piecepart from the holder, as it is
trapped in the pocket or is held rigidly in the part holder. A medium temperature
wax, or other easily removable adherent material can be melted and used to bond
a rough surfaced part to the flat smooth surfaced part holder plate. The flat plate
in turn can be attached to a rotating pivoting arm which is swept across a portion
of the surface of the high speed rotating disk until a smooth flat polished lapped
surface is generated on one side of the piece part. Then the part holder plate
which would have 1 or 2 or many more parts attached to it in a fixed mounting
pattern could be brought into contact with another mounting plate having a flat
surface or a shallow pocketed surface pattern which matches the first part plate.
A higher temperature wax (higher temperature than the first wax) could be
melted at the surface of the parts already lapped and as they were held in flat
contact with the new plate, the original lower melting point wax would melt and
release the parts from the first plate. The parts would be transferred as a group to
the second plate ready to have the rough remaining side lapped as the first plate
is readily removed from this group of parts. High production rates at lapping flat
parts on both sides with good parallelism could be achieved.
Problem: When high speed lapping a 3000 rpm rotating flat platen with fixed
abrasives attached to the platen with adhesives or vacuum, water on the rotating
platen abrasive surface forms a boundary layer between the work piece and the
abrasive media. The boundary layer thickness and shape effect the flatness of the
work piece. The work piece must be allowed to "float" on the abrasive surface to
achieve total flat contact even with this water boundary layer. This is done with
a gimbal mechanism which puts pressure down on the rotating workpiece. It also
allows the work piece to "gimbal" in the horizontal plane while an independent
driver pin drives the work piece around the center line of the work holder shaft.
The amount of down pressure also effects the boundary layer. The work piece
floating on the boundary layer of water allows the abrasive media and the platen
imperfections to be averaged out- high spots on the abrasive do the lapping while
the low spots are filled with water allowing the lapping action to take place and
produce a finished part (work piece) that is flatter than the media and platen.
The work piece will only be as flat as the boundary layer. The problem is how to
control or minimize the boundary layer thickness and control the shape on a
work piece with a small surface area that is not large enough to float on the
boundary layer with a minimum amount of down pressure, yet have enough
water thickness for lubrication and cooling.
Solution: Pump water (e.g., through the work holder) into controlled orifices or
jets in strategic locations that would encourage a controlled boundary layer to
form between the work piece and the abrasive media. The water would also
stabilize the workpiece while presenting it to the rotating platen initially and
while lifting the work piece off after lapping is complete. Water is injected or
otherwise directed to an inside radial area of a piece part holder which is holding
a number of discrete piece parts at the same time. This could be particularly
helpful when an annular distribution of abrasive is used. In this aspect of the
invention, the inside portion of the water would develop a second boundary layer
under the trailing portion of the piece part holder which contains a second piece
part in contact with the narrow annular band of abrasive. Boundary layer water
entering under the leading edge of the holder would tend to lift up that first piece
part and tend to tilt the second piece part downward. This would cause a ground
cone shape to form on the piece part. A second boundary layer would also
develop under the second piece part at the trailing site of the holder and lift it
upward, which would compensate for the tilting of the first piece part.
Collectively, the whole piecepart assembly would tend to lay flat as it would be
supported by both boundary layers at the same time. There would be little tilting
of the piece part toward or away from the platen rotational center as the parts are
in contact with the (e.g., narrow) annular band of abrasive which would only
effect a narrow strip of grinding action. That is, the introduction of liquid
between the piece parts (along an arc [having the center of the platen as the
center of the arc] connecting both piece parts which are in contact with the
annular abrasive areas), reduces any tilting action which might normally occur
because hydroplaning or boundary layer effects from a liquid are introduced at
the relative center of the abrasive sheet only.
Problem: When lapping or grinding a multiple number of small parts or single
small parts each having small surface areas and short surface dimensions in the
approximate size of 0.1 inch (2.54 mm) by 0.1 inch (2.54 mm) and these parts
are positioned in contact with a high speed rotating disk operating at 3000 rpm at
perhaps 9000 sfpm speed, there is not enough surface length to the part to build
up a sufficient boundary layer to float or support the part as it is making contact
with the abrasive disk on the high speed platen. The parts tend to dig into the
abrasive disk and tear the disk and prevent accurate polishing or lapping of the
part.
Solution: Providing a system where an adequate boundary layer can be
generated and maintained while the individual piece parts are being lapped can
easily be done by adding a secondary device to the piece part holder device
which would have sufficient surface area, and dimensional length to develop a
desirable boundary layer. The secondary device is also ground down
simultaneously with the piece parts in a sacrificial way. A typical shape of this
sacrificial contact device can be a disk of metal such as brass which would be
mounted on the inside annular position of a tool piece holder with the to-be-lapped
piece parts mounted inboard or outboard of this device on the periphery
of a round piece part holder. As the total exposed surface area is ground down,
the piece parts are held suspended above the high speed moving abrasive by the
large surface area of the sacrificial disk. A typical disk would be 4 inches (10.2
cm) outside diameter, 2 inches (5.08 cm) inside diameter and about 0.60 inches
(1.52 cm) Thick. It could be easily attached with vacuum chucking and/or
adhesive tape and could be used over and over by loading new piece parts with a
partially ground disk. Other geometry sacrificial plates could be used and
combinations of materials including other metals such as steel or ceramics.
Problem: The annular sheet provides significant advantages to the performance
of many aspects of the present invention, but as with advance, other issues may
develop in performance. Where annular sheets or disks are cut from sheets and
applied to a flat face of a platen, particulate grit and abraded material and/or
liquid lubricant can work its way under the inside edge of the annular section.
Even in the small time periods when the sheet is in use, which may be as short as
ten to fifteen seconds, some particles may lift an edge of the sheet and cause
problems with the uniformity of the flatness of the annular sheet. This would
cause undesirable effects on the lapping process and quality. Additionally, at
extremely high speeds, the annular section becomes wobbly, does not sit
properly on the platen, may be difficult to lay down accurately, and provide other
structural difficulties in securing the annular sheet to the platen.
Solution: There are a number of ways in which a continuous sheet of abrasive
material may be provided, including a flat sheet having an annular distribution of
abrasive material and a continuous middle section without abrasive thereon. The
most expensive way of providing such a sheet would be to coat the abrasive out
in an annular distribution, as by roller coating, gravure coating or screen coating
of the abrasive and binder. An adhesive binder may be printed onto the backing
and the surface dusted with the abrasive grit to form an annular distribution on a
continuous sheet. This type of process would again require a new coating step
rather than providing a means for using existing sheet material. Another less
preferred method of providing an annular distribution of abrasive with a
continuous sheet between the inner diameter of the annular distribution would be
to cut a circular element out of the abrasive sheet material and then abrade away
an interior section of only the abrasive particles (leaving the backing material) to
create an annular element. This would be a waste of significant amounts of
abrasive surface area, but would provide a useful annular sheet on a continuous
backing.
The most preferred method according to the present invention is to cut out
an annular ring of material of the dimensions that are desired and then fixing or
securing a non-abrasive sheet material (hereinafter referred to as the center
portion) within the cut-out portion of the annulus. In providing such a
construction, the following concepts should be kept in mind. The joint between
the annular sheet portion and the center portion should not extend above the
average height of the abrasive particles with respect to the backing material.
This can be done in a number of ways. A thinner sheet material than the backing
material may be used for the center portion. This center portion does not have to
provide any significant structural component to the annular ring, but it can
provide advantages as noted later if the center portion is relatively stiff and
strong (even stiffer and stronger than the annular sheet material section). The
presence of such material, stiffened or not, does tend to make the ring easier to
work with, avoids wrinkling, and makes the abrasive sheet easier to lay down on
the annular work zone. The center portion clearly provides a stabilizing
influence on the sheet as it is being applied to the platen. The material for the
center portion may be chosen from a wide range of materials because of the
minimum physical and/or chemical requirements for the material. Plastic film or
paper is the easiest materials to provide for the center portion. There may be a
centering hole in the middle of the center portion, or even a larger hole than is
needed for centering. The larger hole adds no significant structural advantage,
and should not minimize the stabilizing or edge protecting effect of the center
portion, but some latitude is available in the dimensions of the center portion
with respect to the entire size of the annulus without preventing some of the
benefits of the present invention.
The center portion may be secured to the annular ring by any process
which adheres the center portion to the annular portion. This would include, but
not be limited to, butt welding, fusion of the sheet material to the annular
segment, adhesive stripe between the annulus and the center portion, thermal
welding, ultrasonic welding, hot melt adhesive, etc. The application of an
adhesive may be the most likely to cause raised areas which could be avoided,
but existing process technology makes controls over the dimensions of the
adhesive very effective. Additionally, since the adhesive would be much softer
than the abrasive material, some sacrificial abrading on the inner edge of the
annulus could be performed to lower any edges. Therefore, some conditioning
grinding or lapping at the inner edge of the annulus could be performed before
the abrasive sheet is used for its primary effort at lapping.
Another method for forming such a sheet would be to cut out an annular
ring of abrasive sheet and lay it over another plastic circular sheet having an
outside diameter approximating that of the annular cut-out (it may be somewhat
smaller or larger). This sandwich could be joined together by any method which
would maintain a consistent thickness to the abrasive sheet. since the highest
quality coating methods could be used in joining these layers (the circular and
annular disk), even adhesive securement is useful, where because of process
limitations in the application of adhesive to the platen to secure the abrasive
sheet, adhesive securement would not be desirable between the abrasive sheet
and the platen. Securement might also be made between the annular ring of
abrasive and a backing sheet by thermal welding, ultrasonic welding, or any
other method, particularly those which seal the entire circumference of the
joining line between the annular sheet and the backing sheet to prevent liquid
and particles from entering the seam. A poor seam closure would allow edges to
lift or pull and would be undesirable.
An annular disk provided with a natural raised outside area of abrasive
could be easily used on a flat platen surface. Other structures of abrasive sheets
with attached central areas, where the sheet has a height of the central area and
the abrasive area relatively equally may need a platen with a raised annular area
on the outside of the platen to take the greatest advantage of the annular
configuration. It is to be noted that if the central area were minimally abrasive or
minimally hard (or a later described, completely free of abrasive), contact
between the central area and the piece part during lapping would have negligible
or even beneficial (buffing) effects and the sheet could be used on a flat platen.
The annular band or sheet with an annular distribution of adhesive may be
secured to the platen by a number of different means. Positioning of vacuum
holes or ports or vents in the platen can be effectively arranged. For example,
vacuum holes may be located exclusively inboard of the annular band to assure
that no imprint of the hole is transmitted across the abrasive sheet to the abrasive
surface. With the use of appropriately sized holes, this potential effect has not
occurred, but this positioning of the holes allows for such a distribution of
relatively larger holes or vents if desired. Rows of holes directed relatively
radially through the underside of the sheet from the radial portion into or towards
the center area may be used. Concentric circles of vents or ports may be located,
some or all in the center area or under the abrasive annular distribution. Pressure
sensitive adhesive may be used in limited areas, such as in the center area only,
where there would be no possibility of adverse affects on the consistent level of
the abrasive or buildup effects. The adhesive could be used alone or in
combination with vacuum retention in that area or with the vacuum in areas not
secured by adhesive. Pressure sensitive adhesive could be located outside the
annular area of the abrasive, and thereby not affect the level or evenness of the
abrasive surface. It is possible to have some adhesive under the annular ring of
abrasive, but this would, of course, detract from the evenness and ease of
replacing the sheets.
High friction, rough surfaces may be provided on the platen to assist in the
draw down of the abrasive sheet. When an entire disk (rather than just an
annular ring with no center portion), the vacuum holes or vents are sealed by the
disk, particularly at the inboard portion of the sheet. It is therefore important that
all holes underneath the sheet be in vacuum tight relationship with the sheet to
prevent debris from entering the holes, clogging them, and providing deformities
on the surface of the sheet. The debris can also grind away portions of the holes
or vents, later disturbing the disk surface. The pattern and distribution of the
holes can therefore be important. The best distribution to date appears to be with
a completely continuous sheet (not even a centering hole) and concentric circles
of holes predominating in the center area and minimized (or even absent) from
the annular abrasive distribution area. A problem with the use of a centering
post is related to this phenomenon, in that debris may enter underneath the sheet
around the centering post and gradually cause adverse changes in the holes or
platen surface. Also liquid flow variations and different volumes and sizes of
particulates may be flung outwardly, underneath the sheet, if such materials enter
the space between the platen and the sheet through access around the centering
post.
Problem: The motor driving the platens and/or work piece holders (if they
move) apply vibration to the entire lapping system. The rotation of the platen
itself provides vibration, as does the movement of the abrasive over the face of
the work piece. The flow of liquid over the lapping contact zone (between the
platen and the work piece), especially where there is any hydroplaning or uneven
distribution of the liquid over a moving surface, also creates pressures and forces
which can add vibration into the lapping system. These vibrations in the system
can cause minor instantaneous variations in the relative positions of the platen
and the work piece. These variations, of course, show up in reduced lapping
quality in the product and are undesirable.
Solution: The weight of the frame an the individual elements (the platen and
any moving or stationary work piece holder must be designed to minimize
vibration. The joints between elements and attachments of moving parts must
also be controlled to minimize vibration. The primary method of reducing or
damping vibration is to add mass to the frame and to strategic portions of the
apparatus. The frame of the system should weigh a minimum of 100 kg. Also,
an energy-absorbing member or layer (e.g., a viscoelastic layer) may be present
between concentric tubular structural beam members and between flat plates
where a first of the two flat plates is merely a flat mass unit which tends to
remain stationary in space while the second plate integral to the frame has
vibration excitation induced in it. The thin elastomer layer mutually bonded to
both plates and is sheared across the thickness and, due to its very high viscosity,
will absorb the vibration energy and dissipate it into heat. All of the vibration
damping systems would be designed for a specific portion of the machine,
especially with respect to localized natural frequency, its expected amplitude
multiplication (which can easily exceed fifteen times the oscillation excursion of
the excitation source), the design and characteristics of the vibration
damping/absorbing device, and the different multiple frequencies expected.
Secondary spring-mass systems can also be utilized by positioning masses with
spring supports tuned to the excitation frequency by the formula Wn = the square
root of k/m where Wn equals the natural frequency in Hz, k equals the spring
constant in pounds/inch, and m equals the mass in pounds, with the necessary
constants required for equation units (e.g., such as gravity acceleration of weight
in pounds to mass in slugs). The secondary spring mass tends to oscillate at the
same frequency as the excitation frequency, but out-of-phase, so as to cancel out
the excitation frequency force.
Another vibration prevention device is the use of a large, thick, heavy flat
plate weighing 90 kg or more mounted horizontally in the same plane as the
platen at about the same level as the platen. This mass tends to absorb any
vibration due to imbalance of the platen/abrasive sheet combination assembly.
This prevents the vibration motions from exciting the machine frame in such a
way as to oscillate the piece part being ground or lapped. Adhesively bonding a
viscoelastic layer to this flat mass plate and bonding another large mass flat plate
to it can very effectively reduce the buildup of vibration oscillations,
Some other vibration excitation sources can be the platen system being out
of balance, the piece part spindle being rotated when out of balance, oscillations
being generated by the stick-slip conditions between the abrasive sheet and the
work piece, hydrodynamic fluid-induced vibrations at the moving fluid boundary
layer interface between the piece part and the platen, sudden motion of machine
elements, electrical pulses, etc. Vibrations should be prevented from entering
the system, wherever their source. Adding a large mass ring of heavy, dense
material to the outboard diameter of a (typically) round workpiece holder in a
fashion which allows the center of gravity as close as possible to the moving
abrasive surface is a very effective method of minimizing vibrations in the work
piece. The mass attenuates vibration excursions and oscillatory vibration forces
generated at the abrasive surface contact area. The same mass will also interrupt
vibrations originating from the machine motor drive, and platen imbalance
(insofar as it would travel down to the workpiece support mechanism).
To minimize vibration, it tends to be more preferable that the mass of the
frame comprise at least 200 kg, still more preferably at least 350 kg., and most
preferably at least 500 kg., with no maximum weight contemplated except by the
limitations of reasonableness. The weight of the actual intended commercial
embodiment of the frame of the present invention is about 600 kg. The platen, at
a revolutionary speed of 3000 rpm with a twelve inch (30.2 cm) diameter, has a
natural frequency of about 50Hz. The frame should be designed with a natural
frequency above the frequency of the highest useful speed of the platen (and
motor) to avoid the frame being vibrationally excited by the motor as it is
brought up to specification during operation. For example, with the maximum
designated speed of a lapping apparatus with 30.2 cm platen and abrasive
sheeting being 3000 rpm with a frequency of 50Hz, the natural frequency of the
apparatus frame should be at least 2% above this operating frequency. Greater
differences between the operational frequency (the Hz equivalent of the
rotational speed of the platen) and the natural frequency of the frame would
provide additional levels of vibrational avoidance at the higher speeds, so that
natural frequencies more than 3%, more than 5%, more than 10% or more than
20% of the operational frequency are desirable. Operating equipment used by
Applicant in the practice of the present invention has been made with 3000 rpm
operational speeds (50Hz) and 76 Hz natural vibration frequency. This enables
the frame of the machine to be operated at higher speeds and higher frequencies
(e.g., 3600 rpm and 60Hz, and 4200 rpm and 72 Hz) by increasing the capability
of the motor, replacing the motor, but not significantly modifying the frame. If
need be, weight and mass may be added to the frame after construction to
improve vibration resistance. Damping material, such as elastomeric materials
may also be added at strategic sites within the frame and apparatus, such as at
joints, between a work frame and the main frame, over bolts and nuts (if
present), between legs on the frame and the floor, etc. The purpose of these
features being to mask the vibration or dampen it, as by increasing the natural
vibration frequency of the frame to a meaningful level (e.g., at least 2 Hz or at
least 2%) above that of the operational frequency of the lapping apparatus.
Problem: When a piece part is ground or lapped on a high speed (e.g., diamond)
abrasive disk with surface speeds of about 9,000 sfpm or higher, with a 12 inch
(30.5 cm) diameter platen rotating at 1,500 rpm or 3,000 rpm or more, there can
be an uneven grinding action due at least in part to the liquid boundary layer
between the piece part and the abrasive surface. There can be a thinner layer at
the outer periphery of the circular boundary layer due to the high relative surface
speed at that outer region. The relatively much slower surface speeds at the
inner radial region of the disk will conversely have a thicker boundary layer
because of the slower speeds and the fact that the same volume of liquid is
moving over a smaller area (the area defined by the smaller radius) at a slower
speed. Typically abrasive particles at the outer radius of the rotating platen more
easily penetrate the thinner boundary layer at the outer periphery of the disk and
effect material removal more efficiently in that region than where the boundary
layer is thicker. Therefore, the abrasive activity is affected not only by the
differential in surface speeds between the inner region and the outer region, but
also there is another effect because of the variation in the thickness of the
boundary layer between radially related regions. Thus the abrasive particles
integratly attached to the abrasive sheet may be held away from the work piece
and not remove material as efficiently. This causes uneven wear and lapping on
the piece part due to the boundary layer effect which has not been previously
considered in this technical field.
Solution: The use of an annular ring, with the inner and outer radius of the
center opening and external edge, respectively, being sufficiently close in
dimensions that the relative velocity of the two surfaces, and more importantly
the thickness of the boundary layer at both of these radial positions, are within a
narrower variation than previously used. It is important to note that this effect is
important for the high speed lapping process of the present invention, and would
have had an insignificant effect at the 5 - 200 rpm rotational speeds common to
previous grinding processes. The high rotational speeds create the dramatic
boundary layer changes for which this invention is important. Even if annular
disks had been used with slower speed grinding, polishing or lapping processes,
the benefits of this aspect of the present invention would not have been noted,
even if the benefit was provided by such lower speed annular disk usage. It
would be desirable to have the boundary layer thickness approximate the average
height of the abrasive materials protruding from the support surface (e.g., from at
least about 0.1 micrometers, and for example from about 1 to about 100
micrometers). It is desirable that the boundary layer thickness approximate that
height with a variation of no more than ± 50% of the average abrasive particle
height, more preferably ± 30%, still more preferably ±20%, yet more preferably
±15%, and most preferably within +10% of the average protrusion of the
abrasive particles from the average height of the substrate (e.g., the valleys
formed by the binder). The process may be performed with two piece part
holders, each rotating in a direction opposite (clockwise versus
counterclockwise) from the other. Both holders may be mounted on a common
pivot arm. each piece part holder would tend to stabilize the other and would
also allow each of the piece part holders to stabilize the other across the width of
the platen. A special wobble joint at each piece part holder would allow each to
conform to the slightly uneven boundary layer on the platen. Rotating each
piece part holder would provide the same amount of abrasive material removal to
the exposed surfaces of the piece parts. The normal contact force, surface speed,
liquid flow rate, viscosity, etc. would all be optimized in the entire assembly.
The assembly pivot cradle would be oscillated to obtain even surface wear.
This aspect of the invention can be considered with respect to cutaway
Figure 9. A lapper platen system 130 is shown which comprises a shaft 132 is
connected to a rotation source (e.g., an engine, not shown), a platen face 134 on
which will be secured an abrasive sheet (not shown). The platen face 134
contains ports 136, 138, 140, 142, and 144 through which reduced pressure may
be provided to the platen face 134. A spherical or torroidal element 146
(hereinafter referred to as the "ball 146") with a flattened or flat beveled bottom
portion 148 is secured by a flat internal face 150 to the lower portion 152 of the
shaft 132. The rounded outer surface of the ball 146 is supported by pairs of
spherical- faced bearings 154, and 156, and 158 and 160, which may also be a
pair of torroidal bearing elements with concave spherical faces contacting ball
146. Over said upper spherical faced bearings 154 and 158 are flexing elements
162 and 164. This may be any spring-like elements, coils, or spring washers
which provide a cushioning effect or spring effect between said upper spherical
bearings 154 and 158 and bearing securing means 170 and 168 which help to
secure the upper bearing elements 154 and 158 against movement and provide a
stabilizing and positioning force to the ball 146. A convenient securing means
may be a circular nut with spanner wrench holes, with threads on the sides to fix
into the platen neck 172. A cushioning material 174 and 176 are provided
between the shaft 132 and the interior surface 178 of the platen neck 172. If a
force is applied to the face of the platen 134 and the force is slightly uneven
distributed against the face 134, the face of the platen may adjust to the force and
level itself by pivoting through ball 146. The degree of pivoting is cushioned by
internal resistance of the ball 146, and the elastic resistance of the cushioning
materials 174 and 176. A lubricant (not shown) may be provided in any cavities
180 and 182 which exist between the cushioning material 174 and 176 and the
ball 146. The lubricant may be any preferably liquid lubricant such as an oil.
The cushioning material 174 and 176 may be any flexible composition, such as,
but not limited to, natural or synthetic rubber, silicone or fluorine containing
elastomers, spring elements, or the like. Lubricant may be provided by syringe
injection into the cavity 180 and 182 or may be provided through a replaceable
cap (not shown).
Figure 10 shows a preferred flexing element for use with the present
invention, a Bellview spring washer 190. This element is no more than a
standard washer whose outer periphery has been bent down to form a truncated
cone shape. These Bellview spring washers may be stacked to form a spring-like
element.
It is desirable to limit the degree of pivoting which this aspect of the
invention may undergo. During an emergency, a limitation on pivoting, beyond
that provided by friction and the cushioning materials 174 and 176. One method
according to the present invention is shown in Figure 11. A platen-shaft system
198 may comprise a platen 200 with a front face 202 and an internal anti-pivot
shaft 204. The anti-pivot shaft 204 is separated from the inside face of the platen
shaft 206 by a distance of A. The platen 200 may not pivot any angle greater
than that which would cause the anti-pivot shaft 204 to contact the inside face of
the platen shaft 206. By adjusting the dimensions of the respective elements
(e.g., the length and thickness of anti-pivot shaft 204, dimension A, etc.), the
limits on the degrees to which the platen may pivot can be preset.
This aspect of the invention may be described as a pivoting lapper
workpiece holder system comprising:
The workpiece holder system may have said cushioning means comprise a
flexible composition, and may have said cushioning means comprises an
elastomeric composition, as previously described. As previously noted, said
elastomeric composition preferably comprises a silicone elastomer or a
fluoroelastomer. The workpiece holder system, between said flexible
composition and said at least one upper bearing may have a spring element, and
above said spring element and below said flexible composition may be a
securing element, said securing element being capable of being adjusted in a
direction parallel to said shaft to increase force upon said spring element, said
force on said spring element in turn increasing force of said at least one upper
bearing to press said bearing against an arcuate surface of said pivoting joint.
The workpiece holder system may have at least said flexible composition,
spring element, shaft, at least one upper bearing and pivoting joint creating a
cavity with said workpiece holder system. The cavity preferably contains a
liquid lubricant.
To restrict non-lapping (out of plane) rotation of the workpiece holder, the
workpiece holder system may have an elongate element which is associated with
said workpiece holder so that movement of said workpiece holder, out of its
natural symmetric rotation plane as is used during lapping, causes movement of
said elongate element, said element extending from said back side of said
workpiece holder through an interior channel of said shaft so that said movement
of said elongate element when said workpiece holder pivots will cause said
elongate element to contact an interior surface of said shaft, restricting the
amount of pivoting which said workpiece holder can perform. The elongate
element will contact said interior surface of said shaft when said workpiece
holder is turned less than 30, preferably less than 20, more preferably less than
15 degrees, and most preferably less than 10 or 5 degrees.
The workpiece holder system may use a spring means or spring element
which comprises a stacked array of truncated hollow cone elements stacked upon
each other.
This system is a great advantage over a simple ball bearing type of design
for a number of reasons. Fine abrasive grit can easily get into a ball bearing,
while the pivot center of this design is fully enclosed. Even if some grit does
enter the system, the oil can support it, wash it out, and remove it almost
completely with replenishment of the lubricant. A spindle holder (or the
workpiece holder shaft) is never uniformly and consistently perpendicular to the
workpiece holder. A perfect ball bearing would be very loose and could cause
the workpiece holder to contact the platen in a manner to cause abrasive damage
from the first contact, while the cushioning material (the elastomer) used in the
present invention stabilizes the workpiece holder direction and tilt within a more
controllable range. The use of an elastomer is preferred over spring support of
the shaft because it also provides an added measure of vibration damping.
Problem: Sometimes the extreme liquid pressures and forces can drive the
liquids under an interior edge of an annular disk. Once the edge is lifted, many
undesirable events can occur. The annular abrasive disk presents an uneven face,
since one edge is deformed from planarity. Residue from the abrasive disk and
swarf material from the work piece can embed themselves under the raised edge.
Each of these distortions of the abrasive surface are undesirable and can damage
the workpiece.
Solution: There are a number of solutions to this problem. One basic
consideration is to provide an abrasive sheet which does not have any openings
in its surface. This can be done by having a circular sheet with no holes therein
coated with an annular ring of abrasive material. A circular abrasive sheet may
have the core circle of abrasive scraped or abraded off to leave an annular
distribution of abrasive on an impervious sheet backing. An annular disk with an
opening in the center may be provided with a 'plug" or circular piece that
completely fills the central area. As shown in Figure 5, an annular disk 112
having annular, flat support area 114 with abrasive on the upper surface 116 may
have a plug 118 which abuts (and is preferably secured to) the inside edge 120 of
the annular ring 112. An area 122 between the flat annular surface support area
114 and the inside edge 120 is shown with a bevel, but this is not essential.
Securement between the plug 118 and the interior edge 120 may be effected by
direct fusion (by heat or solvent) of the two pieces, adhesive or the like.
Figure 6 shows a platen 90 with a depressed region 92 and a wall 94
between the flat upper annular support area 95 and the depression 92. A number
of means are available for providing an annular abrasive disk or annular abrasive
work surface (not shown) on this flat portion 95. Figure 7 shows one of these
methods. The platen 90 has an abrasive sheet 100 on its surface. The sheet 100
comprises a backing layer 102 and abrasive material 104. A vacuum port 96 (or
other securement means) retains the back surface 98 of the sheet 100 against the
flat annular surface 95. The reduced pressure will be passed along the back
surface 98 press the sheet 100 against the flat surface 95. The reduced pressure
will also secure the sheet 100 against the wall 94 and the depressed area 92.
The wall 94 is shown with an arcuate slope, but may be more sharp or smooth in
the transition from flat area 95 to depressed area 92. For example, the transition
may be by two right angles or by an S-shaped curve or other form. Figure 8
shows a platen 90 with a plug 93 which is secured to the backside 98 of the
annular sheet 106 with abrasive 106 on it. The location of the abutment 110
between the backside 98 of the sheet 106 and the plug 93 is shown at an
approximately right angle, rather than the edge-on abutment of Figure 5. The
abutment 110 of Figure 8 may be by means similar to those described for the
joining of the plug 118 and the flat annular support 112 at the abutment 120 in
Figure 5.
Problem: Abrasive sheets, even in annular form, tend to wear in a specific
pattern. The precise positioning of the sheets or ring against a work piece causes
the same radial portion of the abrasive surface to be in contact with the work
piece. This tends to cause the abrasive surface to wear down in specific circular
lines or annular areas. As the abrasive surface is not as useful where there is a
discontinuity in the abrasive, the remaining sheet may have to be discarded
because of the absence of abrasive over only 10-20% of the sheet work face.
Solution: Working at high rotational speeds, the centering of the sheet or
annular disk on the platen was assumed to be very important, mainly because the
radial forces would have been thought to be sufficient to create significant
damage to the sheets, literally ripping them apart with the force, or the creation
of vibrations which would effectively distort the relative face of the abrasive
sheet. It has been surprisingly found that not only would the off-centering of the
sheet or annular disk not create damage, but such off-centering could prolong the
life of the abrasive work surface. By positioning the center of the sheet or
annular disk at least 1%, preferably at least 2-5% (even up to 10-20% of the
radius, off-center) of the radius of the sheet or annular disk away from the center
of the platen, the work surface of the sheet or the annular disk would effectively
oscillate, rather than present the exact same radial dimension to the work piece.
This oscillation, since it is unlikely to repeat in a single rotation of the platen,
would expose different areas of the abrasive work surface to the work piece.
Abrasive material would be removed in broader (wider) annular patterns, as
compared to the more narrow annular patterns that would be worn in the work
surface of a perfectly centered abrasive sheet. The degree of off-centering useful
or tolerable in the system is related to the rotational speed and the density of the
abrasive sheet. The greater the rotational speed, the heavier (higher weight per
unit surface area) the abrasive sheet, the less off-centering which may be
tolerated. It is also quite useful to provide a massive (heavy) support for the
work piece and platen. The heavy apparatus pieces will help to dampen
vibrations that may occur by the eccentric rotation of the sheet or annular disk.
Additionally, the abrasive disk could be either intentionally repositioned at
its exact original position or a different position by use of a marker system.
Even a felt-tip writing implement could be used to mark on the abrasive disk
and/or the platen where it was exactly located on the platen relative to the mark,
or a permanent marking system on the platen. An abrasive disk may then be
removed and reinstalled at nearly the identical radial and tangential position on
the platen without requiring the disk to be redressed each time that it is used.
Furthermore, the abrasive disk could be sequentially or progressively or
randomly moved tangentially to align "low" wear areas of the disk with "high"
elevation areas of the platen which would better utilize all of the expensive
abrasive particles of the disk. Small increment tangential repositioning of the
disk would reduce the requirement for re-dressing the disk as many of the causes
which require re-dressing - platen high spots, thickness variations in the abrasive
disk, etc. - tend to then be distributed in areas rather than at specific points which
is more tolerable within a lapping system.
The abrasive disk can also be preconditioned so that high defect spots or
areas are reduced in height to reduce the possibility of local scratching on the
work piece surface. A hard material can be held stationary against the disk
surface (particularly at an edge) or the hard material may be oscillated slowly
and radially to knock off or wear down high spots. Another abrasive material
could be rotated with its own high (or slow) velocity against the surface of the
abrasive disk to remove high spots or loose materials. Any loose or weak
abrasive materials at the inner or outer radius of the disk would be broken loose
by this initial conditioning treatment and would be eliminated from the system
prior to actual lapping of the work piece.
Problem: Because of the higher rotational speeds that can be used in the present
invention, liquids, swarf, removed abrasive and the like is hurled at extremely
high velocity away from the platen. With linear velocities of 20,000 feet per
minute, debris is constantly projected from the surface at over 200 miles (280
km) per hour. This projectile material can cause serious damage to person
around the machine, and upright box-like protective enclosures (particularly with
flat upright surfaces at right angles to the path of the projected materials) are
readily worn away by the projected matter, much of which can be abrasive
material. Additionally, the particulate waste can accumulate against surfaces
and the liquid will also run over any flat surfaces.
Solution: The platen may be enclosed in a sunken box or walled area, with
significant space below the platen to a lower surface for the containment area.
The surface of the platen and the surface which is contacted by the abrasive sheet
should be below the upper edge of the protective walling-in enclosure.
Preferably the plane formed between the work piece and the abrasive sheet
should intersect the wall element at least 1 cm below the highest part of the wall.
Preferably there should be at least 2 cm of such clearance, more preferably at
least 4, 5 or even 10 cm of wall above that plane. The distance below that plane
to the floor of the containment area should be at least 5 cm, more preferably at
least 10 cm, and may be 20-50 below the plane. Abraded material may
harmlessly collect in the floor area, and the area cleaned out from above (around
the sides of the platen or by moving or removing the platen) or from below (by
an access panel or regular drainage system). The collected materials may be
more readily disposed of and collected in this manner. The walls of the
enclosing elements may be metal, coated metal, composite, abrasion-resistant
coated material, or sacrificially coated materials, high friction materials, or
energy absorbing materials. The walls may be sloped outwardly so that
impacting material may be reflected down towards the floor/collecting area. The
entire enclosing structure may be removable most easily down from the bottom
of the work area, there may be constant or sporadic drainage allowed through the
floor area, and the like.
Problem: It is often desirable to control the application of the abrasive material
to a substrate so that a specific pattern and particularly a straight line of lapping
is effected on the work piece. This type of polishing could be done with a
rotating beveled cup abrasive wheel with the beveled side edge coated with
abrasive so that the abrasive action is directed against a plane parallel to the axis
of rotation of the workpiece or piecepart. Sheet material is not naturally thought
to be applicable to such a process unless the sheet material were applied along
such an outer edge. The flat front face of a platen could not create a straight line
contact between the abrasive and a workpiece. Unless a beveled face as shown
in U.S. Patent No. 4,219,972 was used for the abrasive grinding wheel, there
could be no such possibility for any line or flat surface lapping unless an entire
surface were to be treated. That type of configuration would not be expected to
be amenable to abrasive sheet material, as the potential for wrinkling in fitting
the sheet to the outer edge would seem to be significant. Additionally, there has
been no disclosure of the use of sheet applied materials on beveled edges of
lapping or polishing materials as only flat sheets in rectangular and round facial
patterns have been provided.
Solution: A platen 220 is provided with an upper surface 222 (which is shown in
Figure 12 as a flat surface with ports 226 for securing sheets to the surface. On
the beveled side edge 224 are additional air vent ports 230 for securing
subsequently applied abrasive sheet material 228 to said edge 224. A circular
sheet of abrasive material (not shown) or an annular sheet of essentially two
dimensional conformation 228 may be applied to the upper surface 222 of the
platen 220. A flat abrasive sheet (not shown) would be secured by reduced air
pressure through ports 226 on the upper surface 222 of the platen 220. It is to be
noted that because of the beveling of the edge 224 of the platen 220, it is not
necessary that the upper surface 222 of the platen 220 be flat. That surface may
be rough, smooth, arcuate (e.g., spherical segment), or any other shape, with or
without features, since the lapping surface is no longer a face of the platen but is
the beveled edge 224. The edge is beveled at an angle between 1 and 89 degrees
away from the top surface 222 of the platen 220; preferably the angle is between
5 and 45 degrees, more preferably between 5 and 30 degrees. When an
essentially two dimensionally formatted abrasive sheet 228 is applied from
above the platen to the upper face 222 of the platen, pressure (and/or heat) may
be used to conform the sheet 228 to the beveled surface 224. The pressure from
reduced air pressure through ports 230 may not be sufficient to form the sheet 228 and additional pressure as from a mold overlay (not shown) which match the
shape of the beveled platen 220 may be needed. It has been surprisingly found
that the sheet 228 may be formed over the surface without distortion of the
configuration of the sheet. No wrinkles are formed in this fitting procedure. As
one of ordinary skill in the art knows, normally when an annular sheet-like
object in sheet form is fitted over a truncated conical form, the sheet distorts and
forms wrinkles when attempting to conform to the surface. The sheet material
backing on commercial abrasive sheeting has been found to be able to conform
without wrinkles when pressed onto the beveled shape. This is believed to be in
part caused by elastic or inelastic give in the backing material itself. What is
additionally surprising is that with the stretching or reconfiguration of the
backing material, the essentially uniform abrasive surface of the abrasive sheet is
not adversely disrupted. This is particularly surprising since the uniformity of
the distribution of the abrasive material on the surface is so important to the
quality of the lapping process, and the amount of elastic conformation at the
lower edge of the platen may be 10% or more.
The beveling of the edge provides a geometry to the edge that when, as
shown in Figure 13, a workpiece 240 is addressed by the beveled edge 224 of a
platen 220, the beveled edge 224 is parallel to a surface 232 of the workpiece
240. Additionally, a relatively clean line contact is made between the beveled
face 224 and the face of the workpiece 232 so that a relatively flat lapping
contact is made. The shape of the area removed 234 by extended contact with
the edge 224 of the platen would be nearly rectangular (for most purposes), and
only if the lapping were used in more of a grinding fashion would an angularity
in the wall 236 be noticeable while there was only a right angle configuration on
the distal wall 238 of the area 234. An angularity or pitch in the wall 236 while
the distal wall 238 was relatively perpendicular to the face 232 of a ground area
234 would be a fingerprint of the practice of the present invention.
The use of the annular ring with the beveled edge geometry has numerous
benefits and improvements over a cylindrical section or disk element for the
grinding wheel. Systems of grinding wheels with abrasive on the outside
periphery of the wheel (not on the flat face) are known for systems where the
abrasive is part of the wheel material itself (e.g., a grindstone) or coated onto the
edge. An abrasive sheet material does not lend itself to facile application or use
on such an outer edge, both for technical and mechanical reasons. There are
basically three ways in which a sheet material could be applied to the outer edge
of a grinding wheel: 1) coat abrasive on a cylindrical sheet and cut continuous
sections from the sheet which fit the grinding wheel diameter; and 2) cut strips of
abrasive sheet material and adhere them to the surface of the edge. The first
method would involve a specific new manufacturing process and technique to
manufacture such a continuous circular element, and the tolerances for good fit
to the wheel would be quite small. It is possible to have the backing layer of the
circular cut element shrinkable to fit the article more tightly to the wheel, but
adhesive would have been desirable, and this leads to disuniformity. The
vacuum hold-down of the present invention would have helped in this format,
but the new manufacturing procedure would have still been needed.
The second manner of providing an abrasive edge to the wheel would have
required that the strip be attached at its ends to form a circular element. This
would require the formation of a joint or weld, which would be likely to provide
a weak spot, an elevated patch, a wrinkle, or other aspect which would not lend
itself easily to use in the fitting of pre-made abrasive sheeting to the end of
grinding wheel.
The use of the completely beveled edge on the platen in this aspect of the
present invention provides a mechanism for providing a continuous strip of
abrasive sheeting made by existing technology and available as a staple in the
market place as an abrasive surface on a high speed lapping system which can
provide linear lapping and polishing as well as complete surface lapping. It is an
attribute and fingerprint of this aspect of the present invention to provide a platen
with a beveled exterior edge and a continuous strip of abrasive sheet material on
at least the beveled edge. The particle distribution in the abrasive sheet may well
result in a gradient of slightly lesser density of particles in the upper, smaller
diameter region of the beveled face than in the lower, larger diameter beveled
face. This particle density may be as slight as 1, 2, 5, or 10 % depending upon
the angle of the bevel and the degree to which the underlying support sheet has
been shaped by the fitting process. This minor particle density variation has not
been noted as providing any adverse effects on the lapping quality provided by
this configuration, and the important fact is that the shaped annular disk
conforms well to the beveled face and provides a very consistent and smooth
orientation of the abrasive sheet upon the beveled edge.
Problem: Because of the high rotational speeds of the platen and the abrasive
sheet material on the lapping face of a platen, there is uneven wear between a
radial outer area of the abrasive material and a radial inner area of the material.
There are difference in the linear speeds at the two areas, the amount of surface
area each incremental area of the abrasive addresses, and therefore there is more
rapid the wear in the abrasive surface towards the outer edges and likewise more
rapid wear on the workpiece.
Solution: In Figure 14, a workpiece 254 and a platen 250 with an abrasive
surface 252 address each other. The workpiece 258 has an effective center line
A - B. The workpiece 254 is moved so that the center line A - B spends more
time inside the outer edge of 260 of the platen 250 while the abrasive surface 252
of the platen 250 and the workpiece 254 are in contact during lapping. By
distributing or shifting the majority of the time of contact between the abrasive
face 252 and the workpiece 254 towards this interior region, there is less wear on
the outside edge 260 of the platen 250. As the most serious wear and damage to
the workpiece 254 can occur with excessive wear on the outside edge (as
cracking, flaking, and sharp edge features can more easily develop, this is an
important improvement in the wear performance of the abrasive sheet material
252. Figure 13 shows that the direction of rotation 256 of the platen 250 is
opposite the direction of rotation 258 of the workpiece 254. This aspect of the
invention works even better where the workpiece is rotated at the same time that
the platen is rotated, to more evenly distribute the time and position of
orientation of the workpiece and the abrasive surface. Even if uneven wear does
occur, the dual rotation of the workpiece and the abrasive sheet on the platen will
reduce any linear effects or artifacts on the workpiece surface. The rotation 256
258 does not have to be in opposite directions, but this is the preferred mode of
practice.
The time when a workpiece is in contact with an abrasive sheeting is
referred to as the total contact time Tc. The time when the center of the
workpiece is inside (not merely directly aligned with) the outer edge of the
abrasive surface must be at least 50% Tc when operating at a constant speed.
That is if the speed of rotation of the platen decreases, the Tc must be weighted
according to the surface area fanned or covered by the workpiece. Operating at a
constant speed, it is preferred that the workpiece center be within the outer edge
at least 60% of the time, more preferably at least 75% of the time, still more
preferably at least 80 or 90% percent of the time, and it is most preferred and
most convenient to have the center of the workpiece aligned within the outer
edge of the rotating platen at least 95% and even 100% of the Tc.
The combined effect of moving the center of the workpiece inward of the
outer edge and the rotation of the workpiece not only reduce uneven wear on the
abrasive surface, but provides a synergistic effect in reducing the potential
unevenness of lapping/polishing on the surface by both improving the
consistency of the abrasive surface addressing the workpiece and reducing any
linear effects that any unevenness in the abrasive surface could cause in the
workpiece. Additionally, by having an eccentric or non-repetitive movement of
the workpiece with respect to the radial position of the abrasive surface, there is
even less likelihood of any linear uneven lapping effects upon the workpiece
surface.
In the system where the center of the work piece is off-set so as to be
located predominantly inside of the annular ring center line of the abrasive sheet,
the lapping set-up may include multiple workpieces. As the platen carrying the
abrasive sheet is rotated, a workpiece will normally cover or be in contact with
only a very small fraction of the surface of the abrasive sheet. This leaves space
or areas on the abrasive sheet available for additional lapidary work. It is
convenient to have multiple workpieces distributed about the periphery of the
platen carrying the abrasive sheet. At least one workpiece should be oriented as
described above with respect to the relative position of the center of the
workpiece and the annular ring center line of the abrasive sheet. Preferably more
than one of the workpieces and most preferably all of the workpieces are so
oriented. To increase the effect of reduced uneven wear according to the practice
of the present invention, at least two of the multiple workpieces should be
rotating in opposite directions with respect to each other. That is, when viewed
from one direction perpendicular to a platen face, at least one workpiece will be
rotating clockwise and another will be rotating counterclockwise. It is preferred
that with an even number of workpieces, clockwise and counterclockwise
rotation is evenly distributed and alternative between the workpieces, and with
an odd number of workpieces, the numerical distribution would be n+1/2 and n-1/2
for clockwise and counterclockwise workpieces, with only one pair of
adjacent workpieces rotating in the same fashion.
This format of distribution with respect to a lapping surface is useful in the
practice of the present invention whether an entire platen surface is covered with
abrasive sheeting or whether an annular distribution of abrasive sheeting is
provided. The problem of uneven wear occurs in both type of systems, the
potential for damage is present in both types of systems, although it may be
somewhat magnified in the whole sheet system since there is a large variation in
the radius and thus the surface speed of the disk, and so any degree of uneven
wear provides greater likelihood for that uneven portion to contribute to damage
to the workpiece surface. This is simply a matter of probability in that any
damaged area has a greater probability of being in contact with a workpiece
when it constitutes a larger percentage of the total abrasive surface area.
It is also a consideration in the operation of a lapping apparatus using the
conformation of work piece positioning and the outer edge of the abrasive
sheeting to assure that at least some of the contact time of the work piece and the
abrasive platen positions the workpiece over the outer edge of the abrasive sheet,
and if an annular distribution of abrasive, over the inner edge of the abrasive
distribution. The passage of the work piece over the edges of the abrasive
distribution avoids the formation of ridges on unused portions of the abrasive
surface. By rotating the work piece while the platen is spinning, differing areas
of the work piece are presented to areas of the abrasive sheeting. More
importantly, however, buildup of ridges are avoided by the extension of the
edges of the workpiece over the outer (or inner with an annular configuration)
edge of the abrasive distribution. The extension should cover at least 1%, more
preferably at least 3%, still more preferably at least 5%, and most preferably at
least 10% of the effective diameter of the piecepart. (Note that the piecepart
should be somewhat larger than the width of the ring, which is 100% Tc.)
Another operation which proves to be of benefit in the operation of the
lapping apparatus is to precondition the outer edges of the abrasive sheeting
before actual lapping of a work piece. Such sacrificial lapping on the outer edge
for a brief period of time (e.g., less than 50%, preferably less than 25% or 10%
of the actual Tc for the next intended work piece, e.g., for 1-5 seconds) can
remove manufacturing or conversion (cutting) deficiencies in the outer edge.
This has been found to assist in reducing the occasion and occurrence of
particulates being dislodged in the outer area and wedging themselves between
the abrasive sheet and the piece part.
Problem: In initial work with high speed lapping systems, a gimbaled workpiece
holder had been used. This provided unsatisfactory results in that relatively cone-shaped
surfaces were produced. This effect was primarily due to the fact that the
interior region of the lapping abrasive surface is moving slower than the outside
region (radially outside) of the lapping abrasive surface. Less grinding per
rotation was being performed on the interior region, less material was being
removed, and so the interior region of the workpiece was higher in the relative
topography of the surface, producing the cone-like structure. Hydroplaning
effects of liquid between the platen and the workpiece also contributed to an
unevenness in surface smoothness, as did uneven wear in the different regions of
the abrasive sheet surface. The basic system of the platen covered with abrasive
sheet material, rotated at high speeds (e.g., 2,000+ rpm) and a gimbaled
workpiece would produce surfaces with light band uniformity of at best 4-5 light
bands smoothness, and this was attainable only through constant and severe
control of the system.
Solution: The combination of a platen surface with an annular ring of abrasive
material (e.g., with the non-abrasive inner region comprising at least 20% of the
total area of a circle defined by the outer circumference of the annular abrasive
sheet) when used in combination with a gimbaled workpiece holder has been
found to improve surface flatness as compared to a continuous surface of
abrasive material. The light band flatness is reduced to 1-2 light bands. With
the annular abrasive sheet with a gimbaled workpiece, lapping times of from 15-30
seconds at 3,000 rpm are used to with a twelve inch diameter annular disk
with comparable times of 60-100 seconds at 1000 rpm.
The gimbaled workpiece holder is desired in more conventional lapping
apparatus as it is difficult to align the upper workpiece holder perfectly
perpendicular to the abrasive platen surface. Even if it is initially aligned, it
becomes even more difficult to retain that alignment with disturbance from
hydroplaning forces and other machine factors, such as uneven bearings, other
dynamic forces, and the like. The combination of the gimbaled workpiece
holder with annular sheets of abrasive material attenuates or substantially
eliminates some of these effects and problems.
Problem: It is desirable to be able to provide a system where only one of the
workpiece and lapping platen are needed to be moved during operation of the
system. There has been no effective lapping apparatus which has been able to
provide the complete control over positioning of the platen face and the
workpiece face during lapping which would produce high quality smoothness at
high speeds. Because of the high speed component of the present lapping
apparatus, the ability for accurate and fast alignment of the surfaces (lapping and
workpiece) is much more important than in previous systems. The lapping
process for slurries of abrasive or lower speed lapping with abrasive sheet
materials (especially in combination with adhesively secured sheets) would take
hours. The amount of material removed from surfaces with maximum rotational
speeds of 200 rpm was very small and took a large amount of time. In the
lapping process, it is often is not always necessary to replace abrasive material
during the complete procedure. The abrasive had to be changed because first
coarser than finer abrasive material had to be sequenced to rough grind, then
polish, then lap the surface. The slow rotational speeds increased the amount of
time needed for each step. The need to remove abrasive sheets secured by
adhesive was especially slow and unwieldy because of the need to strip the
adhesively secured sheet from the platen, remove excess adhesive, and reposition
a new sheet with new adhesive. Additionally, even with adhesive removal
between sheets, there was a likelihood of adhesive buildup.
Solution: A heavy support frame for the workpiece and lapping platen
(including rotation engine or motor) is provided in combination with a preferably
fixed workpiece holder secured to the heavy frame. The lapping portion of the
system (the motor and lapping platen) is carried on a heavy frame. The
workpiece support or workpiece platen (along with gearing or in combination
with the motor) is positionable in three axes (the x, y and z axes). Each axis is
separately controllable, with an extensive amount of positioning being capable in
the axis controlling the linear spacing between the abrasive platen and the
workpiece (the Z axis), e.g., can be measured in full meters. However, in
addition to any gross maneuverability of the workpiece platen along these three
axes, there may also be a control system in place for at least the y and x axes
(which define the piecepart position parallel to the abrasive platen surface. The
fine controls on the system would require that there be at least one hundred (100
) positions available within any centimeter of movement along either axis, more
preferably at least 250 positions, still more preferably at least 500 or 750
positions available within any cm of movement, and most preferably that there
be at least 100, 250, 500 or 750 positions available for every millimeter of
movement of the platen face along anyone of and all of the three axes of
movement of the platen face. The degree of control may also be measured as
with respect to the rotation of a control element. That is, there may be 36, 72,
120, 144, 180, 200, 240, 300, or 360 individual positions within a single rotation
position of a control or switch. These numbers have been selected merely
because of their relationship to 360°, which is the basic unit for a rotation, but
any other unit or number may be selected, as between 1 and 100,000. The actual
construction the best working model of the present invention uses position
control with a stepping motor having 50,000 step increments per revolution,
which divides the forward motion from a single rotation into 50,000 units of
travel. Units of more 5,000, more than 10,000 and more than 25,000 are
particularly desirable. Each revolution of the control means may have as little
movement of the directed portion of the platen (e.g., one edge moving along one
axis) as less than 0.05 mm, preferably less than 0.005 mm, still more preferably
less than 0.001 mm, and the like.
Positioning along these axes can be effected by any means which can
move the platen face with accuracy. Screw pins and screw drives have proved
easy to configure into the system because the pitch of the screw can be adjusted
to control the amount of linear movement along an axis with respect to any
particular amount of screw rotation. For example, with a screw drive having 1
thread per cm, a 360° turn would advance the screw and any part attached
thereto by one cm. A 36° rotation would advance the screw 0.1 cm. Similarly,
with 5 threads per cm., a complete rotation of the screw head would advance the
screw and any attached workpieces or platens 0.2 cm., and a 36° rotation would
advance the screw 0.02 cm. Thus the sharpness or fineness of the control can be
designed by the threading of screws.
The mass of the frame also has a beneficial effect upon the performance of
the system. As the system is subjected to vibration forces, it is desirable to
minimize these forces. This can be done in a number of ways, but the easiest
way to have a major impact on controlling vibration is to increase the mass of
the support system and the connectors of the workpiece holders and the abrasive
platen. The frame of the system should weigh a minimum of 100 kg. For a
lightweight, small manufacturing model. More preferably at least 200 kg, still
more preferably at least 350 kg. And most preferably at least 500 kg., with no
maximum weight contemplated except by the limitations of reasonableness. The
weight of the actual commercial embodiment of the present invention is about
600 kg.
The apparatus described in this section would generally be a lapper platen
system comprising:
Problem: It desirable to increase the speed of the material removal, obtain better
flatness and surface finish smoothness with a fixed abrsive disk.
Solution: A slurry of abrasive particles can be added to the lubricant, coolant
(e.g., water) which can be used with the coated diamond abrasive sheets. These
loose particles could be larger or smaller than the average diameter of the fixed
diamond particles, and have a controlled size distribution to enhance the
performance of he abrasive disk. Different types of chemical additives could
also be added to the liquid composition provided between the disk and the work
piece, such as surfactant, viscosity modifying (reducing or thickening) agents,, or
acidic or basic solutions, etc. Some selectively chosen foreign matter could also
be added to the slurry mix, such as glass beads, plastic beads, fibers, fluorescent
materials, phosphorescent materials (for examination of the face of the work
piece by other means). The different solid or abrasive materials in the slurry
could perform a surface separation effect to obtain flatter contact between the
work piece and the abrasive sheeting and also additional material removal
mechanism effects. The other additives would have to be considered on an
individual basis as a function or relationship of the type of abrasive used in each
portion of the grinding cycle and the make-up of the work piece and its
compatibility with the chemical make-up of the additives. The combination of
different abrasive particles with the diamond sheeting can provide unique
lapping effects and intermediate effects between traditional lapping with slurry
compositions and the high speed abrasive sheet grinding of the present invention.
Problem: When a piecepart is brought into contact with a moving abrasive
surface, the amount of material that is removed in lapping can be extremely
small, perhaps only 0.1 micron (micrometer) while the typical distance the
piecepart is moved from a typical "start" position to the abrasive is relatively
larger, perhaps 4 to 6 inches. It is desirable to traverse the travel distance for
part loading or unloading rapidly in perhaps 1 to 5 seconds as the actual lapping
or grinding action may last only 10 seconds after contact with the high speed
10,000 sfm abrasive.
Typically the thickness of the material abraded away during one step of a
grinding or lapping process is equal to the thickness or diameter of the abrasive
media particles used in the previous step. A process lapping may start with 50
micron abrasive for the initial grind and be followed with 3 micron particle
abrasive which removes approximately 50 microns of material (although as
noted above, the practice of the present invention may beneficially reduce this
amount of removal to less than 90% of the abrasive particle size). Next 9 micron
abrasive will remove 3 microns of material, 1.0 micron abrasive would remove
1.0 microns of material and 0.1 micron abrasive would remove 1.0 microns
thickness.
Trying to control the contact of the piecepart with the abrasive surface
positionally through the use of geometric advancement devices such as motor
driven screws is very difficult to these very small distances. A fine pitch screw
system with the capability to be moved in 0.1 micron or less increments does not
have the capability to be moved through large distances for initial part loading or
mounting in the machine whereas many other devices which have micro motion
capability such as piezoelectric actuators or thermal expansion actuators are not
capable of large excursions of 4 inches.
A further problem exists with screws in that those using recirculating ball
bearings with inherent large pitches of 3 to 5 threads per inch tend to have
significant position errors relative to accuracies of 0.1 micron or less due to out-of-roundness
of the balls and non-perfect pitch variations of the lead screws used
in conjunction with the balls to advance a carriage when the lead screw is
precisely rotated. These rolling balls result in low drive friction.
Use of a servo motor to drive a lead screw provides fast continuous motion
of the lead screw and the carriage to which the part holder is mounted, but when
the servo motor is stopped at the desired contact position it has a natural
tendency to "dither" or oscillate mechanically and positionally due to its control
system electronics which corrects for the position error sensed. First it will
move past the target, create an error, and then move back again past the target
making a new error and correction.
If a stepper motor is used to drive a screw, then very significant accuracies
can be achieved with micro stepping control architecture where a motor can be
moved in increments of 50,000 steps per revolution. The accuracy of these
micro steppers with ball screws having typical pitches of 3-5 threads per inch of
travel is marginal with respect to the requirements of lapping with 1 micron or
less abrasive media.
Using linear electrical motors directly on a carriage slide device has
problems in that these motors again have a limited number of magnetic poles
which results in minute speed and force variations along the length of travel of
the moving portion of the motor device. Also they exhibit "dither" problems at a
fixed position, similar to rotating servo drives.
An inherent problem of great significance is trying to achieve a smooth
analog progressive grinding event with incremental or digital movements.
Material is progressively ground away from the surface of the piecepart on a
continuous basis as the part is brought in contact with the moving abrasive. The
total amount of material removed is expected to be at a steady fixed removal rate
over a period of time with a constant contact force between the piecepart and
abrasive. However, if a piecepart is moved incrementally by a stepper motor or
an "over-shooting" servo drive, the piecepart will be driven into the abrasive at
initial contact with too much resultant force and therefore excessive and
probably low quality or harmful grinding initially will occur as the piecepart is
ground away during this time period when the part holder is advanced this one
step. As time goes on in this period of the incremental positioning step, material
is removed and the contact pressure is reduced to less than desired until another
incremental step or position change is made in this positional control system.
Typical CNC (computer numerical control) machine tools operate with small or
fine increments of motion and a cutting tool is driven by the strong machine into
the piecepart along a prescribed path with the surface finish and accuracy
outcome a function of the size of the incremental steps and the speed of the mill
cutter. Damage of a submicron layer of the piecepart is not generally a concern
with a CNC positionally driven machine.
Over-aggressive grinding action on a typical lapped part for 1 second or
less can cause considerable submicron damage to the grain structure of these
pieceparts which are usually of great hardness being of such substance as
tungsten carbide, alumina, ceramics, silicones, glass, titanium, carbide and
others. Interstitial grain cracking at grain boundary layers is a common effect as
is localized thermal stress heat cracks.
It is critical that the pressure contact force between the workpiece and the
moving abrasive surface is held at a ;level determined to be best for a given
piecepart material, abrasive type, geometry, etc. The pressure on a given piece
which is defined by the total normal force divided by the surface area would be
quite consistent which means the normal force needs to be changed when the
surface area size of the part is changed to achieve optimal grinding on lapping.
Each piecepart material would have a unique pressure force that results in faster
grinding or better surface finish. This problem would change also as a function
of the period of the grinding cycle. Typically a higher pressure is used early in a
period for greater material removal rates and a lower pressure is used late in the
period for improved smoothness.
Determining the exact position at which a new part of unknown size or
thickness initially contacts a moving abrasive surface is desirable for controlling
grinding process parameters during the grinding process. This initial contact
position changes in a potentially significant amount each time a new sheet of
abrasive is installed for a series of grinding events with progressively finer
abrasive media having a different sheet thickness used for a smoother ground
surface.
Also, it is very important to know how much material is removed from
critical parts and the rate of material removal. The rate of material removal
indicates directly the condition of the abrasive media and indirectly the expected
quality of the surface finish. It is extremely difficult to successfully use an
exclusively position control system to present a workpiece for contact with a
high speed abrasive surface such as the abrasive sheeting used in the present
invention. About 10 micrometers of material from a workpiece surface is
typically removed in about 15 seconds, and machine tool component parts (such
as bearings) typically have fitting gaps larger than those dimensions, and the
high friction that would exist with tighter fitting components would have too
high a level of friction for the smooth movement of equipment necessary for the
best practice of the present invention. An excellent criteria for good grinding or
lapping action is control of the pressure force (which is difficult to measure) by
incremental position steps which are used to create the desired contact force.
Solution: It is necessary to provide a precise, controlled contact pressure force
between the piecepart and the high speed abrasive surface during the whole
abrasive grinding or lapping event. Once the piecepart is removed from surface
contact with the abrasive, then less precise or different means can be employed
to move the piecepart to another more remote location on the machine. A force
based design (as opposed to a purely position-based design) is preferably used
within the lapper system. The contact pressure between the workpiece and the
abrasive surface is controlled by force controlled (and measurable force devices)
devices such as pressure controlled cylinders (as herein described) acting as a
piecepart slide carriage which present a workpiece to be ground to the moving
abrasive.
This aspect of a process of the present invention may be summarized as
follows. A workpiece holder is supported on a linearly movable support (usually
vertically with respect to the abrasive surface). The workpiece is advanced into
contact with the abrasive surface (while the surface is static or while it is
rotating, preferably at a speed that does not cause immediate significant abrasion
( e.g., less than 10 microns grinding in 15 seconds). The parallelity of the
workpiece surface to be lapped and the abrasive surface is preferably adjusted at
this point, as by appropriate adjustment of positioning screws or other alignment
elements, particularly mechanical, position oriented, linearly oriented elements
(e.g., such as those herein described with at least 50 positions settings per
rotation with no more than, for example, 0.05mm linear movement per setting,
preferably no more than 0.01 mm, and more preferably no more than 0.005 mm
per setting) to place the workpiece surface to be lapped in good parallel
alignment with the abrading surface of the sheet. The position is indicated (e.g.,
a program setting, position setting, etc. is indicated within the system, as on a
computer) and the workpiece is retracted and removed from contact with the
abrasive surface. The workpiece is then advanced towards the rotatable surface
of the platen with the abrasive sheeting thereon, with the surface rotating,
preferably at the grinding speeds desired (e.g., greater than 500 rpm with a 12
inch diameter outside diameter platen). The advancement is done with a low
friction carriage so that the movement of the workpiece is relatively slow (e.g.,
less than 0.5m/sec., preferably less than 0.4m/sec., and more preferably less than
0.3m/sec. or less than 0.2 or 0.1 m/sec.) and smoothly progressing. This is best
accomplished by a system of elements herein described. This system of
elements basically operates in a preferred mode by providing both vertical
support forces (e.g., lifting forces as by air pressure, hydraulic pressure,
pneumatic pressure, electromechanical pressure, magnetomechanical pressure,
etc.) and vertical downward (advancing) forces (gravity, air pressure, hydraulic
pressure, pneumatic pressure, electromechanical pressure, magnetomechanical
pressure, etc.). The system may also be inverted, with gravity operating as a
"lifting" force with respect to the vertical movement between the workpiece and
the platen (that is with the platen at a higher elevation than the workpiece and the
vertical "downward" force being a vertical upward force (provided, for example
by air pressure, hydraulic pressure, pneumatic pressure, electromechanical
pressure, magnetomechanical pressure, etc.). The difference between the to
forces (the lifting and descending force) controls the contact pressure between
the workpiece and the abrasive surface at the moment of contact and thereafter.
By accurate measurement and control of these controllable (relatively
controllable, as gravity will be fixed for a workpiece/workpiece holder system)
forces, the contact and lapping operation pressure can be accurately controlled.
One method of solving this positioning and force application problem is to use a
screw drive system to move the piecepart from its remote initial mount
installation position to a new position close to the moving abrasive sheet and
then change the method of controlling the movement of the piecepart from a
position based system to a pressure or forced based system for the grinding event
only. After the grinding event cycle has been completed, then the piecepart
would be removed from contact with the abrasive and then control would be
transferred back to the position based control for a "large distance" physical
move of the part while the next grinding or lapping event is being prepared. An
example of this lapping event change would be to change from a 9 micron
abrasive disk to a 3 micron disk to be used in the next lapping event.
The lapping machine would require a number of other functional devices
(e.g., at least two distinct systems) to allow the easy transition from a positional
mode to a force mode. These functional devices would be used as a part of the
grinding procedure.
A motor driven lead screw would be used for the first positional mode system.
The preferred type of lead screw is not a large pitch acme screw with ball
bearings but rather a standard bolt type 50 pitch per inch of screw length which
gives about 10 times the linear resolution as a 5 pitch (threads per inch) ball
screw. Also by using a standard threaded nut with this screw, there is little or no
variation in the nut-to-screw location at any position because the third contact
element which creates variations, the balls, are eliminated.
Also flexible bellows can be used as short, low friction cylinders for the second,
force or contact pressure based mode. Low friction air cylinders or hydraulic
cylinders are mounted at one end on the screw nut assembly and are connected
on the other end to the piecepart holder lift mechanism. Thus the piecepart
holder can be put into place (e.g., into contact with the non-rotating, slowly
rotating, or high speed rotating platen) by the screw drive and at that time the
cylinders can be activated to lift the part holder up a small distance of 1/8 to ½
inch before significant lapping has been effected. Then the screw drive can be
lowered again until the piecepart is nearly touching the moving abrasive. The
pressure is then appropriately reduced in one of a number of cylinders which
may be used to support the piecepart holder, sometimes one but usually at least
two cylinders, preferably at lest three or four, and up to six offer definite
advantages. In this case, with four air cylinders present, pressure in three of the
cylinders would support most of the weight of the workpiece carriage assembly
and independent pressure to the fourth cylinder can be used to raise and lower
the carriage with a nominal low force of only one fourth of the weight of the
carriage. When pressure to the fourth apparatus cylinder has its pressure
reduced, this allows the piecepart to come into contact with the moving abrasive
at a controlled rate and pressure. The cylinder pressure was changed by a
voltage-to-pressure (E/P) transmitter to provide a very low initial contact force,
which increased as the lapping event progressed, decreased at the end of the
event, and was then changed more to lift the piecepart away from the surface of
the abrasive sheet. There would be a nominal weight of the piecepart assembly
acting down against the force of the cylinders. The force of the workpiece
against the surface of the abrasive surface can be seen as a combination of three
possible forces. There is a support force component (in a relatively vertical
direction) provided by the force mode system (e.g., the air cylinders) and there is
a gravity component (in a generally negative or downward vertical direction).
There may also be a third component (either a separate supporting component or
a driving, downward component) to control the force or position of the
workpiece as it contacts the platen.
After the piecepart is raised adequately from the surface of the abrasive
sheet by the cylinders, then the driven screw lift would be raised which will
allow the cylinders to be lowered to their bottom or home position without the
piecepart contacting the moving abrasive. Non-typical air cylinders such as
AIRPEL brand cylinders with limited air leakage around rigid non-seal inside
rod glass tubes provide very low sliding friction. The process may be generally
described as follows. A workpiece holder with a workpiece thereon is moved
from a first position to a second position which places the workpiece into a
second position comprising contact with or at a distance of less than 2mm from
the surface of an abrasive sheet on a rotatable platen. This second position is
registered within the system which moves or controls the movement of the
workpiece holder (e.g., a computer registers the specific position of the second
position). Movement towards the second position may be done with the platen
fixed, the platen slightly rotating, or the platen fully rotating, but only a very
small amount of material removal is allowed, such as lapping of more than 10
microns for a 50 micron average diameter abrasive particle into the surface of the
workpiece should be avoided in this step. While in the second position,
adjustments in general parallelity between the workpiece and the abrasive sheet
on the platen may or may not be made. After the second position has been
reached, the workpiece is removed from the second position to a third position.
This third position may or may not be the same as the first position, but is a
position which does not afford contact between the abrasive sheet and the
workpiece. This distance may be essentially any distance as the second position
has been registered by the workpiece moving system. The workpiece holder is
then moved from the third position to a fourth position which may be selected by
the operator as approximately before the second position (before with respect to
the workpiece's path of movement from the third position towards the abrasive
sheet surface), to the second position, or where the second position was before
contact had been attained, slightly beyond the second position. The fourth
position is selected so that the actual contact forces between the abrasive sheet
and the workpiece have a maximum pressure of between the desired range of
0.25 and 100 psi, and more preferably within the other ranges of preferred
pressures desired in the lapping process. It is again most preferred that the
pressure control mode used for the movement of the workpiece into contact with
the abrasive sheet surface assures that the contact pressure is within the desired
range. This is effectively done by assuring that the difference in forces (between
the supporting upward vertical forces and the lowering downward vertical forces
is the same as or preferably less than the intended contact force. The chosen
difference forces might have to be smaller than the desired contact force to avoid
the additional, but temporary force that would be added because of the
momentum of the workpiece and the workpiece holder. That momentum would
be absorbed, in part by compressive activities, but the momentum would
definitely tend to momentarily add to the contact force between the abrasive
sheet and the workpiece. By carefully controlling the relative forces (e.g., the
weight is a constant and the air pressure or hydraulic pressure, for example, may
be measured instantaneously or controlled), the contact force, even in the initial
moments of contact can be accurately controlled. The contact forces during
lapping can be accurately controlled by using stress gauges or the like to indicate
the level of forces that must be provided in the support or driving force system
provided in the movement of the workpiece holder.
A hydraulic or pneumatic dashpot or damper or snubber can be used along with
the air cylinders. This device could be spring loaded to raise its plunger or
cylinder rod cylinder into an up position toward the piecepart lift mechanism
arm. When the arm is lowered by reducing pressure to the cylinders which act
against the weight of the piecepart assembly, the dashpot will control the speed
at which the piecepart contacts the abrasive. The dashpot can be adjusted for fast
travel or slow. This can be used to control the momentum in the moving
piecepart and piecepart holder.
Force sensors can be mounted on the end of the lifting cylinders (e.g., the air
cylinders, hydraulic lifters, electronic or electrostatic lifters, etc.) and also be
attached to the piecepart assembly arm. As the force sensors are mounted in
series with the air cylinders, they would sense and indicate the actual pressure
that the piecepart arm is experiencing. If the cylinders are deactivated, the
sensor would still indicate the force that the arm is experiencing directly from
the screw drive. These force sensors are typically strain gauges mounted on
bending beams but may also be piezoelectric or other type devices. The force
gauges may be integrated with the force control and position control devices
through a computer with a program set up to perform specific levels of contact
pressure during each, every or any lapping stage.
This same force sensor can be used to sense the force between the
piecepart and the abrasive disk. As the piecepart arm is lowered onto the moving
disk some of the force supplied by the driven screw on the air cylinders
supporting the piecepart assembly is now supplied by the contact force. The net
result is a reduction in the force on the sensor. If all of the weight of the
assembly were on the abrasive, the force gauge would read zero.
If an additional force were to be applied downward for extra high grinding
force, then the sensor would change signs (if the sensor were initially in a
tension mode) and the total force would be the weight of the assembly plus the
new applied force. This additional force could be used where the differential
between the lifting (supporting) force and the downward force were intentionally
kept small so that the amount of contact force could be actively controlled by a
driving force applicator. This driving force applicator would be any system
which could apply a downward vertical force in controlled amount onto the
workpiece holder. Electric, electronic, hydraulic, magnetic, air pressure or any
other force supply could be used.
The force sensor can be used to establish the location or position of the
piecepart as it just makes contact with the abrasive disk. Here, the abrasive disk
is stopped (and if desired, a piece of paper, etc. of known thickness is laid on the
stationary abrasive) and the piecepart assembly is lowered until it is just in
contact, at which time the force sensor will change its reading to correspond with
the amount of force now being applied to the piecepart. Contact is now used as a
mechanism to establish the position by use of a precision position scale attached
to the piecepart slide arm, or by programming into computer operated controls
on the system.
The force sensor can be a single readout device or multiple units. Use of
multiple units increases the reliability of accuracy in the sense that each of the
sensors should give the same reading for a given equally shared load, so one bad
sensor should give a different reading which can trigger a sensor accuracy
review. Using three sensors mounted in a tripod arrangement gives a "three-point"
natural contact for equal loads to each device. Also, any defective device
would disagree with two others which increases the redundant reliability factor.
The part contact force can be easily read out by "taring out" the weight of the
part holder assembly. Three force sensors reduce the offset deflection of the
bending beam used for mounting an electrical strain gauge sensor.
A linear encoder device such as a Hindenhain brand scale or a LVDT (linear
variable differential transformer) can be used to establish the position of the
piecepart as it is processed by the machine during the lapping process. The
position sensor allows control of the amount of material removed by the grinding
process by comparing the position of the piecepart assembly relative to its fixed
height slide mount to the changing position as the piecepart is ground or lapped.
The Hindenhain brand linear encoder has the ability to determine position
changes of 0.1 microns or less, and therefore is quite useful within the objectives
of the invention. Another device which could be used to accurately determine
position as an alternate to the Hindenhain device is a LVDT device.
An edge finder device used by machinist to physically locate the edge of a part to
be machined for reference input for a CNC machine controller or for manual
machining control may be used to determine that the air cylinder has lifted the
assembly off the bottom home position. Another similar unit may be used to
confirm that the assembly is in a fully raised position. These units typically are
able to locate within 0.001". An edge finder switch can be used to sense liftoff
of piecepart contact with abrasive - establishing the "second position".
Small pneumatic or hydraulic cylinders can be used either to independently
counteract part of the weight of the work holder assembly or be adjusted to
exactly counteract the weight of the assembly or to provide more lift than the
assembly. This last arrangement would then require a downward force to push
the workpiece against the abrasive table.
An electronic pressure sensor can be used with the force lifting mode (or the
position sensing mode) such as with the air cylinders to be used to calculate the
theoretical lifting force of the cylinders.
A variety of slides can be used, including Thompson brand balls on single rods,
Daedal balls on four small rods, and air bearing slides to obtain low friction
forces which act against the piecepart holder. Friction slide forces of typical
slides are generally greater than the desired grinding contact forces which can be
very low, in the 1 to 20 lbs. range for most parts.
A second method of providing pressure force control during lapping or grinding
would be to use a linear motor operated in a position mode control for moving
the piecepart about the machine and then changing the mode of the motor control
just before the part makes contact with the moving abrasive. As the motor
current on a direct current DC motor represents force for a linear motor (or
torque for a rotating motor) the control mode change can be made very quickly
by modern controllers.
The linear motor position mode system would be used with other
functional devices much the same as for the FIRST METHOD using a screw
drive system.
Of particular note is the above described precision position scale which
can be used to establish the position of a piecepart starting the lapping or
grinding process and to follow the size change as material is removed. Here, the
initial position of the piecepart in contact with the abrasive wheel can be
determined by observing a change in the current of the linear motor upon making
contact between the piecepart and the abrasive platen as less force is required to
sustain the weight of the workpiece assembly when part of the weight is borne
by the contact force.
Other combinations of devices may be utilized such as a lead screw; air
cylinders both of traditional design and AIRPEL low friction design; a dashpot
to control descent speed; a force sensor system; or an edge finder switch; or
auxiliary lift cylinders.
A third method that can effect a solution is the use of hydraulics to both move
the piecepart precisely to different positions and also to effect a pressure or force
based contact with the moving abrasive media. A single low friction cylinder
would be used which would have a number of common input fluid sources
which are coupled or decoupled with the use of solenoid valves. The cylinder
would be either connected directly to the work holder lift assembly or connected
in series with a force sensor.
The cylinder and work holder assembly would be positioned very
accurately by the use of high pressure low leakage gear pumps such as those
with the Zenith brand name. The nominal pressure would typically be less than
100 psi even though the pumps would have the capability of generating more
than 1,000 psi. A large capacity gear pump would be used for fast travel and a
very small gear pump would be used to make precise minute incremental
changes in position. Here, the gear pump would be operated by use of a stepper
motor which will allow a fixed increment of fluid to be injected into the cylinder
which would raise in proportion to the surface area of the cylinder piston.
Generally, a 1 inch (2.54cm) diameter cylinder would be used with a pump
which has a volume output of 1 cc or less per revolution and a step motor which
has 50,000 incremental steps per revolution to obtain very small changes in
position per step increment.
When a desired position is reached, then the solenoid valves are closed, which
prevents leakage back through the pumps and holds the part holder assembly in
place.
A precise position measurement device such as a Hindenhain scale or a
LVDT is used to indicate position of the assembly In the event of significant
leakage of hydraulic fluid past the cylinder rod end cup seals, a change of
position is sensed and a corresponding corrective amount of fluid is injected into
the cylinder by an activated gear pump. Large diameter cylinders preferably
would be used to reduce cylinder friction so that the cup seal lips are not held too
firmly against the cylinder wall because the hydraulic pressure is low due to the
large surface area providing adequate lifting force to raise a typical work holder
assembly weighing, for example 30-100 pounds, such as approximately 60 lbs.
To apply a controlled downward pressure to hold the piecepart to the
abrasive surface, the downward force may be controlled by an air/oil (pneumatic
pressurized oil container) source. After the piecepart is positioned very closely
above the abrasive surface, perhaps only 0.050 inches away, the solenoid valves
are controlled so that the hydraulic pressure applied to the cylinder is from an
air/oil source. The air pressure is reduced and the cylinder starts to drop but the
speed is held in control by a separate adjustable dashpot or by orifice flow
restrictors. Contact abrasive pressure during the grinding event is then
controlled by an E/P voltage controlled pressure transmitter such as supplied by
Wats Co. or Rosemount Co. to change it as desired over the duration of the
grinding cycle event. After the grinding event, the air/oil device can be used to
lift the piecepart from the surface of the abrasive and then through the use of
solenoids, transfer can be made back to the gear pump based position control
system.
A ball check valve can prevent formation of fluid bubbles when a vacuum
is generated by reversing a gear pump when a cylinder is bottomed out and can't
move. Mechanical stops can be used to limit the motion of the cylinder. A load
cell force sensor system can also be used in series with the cylinder to obtain an
independent reference of the force which can be compared with a calculated
force based on the pressure readout device sensor which gives the pressure of the
fluid in the cylinder at all times.
Problem: When using disks of abrasive coated material in lapping or grinding
operations, especially when using thin disks of diamond coated plastic which are
round (e.g., circular or annular in shape), there is a problem of positioning and
maintaining the position of the disk, especially during high speed operation (e.g.,
at perhaps 2,000 to 3,000 revolutions per minute). In the past, these disks have
been either positioned with a casual surface tension bond of a water film or also
by use of a aggressive or nonaggressive PSA (pressure sensitive adhesive) layer
which allows disks to be removed and used again. In the probable event that the
disk would be installed even slightly off-center on the rotating platen when it is
stationary, there would be mass out of balance. This would be a significant
problem with high speed rotation of the disk due to the center of gravity not
being positioned at the exact center of the rotating platen. When the platen is
increased in angular velocity, the eccentric centrifugal force due to out of
balance mass is progressively increased by the square of the rpm speed. This
force would have a tendency to move the abrasive disk sheet even further out of
balance with the ultimate possibility of the disk setting up vibrations which
would affect product surface quality or perhaps leaving the platen with potential
operator danger.
Solution: The disk needs to be positioned initially accurately on the platen when
installed and then maintained in that position by at least one mechanical means.
One technique for initial accurate positioning would be to punch a small or
larger hole at the center of the disk and have a corresponding pin or post located
at the center of the platen. By placing the disk on the pin or post, the disk would
be centered and restrained at its true balance position. The disk could be easily
prebalanced with respect to the hole without the necessity of placing it on an
active platen. The existence of a pin or sub post would not materially affect the
use or utilization of the expensive disk or affect the processing techniques of
lapping or polishing as the linear velocity vector at the center of the disk area is
quite small. The center of the disk is seldom, if ever, actively used in polishing.
Competitive techniques using slow rpm (approximately 200 rpm) methods
employ platens with large holes at the inside center and radius. Larger holes,
e.g., greater than 3 cm, may actually be used also. Another technique for proper
initial positioning would be to use a slightly raised outside edge about the
thickness of the disk at the outer periphery to capture the disk and position it. In
both cases, water or water plus PSA or PSA can still be used to temporarily
secure the disk to the platen surface.
Problem: The alignment of the part holder with the rotating platen is critical to
achieve precision flat and parallel grinding of pieceparts which are vertically
positioned in contact with the abrasive and moved laterally in "X", "Y" patterns
along the surface of the rotating abrasive.
Solution: A simple, inexpensive, stable and adjustable mechanism is to mount
the vertical piecepart assembly mounting plates, each of which has a "living
hinge" on one end and 1 or 2 adjusting screws on the "free" end. The adjusting
screws allow the free end of the plate to be pivoted nominally in a pure axis
rotation about the semi-fixed hinged end which creates the ability to adjust the
position of a mounted apparatus in one axis. The use of a second similar living
hinge plate mounted at a position 90 degrees to, but flat to the first plate, allows
the nominal adjustment of the plate about the second axis perpendicular to the
first. By adjusting both plates independently and together as a system, it is
possible to easily align an apparatus precisely perpendicular to a reference plane.
Simple mechanical screws could be used, differential thread mechanical screws
could be used for fine adjustment, wedge slide blocks could be used, as well as
could thermal expansion bolts or other similar devices. In all cases the flat plates
remain flat but some twist out-of-plane could be effected by independently
adjusting two bolts at opposed ends of the free end of the plate. Adjusting could
be done mechanically by hand or by motor driven screws, electrical heat
supplied to thermally expanding bolts or piezoelectric actuators. Adjustments
could be made to achieve precision flatness or perpendicularity or to provide
slight contact angles to create unique grinding efficiencies by closed loop
controllers also.
Problem: Operation of the lapper of the present invention is typically at 3,000
rpm with a 3M Diamond Coated Abrasive disk having a twelve inch diameter.
The disk is held to the steel rotating platen by water film surface tension and
positioned by a 0.5 inch (1.27cm) diameter hole at the center of the disk used
with a 0.5 inch (1.27cm) diameter post at the center of the platen. At the high
speeds, the disk lost its surface tension adhesion and was thrown off the platen
while polishing a tungsten carbide piecepart. The forces on the disk were such
as to lift it off the centering post, and the whole disk was thrown off to the side
of the machine, opening a cavity at the top of the machine post.
Solution: The 0.5 inch (1.27cm) centering post was made larger in diameter to a
1 inch (2.54 cm) diameter or more post. Also, the post could have a non-circular
shape with at least one surface positioned against a center post which would
resist rotation, such as a hexagonal shape or an oval shape which would prevent
the disk from rotating relative to the tangential surface of the disk. The post
could also be made higher so the chance of the destructing disk climbing up the
height of the post would be diminished during this type of event. Another
technique would be to employ a clamp type of device to any of these round or
non-round posts to clamp/hold the disk firmly to the surface of the platen at the
center area of the disks which is not used for polishing because of the slow lineal
velocity in that sector. The clamp could consist of a spline locked washer
pressed on the disk surface with a thread nut engaged with a top threaded post.
Springs could also be used to control the amount of force and to evenly spread
the force uniformly. Ball detent or other snap latch fixturing devices cold also
be employed. As previously noted, since this section of the abrasive sheet would
not be in lapping contact with a workpiece, adhesive could be used in this area to
secure the sheet while vacuum was used in the other area to improve planarity.
Problem: Round disks of minute diamond particle coated sheets of plastic film
on 3,000 rpm spinning platens are difficult to hold in contact with the platen
when running in contact with stationary or semi-stationary workpieces. When an
abrasive disk becomes loose by breaking the water film "adhesive" surface
tension between the disk and the platen, it has a tendency to rip or bunch-up and
wedge between the workpiece holder and the high inertia spinning platen and
can easily damage a workpiece part or can destruct portions of the workpiece
assembly with the possibility of great danger to the operator. This is a unique
problem due to the very high rotational speeds of 3,000 or greater rpm with a
platen of 15 inch diameter (38cm) or more constructed of heavy steel which
could generate explosive type failures As this equipment is operated
horizontally for the most part, the whole surrounding area around the machine is
susceptible to this danger. One method to reduce the likelihood of this
separation problem is to coat one side of the diamond abrasive disk with a PSA
(pressure sensitive adhesive) film to temporarily bond the disk to the platen.
This adhesive creates a flatness accuracy problem in that its normal thickness
accuracy varies greatly around the disk which causes high areas of lapping
contact for this super precision abrasive contact. Secondly, when a disk is
removed, some sectors or pieces of transparent PSA adhesive remains on the
platen and forms a bump when the next abrasive disk is installed in the platen
which then destroys the smooth vibration free abrasive lapping at high speeds.
Solution Use diamond or other abrasive disks without using PSA adhesive and
first position the disk at the true center of the platen by use of a centerhold in the
disk positioned over a post positioned at the center of the platen (or by other
centering means) and then hold the abrasive disk to the platen by use of vacuum
by use of a rotary union on the hollow rotating platen shaft. The preferred area
to apply the vacuum would be at the inner radius of the disk which would seal
out first as the disk is installed at the platen center and also because this inner
one fourth or so of radius is not used much for lapping because of slow surface
lapping velocity. The second most preferred vacuum area would be the outer ½
inch (1.27cm) of disk radius at the periphery of the disk as this would also not be
used much and would have large holding force.
Problem: Fast removal, remounting of disks (10-15 second intervals of typical
use) need to be replaced in the "original" position. When a disk is installed on a
platen it can be held by double stick adhesive tape or by vacuum. A typical disk
is a thin layer of plastic film which is coated with abrasive diamond or other
ceramic type coatings which wear off with use - presenting new fresh sharp
material for fast accurate material removal. Also diamond particles are captured
with metal plating on a film and an additional backing material is adhesively
bonded to this plated film. If the finished product abrasive disk is attached to a
rotating platen with adhesive, the adhesive is usually coated on both sides of
another thin film, all of which have dimensional tolerances so one area of a disk
may be thicker than another and result in non-uniform abrasive wear. All the
variations in thickness of the sticky adhesive can be eliminated by use of the
vacuum hold-down holes of the platen.
Solution: When either the platen or the disk is uneven, only the high spots of
the abrasive disk will wear down first. When a disk is removed after typically 15
seconds usage (because 10,000 sfpm abrasive cutting is 20-30 times faster than
conventional grinding) and a new finer grit disk is used, there needs to be a
method to accurately relocate the disk the next time it is used. A disk typically
can be used ten to hundreds of times.
By marking a disk with color pen or mechanical cut-outs, notches, etc. and
positioning this disk mark on a corresponding mark on a platen, a disk is reinstalled
at a location where it "fits" and does not have to be reground to size for
the next operation, saving time and disk wear costs.
Problem: Using hold-down vacuum holes, adhesive annular disks at the outer
periphery platen of a high speed rotating platen have special problems of lifting
at the inner radius due to surface water and grinding particles being driven under
the annular film disk by the high rotational speeds. Once lifted slightly, the
raised edge gathers even more water/debris which raises the edge further and
presents this structurally weak disk edge to a stationary piecepart having a
typical sharp edge - which has a tendency to catch or cut the disk edge.
Because of the high speed of the platen, running at from about 1,000 to 10,000
surface ft/min, the disk can become damaged and crumpled and tear and then
either be thrown off the platen or wedge between the platen and the piecepart
holder which can create large dynamic forces which result in dangerous flying
shrapnel. If a vacuum hold-down is used, the vacuum would have a tendency to
suck the abrasive debris particles into the vacuum holes, eroding the hold edge
and enlarging them, which would locally distort the working surface of the
abrasive disk. Also centrifugal force from the 500 to 3,000 rpm 12 inch
(30.5cm) diameter disk would have a tendency to curl or raise up the inside disk
edge.
Solution: It is desirable to provide a full circular disk with a method of "raising"
the outboard annular section so water and debris particles can't get under the
inside radius to start the curl-up. A uniform disk with no annular cutout or even
an inner radius hole would be best because no water or debris can get under the
disk. Because of the high costs of the disk material, an annular ring of abrasive
disk could be adhesively bonded to another uncoated circular (not annular) disk.
This could be done by adhesive securement at the meeting edges of the central
disk and the annular disk, butt welding, sonic welding and any other form of
attachment between the two sheets that provideds a barrier for water or abrasive
grit flow under the annular sheet. The inboard circular disk would be thinner
than the outboard annular abrasive sheet disk.
Problem: It is desirable to have a simple drive mechanism to position a
stationary or rotating workpiece on the outer periphery of a high speed rotating
(approx. 3,000 rpm) disk abrasive for most of the processing time with a small
portion of the polishing or lapping time spent at the inner radius portion of the
abrasive disk where the surface speed is reduced and the abrasive action is
reduced.
Solution: A simple, eccentric harmonic motion, constant speed rotation as
provided by a DC or AC gear motor hub can be used to drive a linkage system
will provide smooth continuous motion of a workpiece with most of the time in a
given hub rotation cycle with the workpiece operating at the outer periphery of
the abrasive disk which has the highest surface speed and highest grinding action
and a very small portion of the cycle time spent at the inner radius, low surface
speed, and reduced grinding action portion of the disk.
Problem: A piecepart may need to be rough ground flat which requires a rigid
(non-pivoting) piecepart holder, but then may need to be processed on a
spherical ball piecepart holder to achieve extreme flatness of 1 to 2 light bands or
less. It is desirable to do this on one single machine using coarse grinding media
of 40 micron particle on the rough finish using the rigid holder and 3 micron
particles using the pivot holder.
Solution: A precision rigid piecepart spindle piecepart holder system can be
constructed with vacuum holding of the piecepart for rough grinding the
piecepart flat. Then a flat sandwich construction spherical ball pivot piecepart
holder can be constructed with an internal vacuum chamber to allow the
piecepart to be held or mounted with the same vacuum source and utilize an
internal spherical ball for allowing the piecepart to "float" on the abrasive surface
rotating in contact with the piecepart holder.
Problem: Constructing a high speed lapper platen rotating at 10,000 SFM
velocity or 12inch (30.5cm) diameter wheel at 3,600 RPM is difficult where the
annular edge of an abrasive disk is raised for use with an annular ring of abrasive
disk. It is necessary to avoid water or debris getting under the inboard radius.
Also when abrasive particles are drawn into the vacuum holddown holes on the
platen, they tend to wear the edges of the holes and enlarge them, which results
in distortion of the flexible abrasive disk sheet at he hole locations.
Solution: The platen can be constructed with an outboard raised circular land
area and have a lower inboard area to avoid contact with the piecepart but yet
have a further recessed (depressed) lip or edge so the inner radius of the annular
abrasive disk is below the inboard area of the platen so that water or debris on
the surface of the platen travels above or on the top surface only of the abrasive
disk and does not raise the inner radius. This is shown in Figure 25, with platen
1400, abrasive sheet 1402, inboard area 1404, and the distance of the inner
radius of the annulus below the inboard area shown as 1406.
It is desirable to make the platen out of hardened stainless steel about
Rockwell "C" hardness 40 minimum or plate with a hard chrome of Rockwell C
65 or harder on steel to reduce the wear of the vacuum holes.
Problem: For high speed lapping, it is desirable to quickly convert from lapping
with a rigid piecepart holder to a pivot type holder, particularly when utilizing a
vacuum to hold the piecepart to the holder for both the rigid mount and the ball
pivot mount.
Solution: A piecepart holder can be constructed as a sandwich of two flat
surfaced plates with a single ball at the center. This ball will transfer downward
abrasive contact pressure force to the piecepart and yet allow the surface of the
piecepart to move freely in contact with the moving abrasive surface so that it is
in alignment with this non-perfect perpendicular mounting between the holder
axis and a normal right angle with the platen surface.
The vacuum present at the surface opening port holes of the rigid spindle
holder can be transferred through sealed internal passages in the sandwich holder
to the piecepart contact surface simply by clipping a flat pancake sandwich
holder to the rigid holder. Because both the rigid holder surface and the
matching piecepart surface is very flat and smooth, an effective vacuum seal is
effected between the two surfaces upon contact. Surfaces need to be cleaned to
obtain a good seal. The ball can be sealed with RTV (room temperature
vulcanizing rubber), sealants or grease or other material. Two concentric rings
of plastic or elastomer can be positioned so as to form a passageway for vacuum
transfer from one surface to another and yet seal the passageway from leakage to
outside the sandwich. The outer ring can be attached to the sandwich by
adhesive or other mechanical or cast-in-place means. The elastomer can flex
with a controlled stiffness to allow angular motion centered about the ball. Both
sandwich plates can be precision aligned perfectly parallel to each other before
attaching the elastomer rings and they would tend to maintain this parallelism for
presenting the piecepart to the abrasive surface. Radial pins in a controlled slot
length will prevent over travel on the spherical ball pivot and also prevent
tangential rotation of one sandwich disk relative to the other for torque input to
the holder unit.
Problem: Pieceparts tend to hydroplane when they are held in contact with high
speed platens using a water film that develops a boundary layer between the
platen and the piecepart. The resultant piecepart is not ground flat because the
boundary layer pressures tip the part upward at the leading edge.
Solution: It is desirable to break up this boundary layer by having abrasive disks
coated in striped patterns such that only short land areas, as measured
perpendicular to the direction of travel, with grooves or spaces in between these
land areas are present to relieve this hydrodynamic pressure. The land areas
could be formed by spiral patterns, by islands of abrasive or other patterns.
Problem: It is difficult to establish the precise distance for moving a partially
ground piecepart down to contact the moving surface of an abrasive disk of
unknown thickness when initially starting to process a piecepart or when
changing to a new abrasive disk of finer grit without damaging the piecepart or
approaching too slow. When using coarse abrasive, a few mils are removed in
10 seconds but when using fine 3 micron abrasive, only a few microns are
removed in 10 seconds. The speed of contact used to start new grinding with a
finer grit abrasive is important, so as not to lose set-up approach time.
Solution: A piecepart can be processed, the abrasive disk changed and the
piecepart brought into close proximity to the moving abrasive disk, perhaps 1 to
10 mils (0.001 to .010") away. At that time, an excessive amount of water
lubricant can be applied to the surface of the disk which would tend to
hydroplane the piecepart without having contact with its abrasive particles. A
force sensing device can indicate when this physical contact has been made with
the water wetted surface. A correlation can be established with the amount of
force sensed and the exact water flow rate to determine the precise distance
between the piecepart and the abrasive sheet. Then the water flow can be
reduced progressively while the piecepart is lowered to the abrasive part surface
until grinding or lapping action starts to take place. In this way the water film
would act as a protective barrier at first contact and allow an algorithm estimate
be made of the necessary vertical action required to remove very limited
amounts of piecepart material, perhaps 0.1 micron per second or less. This
whole procedure could be automated and computer controlled with the
parameters of force, flow rate, rotational speed (or any combination thereof
need) correlated to separation distance.
Problem: When lapping parts, it is typically quite difficult to hold the lapped
parts in a fixture so that they are flat and parallel when presented to and in
contact and when removed from the lapping platen wheel, particularly when the
platen is rotating at high speeds of 3,000 rpm as compared to 200 rpm. If a part
is fixtured by mechanical clamping it is subject to being loose or compliant and
patterns or lack of highly accurate surface finish such as (4) four light bands is
not attained. It is also difficult to quickly and accurately load and unload parts.
Also the surface finish of the part holder on the mounting side may disrupt or
destroy the surface already polished when lapping the other side.
Solution: Individual parts, typically 1(1.27cm) to 2 inches (5.08cm) in diameter
or rectangular which may be thin (.010 inch, (0.0254cm)) or thick (0.500 inch,
1.27cm) can be fixtured to a precision flat steel, other metal, or other material
plate by use of paraffin wax as a bonding agent. Here the plate or part can be
coated with wax or wax simply melted on the plate between the part and plate
and the part placed on the plate, heat applied and the two would have a fully
wetted surface of molten wax. The parts could be positioned by mechanical or
other means of uniform pressure or force so they lay flat with a uniform and
controlled thickness of molten wax. The mechanical alignment pressures should
equal or exceed the pressures to be encountered during lapping to assure that
there is no movement under the lapping pressure. Upon cooling the part/plate
assembly, the parts would be positioned accurately and strongly to the plate
ready for lapping action. Then the plate could be attached to a piecepart holding
device by use of a vacuum chuck or by use of magnetic chuck if the plate were
steel. The piecepart holder would have a ball type pivot close to the lapping
action surface. Plates could hold one or many individual parts. Upon lapping
one side, the plate/part assembly could be heated, the parts removed and if
desired, the parts could be reassembled with heated wax on the plate with precise
parallel alignment with no danger of damage to the lapped surface because of
separation from the plate with the wax In this way many plates could be
preassembled for high production rates with a single lapper.
Problem: It is difficult to hold small hard parts which are thin (typical size: 1 x 1
x 1/8inch, 2.54x2.54x0.32 cm) in such a fashion that both surfaces of the flat part
can be polished by lapping action by a high speed 3,000 rpm rotating disk with a
diamond abrasive disk exerting substantial lateral force by the moving platen
powered by a 2 HP motor for a 12inch (28.5 cm) diameter disk when subjected
to about 10 lbs. (4.3kg) of normal clamping force when subjected to surface
water spray.
Solution: These small parts can be fixtured to a flat surfaced piecepart holder or
a holder which has small shallow pockets, just larger than the length and width
of the flat part so that the exposed surface of the part protrudes away from the
holder. In this way, the abrasive disk polishing action is applied to the piecepart
and not the holder. A medium temperature wax can be melted and used to bond
a rough surfaced part to the flat smooth surfaced part holder plate. The flat plate
in turn can be attached to a rotating pivoting arm which is swept across a portion
of the surface of the high speed rotating disk until a smooth flat polished lapped
surface is generated on one side of the piecepart. Then the part holder plate
which would have 1 or 2 or many more parts attached to it in a fixed mounting
pattern could be brought in contact with another mounting plate having a flat
surface or a shallow pocketed surface pattern which matches the first part plate.
A higher temperature wax (than the first wax) could be melted at the surface of
the parts already lapped and as they were held in flat contact with the new plate,
the original lower temperature melting point wax could release the parts from the
first plate and upon cooling somewhat, the parts would be transferred as a group
to the second plate ready to have the rough remaining side lapped as the first
plate is readily removed. High production rates of lapping flat parts on both
sides with good parallelism could be achieved.
Problem: It is difficult to quickly load pieceparts on the piecepart holder for use
with a high speed lapping and polishing system. Also it is difficult to generate a
flat parallel system of polishing parts where .001 to .002 inch (0.025 or 0.05 mm
or so) material is removed from a side to make them smooth, perhaps to 4 light
bands, flat and parallel. Much of the time, hot melt adhesives are used which are
slow and cumbersome to apply and also difficult to remove because of
contaminating the precision surface of the piecepart for later use of the part.
Typically the piecepart holder has a gimbaled spherical ball end to freely allow
the part to move about radially to self align the pieceparts (one or more) with the
surface of the rotating abrasive platen.
Solution: A piecepart holder can be constructed out of a heavy metal such as
steel which has substantial mass very close to the surface of the abrasive disk.
The unit will be allowed to move freely with the surface by the ball-end holder.
A substantial hole can be made within the ball-end device which would allow
vacuum to be coupled to the piecepart holder individual part pockets to firmly
hold the flat pieceparts up tightly against the tight fitting pocket. To create and
maintain a good vacuum, a thin layer of oil or grease can be applied to the
piecepart to seal any leakage paths. In this way, by simply removing the vacuum
applied to a rotary union to the driven shaft open inside diameter, the part is
released, can be turned over and the opposite side lapped to produce a high
quality surface not damaged on the already done side because intimate part-to-holder
contact is not made because of separation by the film of oil, yet is stiff
enough for good polishing action.
Problem: When using a diamond (or other) abrasive disk rotating at very high
surface speeds of 10,000 fpm, most of the abrasive cutting action takes place at
the outer periphery of the disk. The inside area of the disk has low surface
velocity and low cutting action and also low wear rates so that when a piecepart
traverses the disk in a sweeping motion to prevent wearing of tracks or grooves
on the abrasive, there is uneven wear between the outer and inner surfaces of the
disk. There is typically a small 1/2 inch (1.27cm) diameter hole in the inside of
the disk at the center to act as a positioning agent to apply the abrasive disk at
the center of the platen to obtain good balance of this very high speed system. A
larger diameter round section could be removed from a disk to create an annular
ring of active abrasive material somewhat larger than the piecepart which
eliminates the inactive (and raised) uneven section but then the centering
registration hole for positioning the disk is lost.
Solution: A disk can be fabricated with abrasive coating covered on the whole
surface of the disk. Then the inside section of the abrasive toward the center of
the disk could be removed by grinding or peeling it off leaving the backing
material intact with the centering hold. Here the piecepart could be in contact
with the raised section of the abrasive on an annular outer ring only as the
abrasive is raised (by coating) from the disk backing material (usually plastic
sheet). Another way would be to punch out the center ring of the disk for
separate use and then use a centering plug with a small locating hole so the plug
could be centered on a platen center post and the annular disk centered on the
plug, become fixtured by the vacuum grip platen and the plug removed for
complete freedom of movement of pieceparts over a disk as the post could be
removed from the platen also.
Problem: When a wobble plate is used for polishing, grinding or lapping, a
piecepart must be presented exactly parallel to the moving abrasive surface
without a leading edge hanging down where it will be the first section to contact
the abrasive. This could tend to jam the piecepart into the abrasive and damage
the outside edge of the piece part. This problem is made worse by having a
heavy piecepart mounted off-center with the mass center of gravity outboard of
the center axis of the wobble plate. This would tend to dip the heavy side down
and create an out-of-parallel presentation to the moving abrasive. Also any
friction on the wobble plate ball or an out-of-balanced spring center system will
result in dipped edges of the part.
Solution: A ball is used to support the applied contact force of the wobble plate.
The ball is constrained in a cylindrical hole such that the ball is free to fall loose
with the weight of the lower movable section of the wobble plate and the weight
of the piecepart combined. There may be 3 adjustable screws at 120 degrees
apart which act as parallel location stops to hold the lower piecepart parallel to
the wobble plate spindle bottom flat surface. This results in the piecepart being
parallel to the moving abrasive surface. The loose ball will allow the free lower
section of the piecepart and holder to be held accurately by the 3-point screws.
Then when the piecepart is lowered into contact with the moving abrasive, flat
contact is initially made but the free motion slack in the ball holder is then taken
up (perhaps .010 inches, 0.25 cm) so that the wobble plate is free to move in an
angular fashion and the ball surface is in contact with a hard flat surface which
results in very low friction. An anti-rotation leg is used also. One, two or three
legs can be used for anti-rotation with clearance for gimbal wobble angle action.
Problem: It is difficult to quickly lap hard metal or ceramic or other materials
with conventional lapping techniques using disk platens which are 12(28.5 cm)
to 48inches (114 cm) in diameter operating at 200 to 300 rpm using loose
abrasive paste media. Larger diameter platens are potentially dangerous at high
speeds.
Solution: A high speed lapping system can use fixed diamond abrasive coated or
plated on a disk sheet of material and be used on a rotating platen disk with a
diameter of 12 inches (28.5 cm) when operating at 3,000 rpm which gives a
surface speed of about 9,000 feet per minute. If a larger diameter platen wheel
of 15 inches (38.1 cm) diameter is used, the rpm can be lowered somewhat to
perhaps 2,800 rpm to achieve the same 10,000 (or 9,000) feet per minute (fpm)
and if the wheel diameter is 18 inches (47.7 cm) diameter, then the speed can be
further reduced to produce 9,000 - 10,000 fpm at the outer periphery of the disk.
Any reduction of rotational speed for large diameters is desirable because of the
potential danger of a high inertia wheel creating problems if a disk is damaged or
comes loose.
Problem: The surface finish smoothness and flatness of hard parts made of
metal or ceramic or other materials vary as a function of the work force on the
piecepart as the workpiece is held against the surface of a high speed 9,000 to
10,000 fpm abrasive lapping action.
Solution: It was found that the amount of coolant and lubricating water or liquid
applied to the surface of the high speed rotating disk affects the quality of the
lapping action. If a reduced flow rate of water is applied, the abrasive cutting
rate is increased as the boundary layer of water is decreased between the
piecepart and the rotating disk, better allowing the tips of the exposed diamond
particles to be in more direct contact with the piecepart and thus are more active
in removing material as they penetrate deeper into the surface of the material.
Excessive water covers the abrasive particles and keeps the abrasive from
contacting the piecepart surface. Here if the water flow rate is increased and the
piecepart is more "flooded", then a thicker boundary layer of water or liquid
builds up between the part and the surface of the abrasive disk. A moderate
amount of water will tend to keep the diamond abrasive particles away from the
piecepart some fraction of their maximum penetration which results in a
smoother and flatter surface on the part. One method of utilizing this
performance is to have reduced water flow at the first portion of the lapping
period for more aggressive material removal, but with a resultant increased
roughness of the surface. Then the water flow is increased somewhat during the
middle portion of the abrasive cycle to get better surface finish and yet have a
medium material removal rate. Finally the water flow rate is substantially
increased at the end of the cycle to produce a very smooth and flat surface with a
low rate of material removal. Changing of the water flow rate to alter the
material removal rate and to change the surface smoothness could be easily done
with an automatic water flow rate control system which varies the flow rate
during an abrasive cycle.
Problem: When doing abrasive lapping at high surface speeds of 9 - 10,000 fpm
on round platens rotating at 3,000 rpm with diameters of 12, 15 and 18 inches
(28.5, 38.1 and 47.7 cm), there is substantial danger when a piecepart is broken
off its holder (as it normally is held with a weaker adhesive or mounting system)
and the piecepart being thrown off the platen or getting stuck on the platen and
ripping the diamond or other abrasive disk causing further possibilities of fast
destruction of parts of the machine with parts thrown out and endangering an
operator or others or equipment due to large kinetic energy contained in the
rotating disk.
Solution: The rotating platen is round in shape with about a 12 or 15 inch (28.5
or 38.1 cm) diameter. A rectangular corner box is constructed as described
earlier to deflect explosively propelled pieces downward into a collection area.
The deflection may be from a vertical surrounding surface coupled with a lip or
partial cover which reduces the amount of shrapnel which can move vertically
out of the work area, as described above. The box is desirably constructed of a
soft plastic (or rubber) such as ½ inch (1.27 cm) thick high density polyethylene
which would tend to absorb impact from a heavy metal free flying broken-loose
part without ricocheting the part back into contact with the rotating disk which
prevents it from being thrown again or damaging the part. Also, the "square"
corners provide a remote area to trap the part and to contain the part as it stopped
moving by being impacted on one or more mutual walls. Having a distance
between the flat walls and the rotating disk which is somewhat larger than the
largest size of the piecepart, centrifugal force would tend to drive the part off the
disk radially and allowing it to eventually roll or move tangentially to a neutral
corner of the box away from the disk. In the same way, crumpled abrasive disks
are collected by the neutral open corners. Having a ledge over the inside portion
of the box also helps trap the parts.
Problem: When a workpiece holder is held up by an air cylinder to provide
normal force on a workpiece against a high speed 10,000 sfpm rotating disk by
moving vertically up and down to load parts and lap them, there is potentially
great danger if air pressure is lost due to air line leaks or electrical failure. If this
load of the disk rotating motor assembly, which may weigh 30 lbs. Or more,
drops on the 12 inch (28.5 cm) heavy rotating disk operating at 3,000 rpm, there
is great danger in that the abrasive disk can be torn or cut, jam up and create
danger to the operator or severely damage pieceparts which may have great
value.
Solution: The vertically moving piecepart assembly can be mounted on vertical
slides and a chain or cable used with a counterweight which is perhaps 10 lbs.
(4.6 kg) heavier than the 30 lb. (13.8 kg) assembly. Upon loss of electrical
power which would interrupt power to the normally used suspension air cylinder
or a line leak to the cylinder, the piecepart assembly would simply and quickly
retract to the upper position, out of contact with the rotating platen and thereby
reducing the chance of danger. This would also be more assured when using an
E-stop (Emergency Stop) action switch which would then not require power to
obtain safe action.
Problem: When lapping or polishing at very high surface speeds of about 10,000
surface feet per minute, it is difficult to mount pieceparts to a rotating holder for
contact with an abrasive disk surfaced rotating platen in a way to hold the parts
rigid enough they are not broken loose from their mount. Also it is desirable to
avoid a localized vibration of the typically thin flat piecepart (which vibration is
induced by the high speed contact with the rotating platen) as patterns of uneven
polishing takes place on the surface of the precision part if it vibrates during
grinding. It is further desirable that one or more pieceparts be processed at a
time and that unloading these parts and remounting new parts is done quickly
and easily to provide cost effective polishing rates of production. Additionally, a
method of changing parts quickly so that one side of a piecepart can be lapped,
the part turned over and the second flat side be lapped to be parallel to the first
side. Typically .001 inch (0.025mm) to .002 inch (0.050 mm) or less is removed
from each side.
Solution: Thin pieceparts of about 1 x 2 x 0.080 inches (2.5x5.1x0.16cm) can be
mounted on to an individual piece of pressure sensitive adhesive (PSA) tape and
this taped piecepart can then be held by a vacuum to a workpiece holder. The
surface characteristics of the nonadhesive side of the tape would be controlled by
selection of tape backing material or by surface conditioning to provide a high
friction which would resist lateral dynamic forces in a plane along the surface of
the thin workpiece as the nominal 14 psig (25 inches Hg vacuum) would apply a
normal force holding the workpiece to a rotating holder. A large section of tape
could also be used to hold a number of workpieces at once which would be fast
and easy to install by hand or with a robot. This flexible group assembly of PSA
bonded workpieces could then be held into position against a precision flat
surface of a workpiece holder having random vacuum holes over its surface
which would all be sealed by the wide and complete expanse of tape covering all
the vacuum holes and at the same time firmly holding the individual workpieces
to the holder. To process the other side, the group would be removed, tape
applied to the lapped surface side and the tape on the unprocessed side would be
easily peeled off. The tape would not only fixture the parts but would protect the
precision lapped side from scruffing action of rubbing on the holder.
Problem: When holding pieceparts on a rotating holder in contact with a rotating
abrasive coated platen rotating at a surface speed of 10,000 sfpm, it is difficult to
create a gimballed, free wobble motion so the contacting surface can
continuously align itself to the flatness of the rotating platen and yet be held
stiffly enough in a nominally flat position when first lowering the workpiece
holder to the abrasive surface while rotating so as not to have one corner of a
workpiece contact first and be preferentially abraded away thereby producing an
uneven workpiece surface. Vacuum piecepart clamping hoses could also create
problem forces.
Solution: A coiled spring can be used to apply a self correcting force between
the workpiece holder plate having a gimbal spherical bearing and the rotating
drive shaft of the rotating piecepart holder. This spring would be made of metal
or plastic material which would allow the straightening action to be applied but
also would introduce vibration damping for excitation vibrations set up by the
high speed contact abrasive action. One or more solid plastic coupling bars
could provide damped spring action also. If a vacuum hose were to be used to
provide vacuum clamping of the piecepart to the piecepart holder through a
hollow drive shaft, this type of hose could extend from the shaft and be coiled
with perhaps one or less on multiple turns which nominally lay flat with the
upper surface of the workpiece holder which would minimize the creation of
uneven "normal" direction workpiece contact forces as the workpiece holder
turns.
Problem: It is difficult to adjust the small diameter upper piecepart holder
surface to be precisely parallel to the platen large diameter surface and thus the
finished ground pieceparts may have a coned surface if outside edges of the
piecepart are ground more than inboard areas..
Solution: The abrasive sheet carrying platen is mounted on a thick heavy steel
support plate with leveling jack screws on the four (or three) outer corners to get
a nominal axis alignment of the platen with the axis of the piecepart holder to be
coincident with the axis of the platen abrasive spindle. Then a swing arm is
mounted on the piecepart holder which is rotated slowly about the stationary
platen. The swing arm is extended out to the surface of the platen. This
measurement indicates the "Z" axis error perpendicular to the surface of the
platen at different "X" and "Y" coordinate positions on the horizontal surface of
the platen. Adjustments are then made to align the lower platen mounting plate
to the upper piecepart axis. An upper frame can also be constructed for the pivot
arm lapper by attaching the bottom portion of the stiff pivot vertical post to a
round solid steel rod which in turn is attached to the base of the machine frame.
Then two long arms are attached to the upper portion of the post at 90 degrees to
each other, aligned with the "X" and "Y" axis. These arms can be fixtured with
threaded screws on the outer ends and both "X" and "Y" can be adjusted
independently with these screws which are in effect bending this rigid post at the
base. Mechanical clamps hold the post in place after adjustment. This
alignment adjustment could be automated with stepper motor driven screws,
piezoelectric actuators, etc.
There are a variety of different adjustment actuators which can be used.
These include, but are not limited to a threaded bolt, motor driven threaded bolt,
piezoelectric actuator, and a thermal expansion bolt (e.g., electrically heated
thermal expansion bolt). A stepper motor, servo motor, DC or AC gear motor,
and the like can be used motor to drive the alignment arms to different positions
and make corrective adjustments to align both axis of piecepart and platen as
indicated, for example, by an out-of-plane gap sensor.
Problem: When flat circular disks having diamond or other abrasive media are
used on a high speed platen rotating at 3,000 rpm or more to produce surface
speeds of above 2,000 sfpm and even about 10,000 sfpm, the outer periphery of
the abrasive sheet at the outside diameter has a high speed with good abrasive
action but the inner diameter of the disk has a lesser velocity proportional to the
radius and less abrasive action. Most of the abrasive grinding or lapping
material removal from a piecepart is removed by the outer diameter of the disk
which tends to wear down the abrasive media at the outer diameter more than the
inner radius which results in an uneven flatness of the abrasive disk. It typically
is a cone shape with a higher section at the circle center of the disk which
prevents a piecepart from being ground or lapped flat across its surface which is
critical to part surfaces having good enough surface flatness or surface roughness
finish for pump seals, computer chips, hard disk computer components and for
other parts. The unworn inside of a disk is not utilized and therefore there is
inefficient use of the abrasive sheet material which is quite costly.
Solution: An annular ring disk can be used on a flat rotating platen which is
made from the original circular disk of abrasive media by cutting out these rings
in a cookie cutter fashion. Typical rings may be 18 inch OD (47.7 cm) x 15 inch
(38.1 cm) ID; 15 inch (38.1 cm) OD x 12 inch (28.5 cm) ID; 12 inch (28.5 cm)
OD x 8 inch (20.3 cm) ID. A piecepart which is presented in contact with the
rotating ring abrasive disk typically would be swept across both the inside
diameter portion of the disk progressively to past the outer diameter of the
annular ring where both the inner and outer radius of the disk would have
diameters and surface speeds and abrasive action and disk wear, fairly constant
across the full surface of the disk ring thereby reducing the cone effect wear on a
given disk which would produce better flatness and more uniform roughness
surface finish on a piecepart. In this way, expensive diamond particle type of
abrasive disks can be fully utilized for good cost savings and efficient use of the
abrasive media. A pivot arm could be used to sweep the workholder back and
forth across the annular abrasive disk ring with a preferred contact to occur in a
quadrant of the abrasive sheet which provides a stabilizing friction contact force
directed away from the rotating axis of the pivot arm. Also an X-Y table can be
used to sweep the width of the annular ring. A single solid circular disk could be
cut into multiple annular rings and the core center circle could also be all sold
and used as separate units with no manufacturing waste. The disks could also be
cut into ellipse or oval shapes with an annular ring shape where the outer and
inneredges of the disk would be "moving" relative to the piecepart and not have
a tendency to produce nonuniform abrasive wear tracks on the piecepart as much
as a true circular abrasive disk.
To increase the efficient use of the annular rings, the piecepart is also
rotated as it is presented to the abrasive sheet surface and is being lapped. This
assures even lapping address by the surface of the piecepart to various radial
portions of the annular abrasive distribution.
Problem: When using annular ring disks of various sizes on a given circular
high speed rotating platen having a vacuum hold down system, any inboard
vacuum holes are exposed or non-sealed for large sized ring disks and thus the
vacuum hold down system doesn't work. The same is true for using smaller ring
abrasive disks with exposed outer vacuum holes.
Solution: If an 18 inch (47.7 cm) or smaller platen is constructed with
concentric paths of vacuum holes spaced at various radius of the platen, or if
scattered holes are positioned to not create a circular track and to avoid making
abrasive "track" patterns, the exposed holes would be sealed with a pressure
sensitive adhesive thin plastic film on either or both the inside or outboard
portion of the vacuum holes left exposed when applying the nonadhesive backed
abrasive disk material having an annular ring shape with an inside and outside
radius, either circular, oval or other shape. This adhesive backed sealing disk or
ring can be left on the platen for a duration of time and it can be used to register
or accurately position guide the annular abrasive disk onto the true center of the
platen for achieving good dynamic balance of the very high speed rotating
assembly operating at perhaps 3,000 rpm or 10,000 surface feet per minute.
Safety is very much enhanced by good balance and the quality of surface
grinding or lapping is also enhanced by good circular location and strong reliable
vacuum hold down of the abrasive disks which may be constructed using fine
diamond particles or other media. The inboard non-abrasive disks described
above to reduce lifting of the annular abrasive sheet by grit, slurry or water
would also solve this problem.
Problem: Some specialty grinding techniques can be improved by having an
abrasive media disk with a slightly angled surface relative to the normal typical
flat plate surface for high speed (e.g., above 500, above 1000 or about or above
3,000 rpm, e.g., up to and beyond 10,000 sfpm) use of abrasive sheeting such as
fine abrasive particle coated disks such as diamond coated disks.
Solution: Annular rings of diamond or other media coated abrasive disks are
generally fabricated in thin disks with thin metal or plastic 0.005 inches
(0.12mm) thick, more or less, that is locally elastically conformable to a hard
surface. A flat rotating platen can be constructed with a portion of the surface
raised somewhat from the flat circular surface and a cone angle created on this
surface to which an abrasive annular ring is adhesively bonded or held in
position by vacuum holes to this angled raised ring. A piecepart can then be
presented to this cone shaped surface at an angle to the platen perpendicular
which is approximately the same as the abrasive disk cone angle. The piecepart
presentation angle may either be more or less than the abrasive angle to control
the portion of the piecepart surface that is in contact with the rotating abrasive
for optimized grinding/lapping action.
Problem: Achieving ultra flat and smooth surfaces in a milling machine
operation process without subsequent grinding and lapping type steps.
Solution: In a milling machine, CNC horizontal or vertical, a conventional
milling cutter can produce a relatively flat surface with a 16 rms finish. A
special media holder can be clamped in the spindle which has a flat precision
surface perpendicular to the machine spindle centerline. A flat abrasive with a
pressure sensitive adhesive would be attached to the special media holder The
abrasive could be die cut into an annular ring, for example 6 inch (14.3 cm) OD
and 4 inches (10.2 cm) ID. With the spindle running at, for example, 6,000 rpm
and about 9,000-10,000 sfpm, the surface of the machined part can be "high
speed lapped" with the special holder and abrasive media. The abrasive should
be in contact with the work piece. The machine table moved in a crossing
pattern to evenly distribute the lapping action. A supply of coolant fluid should
be used to keep the work piece cool. It could be pumped through the spindle and
special holder if available. A typical material removal piece pass would be
0.0001 - 0.0003 inches (0.025 mm to 0.076 mm) in the "Z" direction. Using this
technique and starting with 125 micron diamond abrasive media and stepping
down to lapping films, 1 micron for example, surface finishes and flatnesses of
very high quality can be achieved in one machined part set-up, eliminating
subsequent grinding and lapping operations with a substantial part handling and
cost savings.
Problem: When grinding or lapping single or multiple pieceparts held by a tool
holder with a typical diameter of 4 inches (10.2 cm) held by a center post and the
tool holder is slowly (or fast) rotated as it is presented down vertically to
uniformly contact an abrasive surface platen rotating at the high speeds of the
present invention, it is important that the piecepart holder be "flat" so that the
pieceparts which contact the abrasive first are not damaged because the holder
has one edge lower than another. Further, with this type of lapping and grinding
it is important that the piecepart holder assembly be held by a ball pivot type of
device located as low as possible (as close as possible so that the central point of
rotation of the pivot is as close as possible to the abrsive sheet surface when
contact is made. It is also best to align the total piecepart assembly so all the
individual parts are floated equally by the thin boundary layer of coolant fluid on
the surface of the disk which may be less than 0.001 inch (0.025 mm) in depth.
With this type of gimbal pivot, this boundary layer thickness has a tendency to
remain uniform even with slight out-of-perfect-perpendicular alignment between
the vertical piecepart holder shaft and the high speed abrasive platen. Foreign
debris contaminates pivot joints and create unwanted friction. It is also important
to control the water boundary layer thickness and shape between a workpiece
surface and the abrasive surface for a small workpiece with a correspondingly
small surface area that is not large enough to be positioned flat on the abrasive
surface with a minimum amount of down pressure.
Solution: A work holder is created with the use of a spherical ball attached to a
shaft which provides a pivot action close to the bottom of the workpiece holder
assembly. A sandwich of washers (between the piecepart holder housing and the
ball) act as a rigid base to transfer polishing normal force downward on the
vertical shaft to push the pieceparts onto the abrasive platen. The washers apply
only a small to prevent slack between the ball and the holder, or the resultant ball
friction would prevent free pivot action on the ball. The pivot action is
restrained by encapsulating the whole assembly (the ball post, ball washers and
ball socket) with RTV silicone rubber which seals the unit from debris and also
provides the function of an elastic restraint that self centers the disk type part
holder perpendicular to the axis of the support shaft, yet the elastic spring which
centers the unit is weak enough to allow conformal pivoting of the assembly
during the lapping action. Thus when little side load is present, as when
lowering the piecepart assembly, the unit is flat aligned, but when subjected to a
normal force, the unit is free to pivot. A piecepart holder with the ball stem and
RTV was constructed and used for lapping of a piecepart assembly for optical
connector devices and appeared to function well.
Problem: When high speed lapping, a rotating flat platen with fixed abrasives
attached to the platen with adhesives or vacuum, water on the rotating platen
abrasive surface forms a boundary layer between the work piece and the abrasive
media. The boundary layer thickness and shape effect the flatness of the work
piece. The workpiece must be allowed to "float" on the abrasive surface which
is partially covered with a boundary layer of water.
Solution: The work piece must be allowed to "float" on the boundary layer.
This is done with a gimbal mechanism which puts pressure down on the rotating
work piece. It also allows the work piece to "gimbal" in the horizontal plane
while an independent driver pin drives the work piece around the centerline of
the work holder shaft. The amount of down pressure also effects the boundary
layer. The work piece floating on the boundary layer of water allows the
abrasive media and platen imperfection to be averaged out, so high spots on the
abrasive do the lapping while the low spots are filled with water, allowing the
lapping action to take place and produce a finished part (work piece) that is
flatter than the media and platen. The work piece will only be as flat as the
boundary layer.
Water is pumped through the work holder and into controlled orifices or
jets in strategic locations that force a boundary layer to form between the work
piece and the abrasive media. The water stabilizes the work piece while
presenting it to the rotating platen initially and while lifting the work piece off
after lapping is complete.
Problem: When lapping or grinding a multiple number of small parts or single
small parts each having small surface areas and short surface dimensions in the
approximate size of 0.25 inch by 0.25 inch (0.63 cm) and these parts are put in
contact with a high speed rotating disk, there is not enough surface length to the
part to build up a sufficient boundary layer to float or support the part as it is
making contact with the abrasive disk on the high speed platen and the parts tend
to dig into the abrasive disk and tear the disk and prevent accurate polishing or
lapping of the part. This problem is again uniqely felt in the high speed lapping
process of the present invention with abrasive sheets secured to the platen.
Solution: A system is provided to effectively extend the too short surface
contact length dimensions of the pieceparts to allow them to be presented flat to
the abrasive surface. Here an adequate boundary layer is generated and
maintained while the individual pieceparts are being lapped by adding a
secondary device to the piecepart holder device. This sacrifical device, which
would have sufficient surface area and length would be mounted outboard of the
piecepart on the piecepart holder device. It would also be ground down
simultaneously with the pieceparts in a sacrificial way. A typical shape of this
can be a disk of metal such as brass which would be mounted on the outside
annular position of a tool piece holder with the to be lapped pieceparts mounted
inboard of these on the periphery of a round piecepart holder. The sacrifical
piece should have a susceptibility to grinding which is within about 50% of the
workpiece (either greater of lesser, preferably lesser) to assist in more uniform
grinding. The susceptibility to grinding can be readily measured by grinding
identical surface areas of the materials, with similar initial roughness, for the
same period of time, at the same speeds and pressures, with the same abrsive
sheeting and comparing the amount (e.g., weight) of material removed from each
sample by the lapping. As the total exposed surface area is ground down, the
pieceparts are held suspended above the high speed moving abrasive by the large
surface area of the sacrificial disk. As the sacrificial device lays outboard of the
piecepart, it is contacted first by the abrasive when the piecepart is tilted and
initially brought into contact. Contact with the piecepart is prevented until the
entire assembly lies flat. A typical disk would be 4 inches (10.1 cm) outside
diameter, 2 inches (5.1 cm) inside diameter and about 0.060 (1.58 mm) inch
thick. It could be easily attached with vacuum chucking and/or adhesive tape
and could be used over and over by loading new pieceparts with a partially
ground disk. Other geometry sacrificial plates could be used and combinations
of materials such as steel, ceramics.
Problem: When a high speed rotating abrasive platen is manufactured and after
repeated usage of the machine, the platen is not perfectly flat as it had been
originally machined or ground (having been damaged by wear or impact) to a
required or desired flatness of less than 0.0005 (0.00127 mm) inch at the outer
periphery with a need for the best performance to reach 0.0001 inch (0.00065
mm) as measured by a dial indicator placed at the outside diameter and the disk
rotated by hand for one revolution to measure the maximum excursion. Any
deviation acts either as a "valley" where the abrasive does not contact the
piecepart or a "high spot" which is the only area that contacts the piecepart.
When the disk rotates at its normal high speed, each high spot will have a
tendency to hit the piecepart and set up a vibration which will reduce the
smoothness of the lapping abrasive action. Localized distortions of the platen
surface will also have a tendency to penetrate the boundary layer of liquid
between the platen (covered with a thin sheet of diamond or other coated
abrasive) and the piecepart and produce a localized scratch or track on the
piecepart surface. Surface defects on the platen structure may be generally
transmitted through the thin abrasive disk and produces a bump or high spot on
the disk.
Solution: An existing platen can be "dressed" on a machine by bringing it up to
full speed rpm and lowering a heavy flat abrasive coated piece unit directly onto
the bare rotating platen and grinding or lapping off the bumps, and high spots.
Even full out-of-flatness surface variations can be removed by first using a
coarse abrasive and progressively using finer abrasive or lapping abrasive media.
The platen, in effect, becomes the workpiece and the workpiece becomes an
abrasive surface or sheet. The typical first abrasive may be 40 micron metal
bonded diamond and ending up with 3 micron or less diamond or ceramic
abrasive depending on if the platen surface is chrome plated, stainless or bare
steel. It is important that the surface area of the abrasive lapper disk be large
enough to cover the total area of the platen with a slight overlap and it could be
oscillated back and forth across the platen, could be stationary or rotating at
either slow speed or rotating at very high speed so the tip speed of the grinding
disk will provide uniform removal of platen material at the low surface speed of
the inner radius of the platen. Different geometries of adhesive disks could be
used. Also a piecepart holder already in use for normal lapping could be used to
perform this function.
Problem: The surface of metal objects are polished for many reasons including
the optical examination of a metallurgical characteristic, to create a smooth low
wear tight hydraulic or fluid seal and other uses. Usually this polishing is done
on low speed 5-200 rpm or so rotating flat platen disk wheels of various types of
construction may be used, such as aluminum, steel, plastic, composite, cloth and
other materials. The wheel surface is very flat and the workpiece to be polished
is held with controlled pressure by hand or work holder against the rotating
wheel with water or other fluid wetted abrasive particles introduced as a slurry or
disks of fine abrasive sheets "stuck" or bonded to the rotating wheel. This
process slowly produces an accurate, highly polished surface and it is labor
intensive and expensive if not automated. Inaccurate platen or shaft machining
or loose bearings or weak machine structure frameworks may cause polishing
accuracy problems.
Solution: It has been found that very high quality polishing can be achieved at a
fraction of the expended time by using microabrasive sheeting, such as 3M brand
microabrasive disk sheets for polishing at the very high speeds of this invention
described above. The process is especially useful with disks about 8 to 10 inches
(20.3 cm to 25.4 cm) in diameter. However, it is critical that the rotating platen
disk run very "true" and flat at the operating speed range to provide a
mechanically stable moving surface against which the to-be polished workpiece
is held stationary at a controlled normal force or pressure (against the fine
particle wetted abrasive). Options also may change the contact pressure
(between the abrasive sheet and the workpiece during lapping) as a function of
process time or the workpiece rotated to distribute polishing across the surface.
A unique method to provide a very "flat" and accurate stable rotating platen disk
surface is to mount the platen to a "weak" shaft which allows the rotating disk
mass to seek a true "smooth" center above its first rotating natural frequency.
The motor drive speed would be increased above the natural frequency of the
rotating platen with abrasive sheeting thereon, the workpiece part presented in
contact for polishing, then removed from contact prior to reducing the disk rpm.
Problem: When lapping or grinding at high speeds producing as much as
perhaps 5,000 or even at least 8,000 to 10,000 sfpm of surface lapping speed
using plastic disks coated with thin layers of diamond or other abrasive material,
it is sometimes a disadvantage to have a uniform flat disk surface in flat contact
with precision pieceparts. This is due in part because the fluid boundary layer
has a tendency to draw the piecepart down to the flat surface of the rotating
platen (by the effects of Bournoulli's principle) and create large fluid adhesion
forces requiring more force to hold pieceparts (e.g., with bigger motors) and the
need for larger and heavier holding devices for the pieceparts, and the need for
more frequent variations in the holding forces because of the variations in the
adhesion forces from fluid flow rate changes. This may also result in uneven
material removal resulting in non-flat parts. Furthermore, when a liquid
boundary layer builds up, it has a tendency to increase in thickness along its
length, which has the effect of tilting the surface of the piecepart relative to the
abrasive.
Solution: A precision ground rotating platen can be fabricated with slightly
raised spiral surfaces having different shape patterns from the inside center of the
platen toward the outer periphery of the platen. These spiral patterns would
create short land areas at the top surface of the platen of varying widths and
shapes with areas between these land areas that are somewhat lower, perhaps
from 0.002 inch (0.05 mm) to .010 inch (0.25 mm) or more. Then a thin plastic
coated abrasive disk that is uniformly coated with precision fine abrasive would
be mounted to the round platen and held in place by vacuum hold-down holes
either on the raised land surface or on the lower surface area or a combination of
holes in both areas. The raised land areas could be produced by manufacturing a
precision platen and acid etching the land area geometry configurations of the
lands. When the abrasive disk is mounted on the platen, only some portions of
the disk would be in contact with the piecepart being ground or lapped. The
boundary layer of fluid coolant would be effected by the length of the land area
under the piecepart, the direction of the spiral or radial or circular annular land
shapes or a combination of these geometries. The effects on the boundary layer
thickness would be the rotating speed of the platen, as related to the vector
speed, including direction for the surface relative speed between the two, the
viscosity of the fluid, the normal force pressure of the piecepart holding it to the
platen. The boundary layer thickness which would vary over the surface of the
piecepart would affect how the individual particles of abrasive normally sticking
1/3 of their size about the bonding agent, either metal plating or plastic bonding,
surface of the abrasive disk. If more liquid is applied, the boundary layer would
tend to be thicker and less abrasive material removed is achieved. Thus the local
pattern of the surface of the abrasive contact area can be utilized for the optimum
grinding action using only one portion of the abrasive disk with the non raised
section between the land areas allowing free passage of grinding debris. When
this surface area of the abrasive is worn, the disk can be unmounted by the
vacuum chuck, rotated to a "fresh" area of the abrasive and grinding continued.
The disk will remain uniform and strong through service. This can be done in at
least two different ways. A grooved pattern with a preselected distribution of
islands on the surface of the platen is created by molding, etching or the like.
When a thin backing abrasive sheeting (as used in aspects of the present
invention) is applied and secured to this textured platen, the backing of the sheet
conforms to the pattern. Continuous boundary layers will be broken up by the
predesigned variations in the surface of the conforming abrasive sheet, which is
very desirable. Since the pattern is chosen (with the highest areas on the platen
being fairly uniform and constant), a planar area of contact between the abrasive
and the workpiece can be maintained, with areas of non-contact or light contact
provided which will break up the boundary layers. It is better to have a flat
platen with a groove pattern existing on the abrasive sheet or by using segments
of abrasive sheet, as described herein. Abrasive sheets, even with diamond
abrasive, are now available from 3M with abrasive islands (e.g., diamonds
within a matrix) having paths where swarf, liquid and the like may flow between
the islands without disturbing the contact between the workpiece and the
abrasive on the sheet.
Problem: When a piecepart is ground or lapped on a high speed diamond or
other coated abrasive platen rotating at high surface speeds, there is an uneven
grinding action due in part to the boundary between the piecepart and the
abrasive surface being uneven with a thinner layer thickness at the outer
periphery being thinner due to the high surface relative speed at the outer
diameter and much less at the inner radius of the platen which is subjected to
liquid water or other fluids. Typical abrasive particles at the outer radius of the
rotating platen penetrate the thinner layer of the boundary layer and provide
material removal quite aggressively there. At the inner radius, the boundary
layer is thicker, the abrasive particles don't penetrate as well through the
boundary layer which "floats" or hydroplanes the piecepart, with the result of
significant material removal at the outer radius of the platen and reduced
removal at the inner radius. This produces uneven wear on the piecepart which
is subjected to both extreme areas of the platen radius and the piecepart is not flat
or the surface is not uniform in surface damage.
Solution: An annular ring of abrasive mounted on a platen is used so the relative
surface velocity at both the inner and outer radius is close enough that the
boundary layer is about the same relative to the height of the coated abrasive
(from above 0.1 or from about 1 to 100 microns). There may be two or more
piecepart holders, both rotating in reversible directions if desired for special
grinding effects, with both mounted on a common pivot arm (either straight with
two piecepart holders or branched with three or more piecepart holders. Each
piecepart holder would tend to stabilize the others across the platen. A spherical
wobble joint at each piecepart holder would allow each to conform to the slightly
uneven boundary layer on the platen. Rotating each piecepart holder would
provide the same amount of abrasive material removal to all the exposed
surfaces of the individual pieceparts. The normal force, surface speed, liquid
flow rate, viscosity, etc. could all be optimized The whole assembly pivot cradle
could be oscillated to obtain even surface wear.
Problem: It is often desirable to have a narrow annular ring of abrasive material
on the outside periphery of a rotating platen to effect fast high quality lapping
action. Production of a narrow annular abrasive disk as a continuous ring of
material from a linear web results in removal of the inner diameter disk of a large
diameter which is very expensive. This inner disk of material may be 8 inches
(20.3 cm) in diameter when producing an annular ring with an ID of 8 inches
(20.3 cm) and of 12 inches (30.5 cm) is also constructed of the same web coating
of fine diamonds or other expensive abrasives. These smaller disks are not
readily sold in the marketplace.
Solution: Cut annular segments having circular curvature from a web and join
these end-to-end in a pattern to form a continuous annular ring. These annular
segments can be adhesively attached or, even better, fused to a common base
material of strong plastic such as polyester or other materials such as hard thick
plastic or metal disks. The long ends of these segments can be butted directly
adjacent to each other, butt welded together or prescribed gaps can be left
between the ends of the segments to allow water/lubricant to better carry away
swarf. Different shapes can be given to the annular rings which may promote the
abrasive lapping such as serpentine shapes or curved radial segments. All of
these shapes can be cut out of linear web material with very little yield loss or
throw away. Short or long segments can be used.
Problem: It is difficult to determine if a piecepart has been brought into contact
with a high speed moving abrasive surface when it is initially presented for
grinding as it is not easy to calculate positionally when this would occur when
first using an unknown sized (thickness) part and when using abrasive disks of
unknown thicknesses and other machine variables.
Solution: The apparatus can have Fast Fourier Transformation spectrum analysis
pattern recognition controls used with an annular ring of abrasive. These
characterize vibration by amplitude as a function of frequency. It has been found
that when piecepart materials such as ALTIC (aluminum tungsten carbide or
aluminum titanium carbide) are brought in contact with high speed platens using
the abrasive sheeting (such as the 3M diamond abrasive disks) operated at high
surface speeds, especially such as about 10,000 sfpm, that a characteristic
significant sound is produced which is quite audible to the human ear at the very
first contact between the piecepart and the abrasive surface. At the time of the
onset of this audible sound, it is possible to very precisely determine the relative
location of the piecepart to the machine frame with the use of a Heidenhain
linear scale and then to commence to remove a fixed amount of the piecepart
surface of about 0.005 inches (0.0064 mm) by motor driving a threaded screw
actuator device which forces the piecepart into contact with the abrasive surface.
The audible signature allows the piecepart to be moved quite rapidly up to the
surface of the abrasive and then to be slowed or stopped for restart to allow a
very slow, controlled motion approach by driving the piecepart into the moving
abrasive surface at a slow prescribed rate with optimized controlled flow of
lubricants for a specific abrasive particle size over a fixed period of time. With
this technique, a piecepart surface will not be damaged by too sudden contact
due to excessive heat generation or impact.
It is difficult to determine if a piecepart has initially made contact with a
highspeed abrasive moving platen surface and also to control the normal (right
angle) pressure between the piecepart surface and the abrasive surface to
optimize the removal rate of grinding. The goal of producing a smooth ground
surface with 2 lightbands or less flatness is difficult to accomplish. A square
piece of ALTIC material about 2 x 2 inches (5.1 by 5.1 cm) was stepper motor
driven in small increments to where the contact force between the workpiece and
the abrasive moving, at 3,000 RPM for a 12 inch (30.5 cm) diameter platen with
about a 1.5 inch (3.77 cm) wide ring of annular shape had an initial contact force
of about 2-20 pounds (0.9kg to 9 kg), usually around 9 lbs (4.1 kg). The first
portion of the grinding period of about 1 minute removed surface material quite
rapidly, but as time went on, the force sensor showed a progressive decrease in
contact force with an unchanging machine incremental position. Also the swarf
of ground debris visually was quite heavy, but decayed in some proportion to the
contact force. A typical amount removed was about 0.005 inches (0.13 mm)
over this 1 minute period. The finished surface of the part was very smooth in
surface roughness, producing a mirror finish and the flatness was better than 1
lightband as measured by a green optical light flatness measuring instrument. As
the machine was not advanced during this period, the spring compliance of the
machine members produced this very successful fast initial removal of ground
material with a proportional or exponential decay of force which resulted in a
progressively more gentle contact at the last portion of the period, resulting in
the desired surface.
Problem: When a piecepart is initially brought into contact with a high speed
rotating (or linear) high speed moving abrasive surface, there exists the
possibility of one portion of the piecepart contacting the surface of the abrasive
in such a way that it will get caught or impact the high speed abrasive and either
harm the piecepart due to uneven grinding or jam it into the moving abrasive
surface which generally has very high inertia and momentum which can then
cause a virtual explosion with fracture of the piecepart, the holder, and the
abrasive media, either in sheet form or bonded abrasive. This can result in great
danger to the machine operator or significant damage to expensive parts being
precisely ground to size, finish or flatness. Also perfect alignment between
piecepart and the moving abrasive surface is difficult to achieve.
Solution: A multiple piecepart holder can be constructed such that the piecepart
is held rigidly and precisely on a flat surface by vacuum or other means such as
adhesive, melted wax or be established by mechanical measuring equipment and
process techniques so the piecepart can be lowered (vertically) so it is just barely
within 0.001" of the moving abrasive surface and then when contact is made by
further motion, the piecepart holder then is allowed to move freely by use of
weak springs which allows perfect flat alignment between the piecepart surface
and the grinding surface. For rigid grinding to obtain initial flatness of the
piecepart surface, small air cylinders can be used to clamp the piecepart
mechanism by driving a lower wobble plate portion of the piecepart (workpiece)
holder against adjustable mechanical stops. These stops align the piecepart
adequately parallel for the initial grinding contact and/or activity. These small
air cylinders are strong enough to overcome the weak springs. The weak springs
are used primarily only as the wobble plate is allowed to pivot. The air cylinders
prevent the wobble plate from pivoting. In this way the "floating" piecepart
holder device can be used to initially rough grind a piecepart by cylinder
clamping and then use the floating springs to continue grinding or lapping to
produce typical mirror finishes with flatness better than 1 or 2 light bands. The
air (or hydraulic) cylinders are only activated during rigid grinding but they
could also be used to apply a varying pressure to hold the piecepart against the
abrasive depending on the grinding process cycle events.
This entire section relates to a combination piecepart holder which allows
spherical pivoting (for finish grinding) and is also able to be supported in a rigid
position (for initial grinding). The piece part does not have to be changed, so
there is no set-up time needed for changing from these grinding modes.
When the pivot workpiece holder is used for rigid grinding of a part, the free
moving spherical section is moved against mechanical stops which rigidize the
unit. Moving this portion of the pivot part (workpiece) holder can be effected,
for example, by a variety of devices which include (but are not limited to)
springs, flash cylinders, electric solenoids, linear electric motors, thermal or
electrical screw devices, and the like. The important function is to hold the
piecepart holder against local stops to rigidize it, and then the entire rigidized
assembly is lowered to present the piecepart in rigid contact (non-pivotable
contact) with the abrasive surface (e.g., the abrasive sheet on the platen). This
rigid piecepart holder can be rotated axially, but does not have a spherical
pivoting action at this time. When a piecepart has been initially ground, it can
then be followed by conformational spherical grinding without changing to a
different lapping apparatus. It is very important with these relatively thin sheets
of coated abrasive material that the piecepart be presented to and contact the
abrasive with controlled pressure and force rather than attempting just a position
controlled presentation. The following equipment and procedures may be used
to effect this result.
A center slide (may be spring retained or activated by a cylinder or an
electric solenoid). Pressurize bottom of cylinder to lock part holder "up" against
ball for rigid grinding. Spherical joint for cylinder
Can use frictionless "air pot" brand cylinders with small air or oil gap between
cylinder wall and piston which allows fluid leakage but no stick (friction break
away). Center ball - can be held in a fixed position or allowed to slide vertically.
Multiple metal flex bellows with vacuum applied to draw "up" against ball stud
for initial grinding to flatten piecepart parallel or to initiate presentation of
piecepart to abrasive platen. Hollow metal of plastic flexible disk bellow stack.
Bellows can also be given a positive pressure to hold piecepart flat against the
abrasive platen surface with controlled contact force or pressure. Metal bellows
disk can be single annular unit or a multiple number such as three each at 120
degree increments.
Problem: When lapping at high speed with a rotating platen it is very difficult to
align the rotating piecepart holder precisely perpendicular to the platen abrasive
surface and to accurately bring a piecepart into contact with the high speed
moving abrasive without impact.
Solution: Construct a lapping machine which has the possibility to micro-align
the axis of the piecepart holder mechanism and the abrasive lapper platen. Also
use a fine pitch (40 threads/inch) screw to move the piecepart down into contact
with the abrasive with a stepper motor having 50,000 steps per revolution.
Further, the screw is attached to an in-line force gage which senses when the
piecepart comes in contact with the abrasive surface and this position is sensed
very accurately with a precision linear encoder device. A linear actuator with a
stepper or other motor is used to position the piecepart holder on the annular ring
of abrasive of the platen in the quadrant of the platen where the grinding or
lapping force action is the most stable depending on the direction of the platen
rotation.
One method is to first align the platen baseplate with 4 corner jack screws then
align the pivot post, then align the pivot arm.
Then mount the piecepart, find its contact position with stationary abrasive
platen, grind flat, finish lap with wobble plate.
The use of a sacrificial outer ring, square, segment pieces or ring with water
inlet/outlet slots, with the sacrificial parts made of various different materials:
plastic, metals, ceramics and metal/other composites, combinations, can assist in
assuring that the initial piecepart contact with the platen is level. By having the
sacrificial parts at a higher elevation with respect to the approach path to the
platen abrasive surface (usually by being outboard of the piecepart), the
sacrificial material will contact the abrasive surface of the platen before the
piecepart. This initial contact with the sacrificial part will level out the
workpiece while the sacrificial part is being lapped, without any damage to the
workpiece. This causes a touch down on the outer ring of sacrificial material
first, to "level" the workpiece part. Examples of sacrificial material could be
substantially anything that would not interfere with the lapping (e.g., explosive
materials, highly abrasive material that would destroy the abrasive surface, etc.),
such as porous material filled with lubricant. This technique may be used rigid
mounts or spring mounts on the piecepart holder.
Flooded Wedge Angle: One can also present the piecepart at angle tipped to
raise an edge toward incoming abrasive and water. Water will develop a high
pressure under the back (downstream) portion of a flat workpiece and lower the
workpiece flat. This will keep the piecepart from being presented with the
leading edge contacting first and "camming in" due to friction or water pressure
which destroys the leading edge as the piecepart is ground or lapped.
Boundary Layer Lifting: The use of a finite element dynamic fluid flow
computer program (FIDAP, by Fluent Company) shows that where a boundary
layer of water is uniformly flat under the full downstream length of the piecepart,
there is little tilting force on the piecepart. However, if excess water pushes up
to form a "dam" at the leading edge of the piecepart, a dynamic pressure head is
created under the first portion of the piecepart which tends to tilt the part on the
abrasive surface. A leading ramp knife edge can be used to reduce the dam
pressure build-up effect. Large leading edge pressure head lifting results if there
is a raised front edge or a big dam head of water on front leading edge of the
piecepart.
The tapered ramp knife edge is used at the front to cut off the water dam by
lifting it up (as with a snow plow), forcing the front of the piecepart down due to
reactive forces. The best procedure is to only use enough lubricant to wet the
valleys in abrasive mountains plus a little extra.
Change Down Pressure: By using speed control, downward normal force is a
function of surface speed, with greater downward force being used with greater
speeds to counteract the lifting or tilting force of hydroplaning of piecepart.
One should use very small down pressure at first contact, then increasing the
pressure after contact has been made, then again reducing the pressure very fast
with lift off from the moving platen.
Stationary Platen Start-Up: The platen is started only after the piecepart is in
contact with the abrasive sheet surface, using a start slow acceleration, then a
quick ramp up to full speed. The platen would normally be brought from a
stationary position (zero speed) to a full 3,000 rpm in about 15 seconds, or at
least about 100 or 200 rpm/sec. acceleration.
Option 1: Have the piecepart stationary until some minimum platen speed (e.g.,
at least 200 rpm) is reached.
Option 2: vary the speed of piecepart rotation before the platen start-up and also
during processing of grinding event. The piecepart could be rotating or
stationary at the time of the piecepart removal. Removal could be made with
platen at full speed, partial speed or slowed to a stationary state. The piecepart
will tend to stay conformed, flat to the platen at low speeds or stationary and
therefore it will not damage the leading edge of the workpiece.
Water or lubricant can be varied during the process, with large excess amounts
used during start-up initial contact or during removal at low platen speeds or
stationary platen. In the case where it is desired to intentionally tilt the piecepart
spindle relative to the abrasive platen to produce a slight cone shape on the
piecepart surface, the platen can also be started from a stationary position after
the piecepart is placed into contact with the abrasive. An initial "motor mat" tilt
angle can also be used with stationary start-up or lift off.
Add a loose material as a contact initial barrier such as powdered plastic,
abrasive particles or other materials. These would be used either as pre-coating
on piecepart surface or as constant flow input with water lubricant source during
initial contact, but stopped or eliminated during normal grinding. Their addition
can be restarted prior to lift off to develop a film or layer between the piecepart
and platen. The material could also be a thick liquid, such as a polymer solution,
grease, etc.
Problem: It is desirable to prevent tipping of the piecepart of a wobble pivot part
holder as it first contacts the abrasive which grinds of the leading edge of the
piecepart.
Solution: Use a sacrificial contaminant ring surrounding piecepart so that the
outboard sacrificial ring makes the first contact with the abrasive. Also the
piecepart could be potted in an adhesive, epoxy-like device which encompasses
the piecepart.
Piecepart pressure from high speed air jets across the top surface directed under
the surface to create an air film under the piecepart. Water jets impinging around
the piecepart on top of the piecepart surface to provide uniform pressure across
the piecepart surface to form a water film under the piecepart.
A stationary hollow holding ring can be held in a fixed position above the
abrasive surface and a piecepart which matches the ring opening can be dropped
into the ring to be in contact with the abrasive.
A dead weight may be placed on the piecepart top surface. A dead weight with
spring between weight and top surface may be used. One may also use a dead
weight with resilient spring material which is filled with vibration damping
material to reduce vibrations. Damping can be from liquid in foam or from
motion induced shear action within foam material itself where high local
velocities from vibration of piecepart introduced by unstable hydrodynamic
forces are alternated by local damping. It is also possible to use diaphragm
pressure on vacuum pistons to produce uniform pressure across free weight by
use of conformal diaphragm membrane in contact with piecepart top surface.
Use heavy or light piecepart ring with open center hole to mount piecepart
and have an extended outer portion with a low outboard bearing contact ring
having a spherical shape.
Two or more stationary standard roller bearings would be mounted to contain the
piecepart ring as it is forced against the bearings by the forces induced by the
moving water coated abrasive. The low position of the extended spherical
portion results in reactive forces kept low toward the abrasive surface and
minimizes upward tipping forces on the piecepart. A spherical surface on the
extended portion assures only point contact with the support bearing outer-flat
surfaces.
Another variation is to use support bearings with spherical surfaces to get
point contact. This point contact feature minimizes lifting or tipping forces on
the piecepart ring.
Gear teeth can be used on the outer edge of the piecepart ring so the ring
can be turned by a motor driven gear matching contact with the ring gear.
Other mechanical ring rotation drive mechanisms can be employed such as
engagement pins with contact "dog" arms, universal joints, magnetic couplers,
roller drive wheels, air or fluid contact impingement jets, inductive magnetic
electrical fields.
Another drive mechanism is the differential speed of the outer periphery of
the rotating platen abrasive having a greater contact force than the inner radius
abrasive contact thereby setting up a relatively slow differential rotating velocity
of the piecepart ring.
Problem: When a piecepart is ground or lapped using an annular ring which is
less wide than the piecepart, there is a center portion of the piecepart which is in
constant grinding contact with the abrasive, while other parts of the piecepart are
not in contact with an abrasive surface. This central area receives more grinding
action than the outboard portions of the pieceparts (which are typically rotated)
that leave contact with the abrasive. This center section typically has a circular
shape as the piecepart is rotated. If the piecepart is not rotated, then a groove
would be ground into the piecepart and it would have a width equal to the width
of the annular ring. The heat which would be generated by the friction contact
force with the abrasive is at a greater amount at the inside circle, and this also
tends to swell and raise this circle due to greater thermal expansion in the
inboard (central) area than in the outboard areas which leave contact with the
abrasive and are water cooled. When the raised, thermally swollen surface is
ground level and cools off and shrinks, the circle will be a "low" spot on the
piecepart.
Solution: The annular ring can be changed from an essentially uniform (evenly
distributed particles over any given significant area) surface to one of smaller,
parallel, concentric rings with free space grooves between the raised abrasive
which is flooded with water coolant. All portions of the piecepart then would
leave contact with the raised abrasive as it is rotated. The annular ring could be
made with raised tangential abrasive segments with gaps between staggered
adjacent inner concentric rings to grind-cool-grind a given area. Also the
piecepart rotating axis can be moved sideways during the grinding so that a
selected area can be moved out of contact with the abrasive surface.
Problem: When a lapper wobble spherical ball gimbal pivot plate is used to hold
a piecepart in intimate flat contact with a high speed rotating abrasive surface to
compensate for small minute misalignment between the piecepart support
rotating shaft and the platen shaft (collectively called the spindles), there is
generally sufficient friction in the antirotation mechanical device used to keep
the lower part holder portion of the wobble plate from torsionally rotating
relative to the upper portion which is attached to a spindle. As these two
portions of the piecepart wobble plate must move freely in a spherical pattern,
rotating about the spindle center, any friction from an outboard antirotation
device will impede the free spherical movement of the piecepart as it attempts to
align itself perfectly flat to the abrasive surface with a small nominal downward
contact pressure force which holds the part surface to be ground in flat contact
with the moving abrasive. A typical piecepart is ½ to 8 inches in diameter,
typical downward contact force is 0.5 to 20 lbs. and more, and the amount of
ground off material is typically 0.0001 (0.0025 mm) inch to 0.003 inch (0.0077
mm) to obtain a flatness of typically 1 optical lightband or less. Usually a post
with a stationary ball on one end is used where the ball spherical surface is in
rubbing contact with a flat surface and the frictional contact force between the
ball and the flat surface increases with increasing piecepart rotational torque.
This friction prevents easy movement of the ball against the flat surface which is
required to allow the spherical movement of the piecepart, and this friction is
further increased when the flat wall is contaminated by grinding debris or swarf.
Solution: The stationary ball post is replaced with a roller bearing, either a low
friction needle bearing, ball bearing, roller bearing or air bearing and this bearing
is constrained between two round stationary posts mounted on the opposing plate
which act on either side of the bearing so the piecepart can be torsionally rotated
in either direction. The outer cylindrical surface of the bearing will be self
cleaning as there is only point contact between the bearing surface and the posts
during sliding oscillations of each piecepart revolution.
Problem: A wobble spherical pivot plate that is made in two plate sections
attached to each other by use of a free floating trapped spherical ball needs to be
restrained or have the two plate sections coupled to each other to transmit
rotational torque from the upper plate to the lower plate. A typical "dog" type of
system where a post on one plate contacts a surface on the other provides
rotational torque, but has the disadvantage of having sliding friction on the ball
post to flat surface area which impedes the free pivoting action of the wobble
plate which is moving in an oscillating motion to maintain the wobble plate
piecepart surface flat to the moving abrasive surface as the wobble plate is
rotated during a grinding or lapping action. This friction can create undesirable
patterns of uneven ground surfaces in the piecepart, as the spherical pivot action
will tend to stick, break loose or stick again due to changing from the high forces
of static friction and lower forces of dynamic sliding friction which occurs at
each piecepart revolution.
Solution: A linkage bar with pin pivots at each end can be used to couple the
upper plate with the lower plate to obtain good torsional coupling with free
motion of the spherical pivot action of the wobble plate. The pins would be
solid with a small diameter which are periodically lubricated or they may have
ball, roller or sliding bearings at the pivots. The longer the bar and the more
horizontal the bar, the less incremental rotation of the lower plate relative to the
upper plate with the pivot action. Another method to accomplish the reduction in
"stiction" (jumpy dynamic friction) is the use of a hinge linkage system or a
living hinge solid flexible spring that is wide to be stiff for rotational forces but
weak for spherical pivot.
Figure 3 shows some of the features of apparatus of the present invention
in a segmented view of the apparatus 1200. This apparatus 1200 comprises a
rotatable platen 1205 with an annular ring of abrasive 1201 located on an upward
face of the platen 1205. The workpiece holder assembly 1230 comprises a rigid
shaft 1232 and an assembly housing 1234. Two of three air cylinders 1202 and
1203 (the third is removed by the segmentation of the figure) are attached to the
housing 1234 by pivoting connections 1236 and 1238. The air cylinder 1202 is
shown by further segmentation to be a spring air return cylinder. The cylinder
1202 is connected through a shaft 1240 to an intermediate plate 1242. An "up"
stop screw 1244 with a ball end 1208 is positioned below the intermediate plate
1242. A "down" stop screw 1206 is positioned at another position on the
intermediate plate 1242. The rigid shaft 1232 which is driven by shaft bearings
1204 is rigidly attached to the inside surface 1246 of the housing 1234. A
second rigid shaft element 1248 is rigidly connected to the underside 1250 of the
housing 1234 to slide or telescope within the first rigid shaft 1232. This creates
a rigid connection from above the housing 1234 to the pivot ball sleeve bearing
1212 below the housing 1232. A sleeve bearing 1212 for a pivot ball 1211
radially restrains the second rigid shaft element 1248. The sleeve bearing 1212
is connected to or at least associated with a piecepart holder 1252. The ball nut
1214 is adjustable to allow the telescoping gap distance to be set. This
connection or association may be accomplished in many different ways, the
requirement being that the piecepart holder 1252 spherically rotates around the
pivot ball 1211. A piecepart 1209 is fixed on the bottom of the piecepart holder
1252. There is preferably an antirotation ball pin and stop 1215 limiting the ease
of rotation of the piecepart holder 1252 with respect to the bottom surface 1250
of the housing 1234. A spring element (not shown) may be used with the ball
nut 1214 to control the axial gap movement. A segment of a spherical mass of
elastomeric material 1213 such as a room temperature vulcanizing rubber can
effectively perform the function of sealing the ball joint from grinding debris and
also seal in a ball lubricant. This configuration allows for the solution of a
uniquely difficult problem in alignment of the lapping apparatus 1200.
To be optimally effective in performing the function of proper alignment
of thee workpiece or piecepart 1209 to the abrasive annular ring 1201, the
piecepart holder 1252 must first act in a wobble or adjustable mode to place the
piecepart 1209 into alignment with the abrasive ring 1201. To assure the best
high speed lapping, during the actual lapping process, the piecepart 1209 is best
held in a more rigid alignment with the abrasive annular ring 1201. The
configuration in Figure X allows this adjustment in modes. When the piecepart
1209 is placed into contact with the abrasive annular ring 1201 in a non-lapping
contact according to a preferred method of the practice of the present invention,
the initial contact is made between the piecepart 1209 and the abrasive annular
ring 1201, the force on the top surface of the piecepart holder 1252 is provided
by the two air cylinders 1202 and 1203 and the ""up" stop screws 1207 and 1244
with the ball end 1208. These "up" stop screws 1207 and 1208 (the third or
more is not shown because of segmentation of the drawing) are able to move
independently and are allowed to move independently to allow the piecepart
holder 1252 to wobble or move spherically about pivot ball 1211 with the air
cylinders 1203 and 1202 mount pivoting connections 1236 and 1238 and find
proper alignment with the abrasive annular ring 1201. The pressure on the
contact is minimal as the air cylinders 1202 and 1203 are precisely controlled.
When this first, non-lapping contact controlled by the "up" stop screws 1207 and
1208 is made, further force is applied to the housing 1234 by lowering shaft
1232 so that it drops further. The piecepart holder 1252 moves towards the
bottom surface 1250 of the housing 1234. Contact is made between the ball end
1208 and the piecepart holder 1252. The bottom end 1256 of the "down" stop
screw 1206 makes contact with the top surface 1246 of the piecepart holder 1252
to equal the axial gap between the pivot ball 1211 and the ball nut 1254. Each
individual "down" stop screw (e.g., 1206) is adjusted so that in this static
position of contact between the piecepart 1209 and the abrasive annular ring
1201 in a non-lapping contact, the "down" lock screws 1206 are in the exact
alignment position desired when the piecepart 1209 is eventually brought into
contact with the abrasive annular ring 1201 during lapping. Therefore, the initial
contact between the piecepart 1209 and the abrasive annular ring 1201 during the
lapping process, when the platen 1205 is rotation at greater than 500 or more
revolutions per minute and at high surface feet per minute speeds, the piecepart
holder 1252 will be rigidly held in place in proper alignment by the rigid support
between the bottom 1256 of the "down" stop screw 1206 and the top surface
1246 of the housing 1234 as the housing 234 is pushed down by the air cylinders
1202 and 1203. If the air cylinders 1202 and 1203 are deactivated, then the
piecepart holder 1252 is allowed to wobble with the pivot ball 1211 in contact
with a hardened contact plate 1210. Vibration of the piecepart 1252 is prevented
by insertion of a vibration damping agent or damping device 1261 which
provides a connection between the piecepart holder 1252 and the housing 1234.
In this manner, the apparatus will be able to shift from a wobble or floating mode
to a rigid lapping mode during the rapid operation of the equipment. This
configuration is best performed with three sets of "up" and "down" stop screws
and three sets of air cylinders. Two, four or more can be used, but three has been
found to provide the best results to date.
Another issue which may have to be addressed is the fact that when
annular rings are cut from round sheets of abrasive disks, there can be significant
waste of material from the central round area cut from this disk. This is one
reason why printing of patterns of abrasive on a sheets is desirable. However,
because the sheets of abrasive are most commonly available in round sheet form,
the cutting out of annular rings is the most likely source of the annular rings. For
this reason, this invention also describes an annular distribution (to be included
within the meaning of the term "annular rings") of abrasive sheet material which
can use the residue of the process where a single piece, continuous annular ring
was cut from a round sheet of abrasive. As shown in Figure 15, segments or
pieces of abrasive sheeting may be lain in an annular distribution within the
abrading surface area of a rotating platen. In Figure 15(a), two segments 1301,
each of which is a half of an annulus, have been cut from the remaining material
from the original round sheet of abrasive material (not shown) and then placed
end to end to form the annular shape. The vacuum hold down of the platen (not
shown) can secure the individual piece 1301 into a secure position onto platen
1320. The individual pieces 1301 may be secured together at their intersection
1304 by adhesives, fusion, butt welding or the like. The center area 1306, as
with a single piece annular ring, may be left open or may be filled with a central
round sheet (which may also be physically joined to the two segments 1301 to
prevent flow of material under the segments 1301 and add support. Figure 15(b)
shows a multiple number (5) of arcuate segments 1308 aligned around the platen
1320 in an annular distribution. Any number of segments may, of course be
used, but the fewer the number of segments, the less work is needed to align
them.
Figure 15(c) shows a number of distinctly different shapes of abrasive
sheet segments on a platen 1320. There are three sets of abrasive materials, each
with distinct shapes, grouped as multiple wave forms 1322, kidney shaped 1325
and smaller arcuate 1324. An important feature of this configuration is the fact
that there are physical gaps 1326 between one of the pairs of segments 1324.
One of the problems previously discussed was the effects of removal and passage
of detritus, swarf and liquids away from the lapping contact area, especially the
problems associated with boundary layer thickness changes, channeling of liquid
flow (with or without swarf included), and other effects on the alignment or
pressure or exposure of particulate abrasives to the workpiece. This Figure 15c ○
shows another benefit of the use of non-butted and non-smoothly joined
segments form a residual cut-out sheet. Because the segments allow spaces
1326) to exist between the abrading or lapping surfaces (e.g., 1324), natural run-off
areas are provided which can carry away material without its moving
completely within the lapping contact area (e.g., on the surfaces of the segments
1322, 1324 and 1325). The dimensions of this gap 1326 are defined by the
surface of the platen 1320 and the height of the segments (e.g., 1324).
Figure 15(d) shows other configurations of segment areas which provide
fluid or swarf removal capability. The platen 1320 may have many various
configurations of abrasive sheet segments on the platen 1330. For example,
segments 1331 have holes 1332 in them which can trap material, rather than just
letting it flow away in the gap 1334. Segment 1336 has serpentine paths 1338
without abrasive thereon to form the flow paths. Segment 1340 has both central
open areas and an outlet area 1342 in a single design. This enables both some
collection and a flow path for material. As the most significant area of potential
damage from material on the surface of segments (e.g., 1340) is on the outer
areas, this configuration is very efficient. Segment 1344 has straight open lines
1346 between the areas of abrasive 1348. The segments radially curved 1350 are
smaller arcuate pieces which provide a significant flow area 1352 between the
arcuate pieces. It is to be noted that the segments may be touching (as in (a)) or
not touching (as in Figure 15 (d)) or combinations of these may be used. By
having non arcuate segment elements such as segments 1336 and 1342 contact
each other, flow passages which allow the movement of material from the center
of the equivalent annular abrasive ring to the outside of the ring would be
provided.
Another significant problem in the design of the equipment is the effect of
vibration on the workpiece holder and workpiece. As the finished piecepart
dimension specifications desired for the lapping process are so small, anything
which dynamically moves the abrasive sheet, the platen, the workpiece or the
workpiece holder, or shifts their relative positions is undesirable. As the platen
is quite massive, there is seldom any significant vibration in that element
(especially since designing the weight and construction of the assembly have
made considerations for that problem). However, the workpiece may vary from
job to job, the workpiece and workpiece holder do not have as great a mass as
does the platen and its housing, and vibration is much more likely to occur with
the workpiece holder, especially when in contact with the abrasive material
rotating at the high speeds of rotation of the present invention. Figure 16 (a) and
(b) shows mechanisms for reducing vibration on the workpiece holder and
consequently the workpiece. A shaft 1360 is shown attached to a workpiece
holder 1362 with a workpiece 1364 attached thereto. A vertical vibration
damping assembly 1366 is shown on the workpiece holder 1362. A leaf spring
1370 comprising a sandwich dual spring 1368 with a viscoelastic damping layer
1372 is shown. A mass 1374 is on the outer edge of the vertical vibration
dampening assembly 1366. The natural frequency of the unwanted natural
frequency vibration can be ascertained and a secondary spring mass vibration
absorber can be designed and installed to combat these vibrations. In Figure 16,
a spring constant for the leaf spring vibration damping assembly is designed and
installed to combat these vibrations. The spring constant is selected to be
matched with the discrete mass 1374 so that its natural frequency, as described
by
Wn = (K/M)½
is equal to the undesired natural frequency oscillation, wherein Wn is the natural
frequency, K is the spring constant, and M is the mass. This secondary spring-mass
will vibrate 180 degrees out of phase with the unwanted natural frequency
of the workpiece holder in a direction which is perpendicular to the abrasive
surface (this is why it is referred to as a vertical vibration dampening element)
and will not be affected by the rotation of the workpiece holder. This is because
when a flat spring is used, it flexes in only one direction, which is substantially
perpendicular to the abrasive surface. It is desirable that at least two, preferably
three, and possibly more of these units would be installed, most preferably
approximately symmetrically around the piecepart holder circumference. When
the most preferred arrangement of three vibration dampening elements are used,
they would be installed circumferentially with about 120 degree spacing between
the elements. The most preferred element construction, primarily from a cost
and convenience standpoint, is the use of two metallic layers (e.g., lead spring
layers) with a vibration dampening material (e.g., a viscoelastic material) acting
as a dampening agent between the two springs.
Figure 17 shows a configuration, previously discussed herein, for reducing
swarf, detritus and liquid movement problems within the system while it is
lapping at the high speeds of the present invention. A lapping system 1400 is
shown which comprises the workpiece holder 1401, a workpiece 1410 and the
high speed rotatable platen 1403 with an abrasive sheet 1405 secured onto the
platen 1403. The abrasive sheet 1405 makes contact with the workpiece 1410 in
a narrow region of contact 1403. The surface of the platen 1414 after a
significant flat area of contact 1403 has been effected, slopes away from this
contact area to a lower region 1422. This lower area 1422 has a ledge
indentation distance 1406 which is the difference between the level of the lowest
point 1422 and the interior surface 1416 of the platen 1402. The abrasive sheet
is shown to be secured to the platen 1402 by vacuum passages 1404. Debris and
liquid 1408 move over the interior surface 1416 towards the contact area 1403
between the abrasive sheet 1405 and the workpiece 1410. The level of this
surface 1422 is preferably lower than the height of the surface of the abrasive
sheet 1405 and more preferably below the height of the platen 1402 within the
contact area 1403. The liquid and debris 1408 move radially over the surface
1416, but are propelled to due centrifugal forces to jump over the ledge
indentation's distance 406 gap and continues on radially to contact the top
surface of the abrasive sheet 1405 and thus avoid the inside radial edge of the
annular abrasive sheet 1405 and prevent lifting of this inside radial edge of the
abrasive sheet 1405. Even the high centrifugal forces will not force the liquid
and debris between the abrasive sheet 1405 and the platen 1402. Figure 14c ○
shows a sharply stepped ledge indentation distance 1406 which prevents liquid
and debris from being forced by centrifugal action under the abrasive sheet 1405.
Figures 17 (a), (b) and c ○ all show how contact with the inside radius cuts off the
annular abrasive sheet 1405 which potentially has loose particles from the platen,
the center of the surface area of the workpiece does not align with the
geometrical center of he curved annular segment of he abrasive which contacts
it. However, the vacuum removal passage 1420 is a desirable assurance against
such movement.
Because of the use of an annular distribution of material on the rotating
platen, previously unknown geometrical effects have been introduced into the
system which have been first addressed in the practice of the present invention.
When a workpiece is being lapped, it is natural to place the geometric center of
the workpiece within the center of the rotating abrasive surface. It has been
found in the practice of the present invention that this natural positioning is
somewhat less preferred than another orientation. Because of the arcuate nature
of the annular ring of abrasive where the portions of the annular section which in
contact with the piecepart surface "break away" to the center of the platen, the
center of the surface area of the workpiece does not align with the geometric
center of the curved annular segment of abrasive which contacts it. Because
these two centers are not perfectly aligned and a contact force is applied to bring
them together for lapping, there is a subtle tendency for the piecepart to tilt out-of-flat-contact
to the radial outside of the platen. This happens because there is
less contact area support under the workpiece at the outside portion and more
contact area on the inside portion. This deficiency can be corrected by a slight
radial repositioning of he workpiece area center relative to the center line of the
annular ring. It is therefore desirable to shift the position of the workpiece
towards the inboard area of the annular abrasive sheet. This shift of the
geometric center of the workpiece should be at least 1%, preferably at least 3%,
more preferably at least 5% of the theoretical matching radial dimension location
of piecepart area center and the area center of he contacted segment of the
annular abrasive sheet dimension of the workpiece which addresses the abrasive
sheet surface. The exact percentage of shift of the geometric center of the
workpiece can be precisely calculated by simple arithmatic means, but has not
been done so here as it would have to be done for each annular shape (e.g., ID
and OF considerations). The speed of rotation does not by itself affect this
calculation.
Another factor in the movement effects of the workpiece holder (and
consequently to the workpiece) shifting during the high speed lapping of the
present invention is the forces being applied to the workpiece (and consequently
to the workpiece holder ) by the high rotational speeds of the workpiece holder.
The forces caused by debris and liquid flow under the workpiece also contribute
to this effect. These forces can cause the workpiece holder to want to swivel
about the ball pivot joint, or other pivoting joint, which secures the second rigid
shaft member to the workpiece holder. This problem is again unique to the high
speed rotation of the lapping system, particularly in combination with the
abrasive sheet which is less forgiving to shifting of the workpiece than a liquid
slurry on a slower speed rotating platen. The extent and seriousness of the
problem can be reduced by making at least one geometric reconfiguration of the
relationship of elements. It has been found that to correct for out-of-balance
swiveling of the workpiece holder due to rotation of the workpiece holder with a
mass center of gravity located below (or above) the pivot can be reduced by
moving the center of the pivot joint closer to the center of gravity of the
workpiece holder. It has been found that to correct for out-of-alignment
problems due to the dynamic abrasive contact friction forces on the surface of
the workpiece that it is desirable that the location of the workpiece gimbal axes
be located as close as possible to the surface of the abrasive sheet.
Figures 18 and 19 show constructions which address solutions to this
problem and which move the center of gravity of the workpiece holder closer to
the rotational center of the pivot connection to the shaft. Figure 18 shows a
lapping assembly 500 which addresses this problem. The shaft 501 is connected
to a primary support plate 502 having X and Y axis pivoting connections such as
gimbal bearings and pivot shafts 506 and 508 connected to downwardly
extending arms 504 on the primary support plate 502. A pivoting second support
plate 510 is connected to the workpiece holder 512. The workpiece 516 is
connected to the workpiece holder 512 and is in contact with the abrasive sheet
520 on the rotating platen 518. The abrasive sheet happens to be shown in this
configuration as larger than the workpiece, but that is not required. In many
instances the abrasive sheet 520 may be the same or smaller in the radial
dimension or radial direction (with respect to the platen) than the workpiece 516.
The workpiece holder 512 is shown with arms 514 which carry mass upwardly,
even beyond the line of the pivot shafts 506 and 508. This mass distribution
keeps the center of gravity closer to the plane of the gimbal bearings 506 and 508
than using a workpiece holder which was flat on all sides (e.g., a slab with
rectangles on all sides). Another configuration that would work is shown in
perspective in Figure 19. In this configuration, the lapping assembly 530 is
shown with a shaft 532 attached to a first external gimbal arm 534. The first
external gimbal arm 534 is attached through gimbal bearings and pivot shaft 536
to a second external gimbal arm 538. This second external gimbal arm 538 is
connected through gimbal bearings and pivot shaft 540 to a piecepart holder 542.
The piecepart holder 542 holds the workpiece 544. By having the piecepart
holder sitting within a volume of space created by the combination 546 or 534 of
the first external gimbal arm 534 (and the second external gimbal arm 538), the
center of gravity of the piecepart holder is maintained in a position which is
relative close to the line of rotation of the gimbals 534 and 538 through the
gimbal bearings 536 and 540 to reduce tilting of the workpiece holder 542 due to
the rotating speed of the workpiece. In addition, this configuration also
demonstrates a method for lowering the plane of the axes of the pivot gimbal
running through the gimbal bearings 536 and 540 close to the abrasive contact
surface of the workpiece 544. This geometric orientation reduces the tilting
torque on the workpiece and assists in the maintenance of proper alignment
within the lapping system.
Another benefit of the present invention, particularly with the use of
annular rings, is the ability to lap multiple pieces and even use multiple piecepart
holders at the same time. Figure 20 provides a description of this aspect of the
invention. A lapping system 550 is shown with an annular abrasive sheet 552,
an arm 554 carrying two piecepart holders 556 and 558. Each of the piecepart
holders 556 and 558 support a multiplicity of pieceparts 560 and 562. The
piecepart holders 556 and 558 rotate so that the individual pieceparts 560 and
562 are exposed to the abrasive sheet 552. Each of the piecepart holders 560 and
562 are aligned on wobble plates (not shown) and are operated by the processes
described above in the practice of the present invention. The arm 554 may also
have alignment mechanisms associated with it to assure proper alignment with
respect to the annular ring 553 and the rotatable platen (not shown). In this
system, the different pieceparts 560 and 562 do not even need to be of the same
size or cross section. For example, one set (e.g., 560) could be round, and the
other set (e.g., 562) could be square or triangular in cress-section. It is equally
useful to have a three arm central support piece for three separate workpiece
holders. It is desirable to process each piecepart for an equal amount of time to
make the surface treatments equivalent. Therefore, pieceparts located at the
center of the piecepart holder, such as pieceparts 566 and 564 may be eliminated
in this grouped set-up of pieceparts. If this were not done, pieceparts 566 and
564 would be continually lapped over the process, while other parts located in a
ring, such as shown for parts 560 and 562 would be processed only
intermittently.
In positioning an abrasive sheet material in platen with an annular raise
area on the outboard edge of the platen, it is often convenient to use a sheet with
larger dimensions (especially with respect to the radius) than the raised annular
area. When the support layer (and even when it is a continuous sheet of abrasive
with polymeric or other binder) is position over the flat central area of the platen
(or a part thereof) and then fitted over the annular raised area, the sheet of
abrasive shows a tendency to crinkle and lift at the transition from the central
area to the annular area. This is shown in Figure 20(a), shown with the platen
600, raised annular area 602, vacuum hold down holes 604, abrasive sheet 606,
and central area 608. As the abrasive sheet 606 moves up the step-up distance
610 with section 612 of the abrasive sheet 606, a crinkle or fold 614 forms at the
point 616 at the raised annular area 604. Figures 20(b) and c ○ show alternative
platen shapes 620 and 622 which provide sloped transitions 624 and 626 from
the central areas 628 and 630 to the flat raised areas 632 and 634. The slopes
should never present an angle that would bend the abrasive sheet past an angle of
65 degrees (e.g., forming an apex of less than 65 degrees by bending it more than
25 degrees away from horizontal), preferably not past an angle of 70 or 75
degrees, and most preferably not past an angle of 75 or 80 degrees, or more than
85 degrees. By reducing the angle that the abrasive sheet must be bent, the
possibility of any crinkling is avoided. As the placement of abrasive sheets over
an annular raised area is another unique aspect of the invention, this solution is
unique to the field of the invention.
In Figure 3, two separate supports 1253 and 1252 (the housing) form the
substance of the wobble plate. To further reduce vibration, a cushioning,
compressible element 1261 is provided between the wobbling piecepart holder
1252 and the bottom 1250 of the housing 1234. The compressible element 1261
should make contact between both the wobbling piecepart holder 1252 and the
bottom 1250 of the housing 1234. Viscoelastic material, springlike elements,
elastomers, rubbers, and layered structures may be used. In the Figure 3, double
sides polymer backed adhesive tape was rolled into a tube and cut to the proper
length. The tube was placed between the wobbling piecepart holder 1252 and
the bottom 1250 of the housing 1234. As they are brought together, the two
surfaces compress and flatten the cushioning, compressible element. This
element assists in reducing the vibration within the wobble plate element and the
piecepart assembly.
In the movement of the workpiece holder and the workpiece towards and
into contact with the rotating abrasive sheet covered platen, the contact force
application has been repeatedly identified as a desirable focus of control within
the practice of the invention. An additional aspect of this control is the speed
with which the workpiece (and the workpiece holder ) approaches the rotating
platen. As initial contact forces tend to be higher because of momentum,
reactive forces from the stationary surface, and elastic forces, control of the
speed of the movement of the workpiece and work piece holder are desirable
ways of controlling or moderating the initial contact force. Thus, as generally
mentioned herein, velocity control devices, such as fluid dampers (oil dampers
preferred, but other fluids, including gases, may be used). These velocity control
devices may be used with the cylinder contact force system to prevent the
workpiece from 'slamming' into the abrasive at a speed which would cause an
undesirable level of contact force initially. Therefore, a somewhat distinct or
auxiliary speed control or speed dampening system should be overlaid on the
cylinder contact force system to provide a second aspect of control to the contact
force aspects of the present invention. This speed control or speed dampening
system may also be used to lock the workpiece holder at a desired vertical
position at any time during the process (as for example after the removal of the
workpiece from contact with the abrasive sheet and platen element).
While the abrasive sheet and platen are rotating at the high speeds of the
present invention, it has also been found to be desirable to rotate the workpiece
(usually by rotation of the entire workpiece holder, although with multiple
workpieces in a group holder, the individual workpieces may also be easily
rotated). It is desired and has been proven to be beneficial to the flatness and
especially the smoothness of the work piece to have the workpiece rotated during
the lapping process. The workpiece should be rotated at least 1 or 2 full
rotations during 10 seconds of active grinding, especially at the point where the
finer abrasive particles are being used. The workpiece be rotated at a rate of at
least about 100 rpm, preferably at least 150 rpm, and more preferably at least
200, at least 300 rpm, which for a 30.8 cm diameter disk at 500 rpm, there
should be at least 3 to 4, and preferably more than 4 rotations of the workpiece
during 10 seconds of lapping. It is preferred that the workpiece be rotated at
least 3 or 4 times in a 10 second interval during lapping in the practice of the
present invention. The work piece may be rotating as it is brought into contact
with the abrasive sheet surface.
As has been previously noted, it is desirable to only fill the valleys
between the peaks of the abrasive particles (the peaks protruding rom their
binder support on the backing sheet) by from 50% of the protruded height to
perhaps 110 to 150% for an abrasive sheet with an essentially continuous
(uniform) coating or covering of abrasive particles. However, where the
provided abrasive sheet is provided with island areas of abrasive or other broken
or less continuous or less uniform distribution of abrasive particles, then part of
the water or coolant flow will lie in the river valleys which are relatively lower
than the protruding mountains of the abrasive islands. The water will therefore
be much deeper (a thicker boundary layer) than with a continuous and uniformly
coated abrasive sheet, and the piecepart will not hydroplane. In fact, the more
water that is present, the better is the grinding, as more heat is also carried away
by the larger volume of coolant water.
Claims (20)
- A process for lapping a surface comprising:a) providing a work piece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having I) a back surface and ii) a flat surface which can be adjusted to a position parallel to said at least one surface of said work piece,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said flat surface of said platen, and1. rotating said platen at a rotational speed of at least 500 revolutions per minute, and a surface speed at an outside edge of said sheet of abrasive material of at least 1500 surface feet per minute, and2. contacting said abrasive face and said at least one surface of said workpiece to be lapped.
- A lapper platen system comprising:a) a shaft which is connected to a rotatable platen having vents for air on a front surface of said platen, said platen having a back side to which said shaft is connected and a flat front side on said platen to which can be secured an abrasive sheet by reduced air pressure conveyed through said vents;b) a frame having a total weight of at least 200 kg supporting a work piece holder;c) said work piece holder is movable on said frame;d) said work piece holder is attached to a movable element on said frame, said movable element being capable of moving in a direction towards and away from said platen to perform lapping of a work piece held on said work piece holder;e) said work piece holder having at least one control element thereon which allows for independent movement and alignment of said work piece holder along three perpendicular axes so that a work piece on said work piece holder can be adjusted and oriented towards parallelity with said platen so that a work piece can be lapped; andf) optionally said control elements having at least 50 settings per rotation, each setting moving said workpiece holder along one of said three axes by a dimension less than 0.05 mm.
- A process for lapping a surface according to claim 1 comprising at least one of the following sequence of steps:Sequence of steps A comprising:a) providing a work piece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having I) a back surface and ii) a flat surface and providing a workpiece which can be adjusted to a position parallel to said platen, said flat surface of said platen having openings therein through which air may flow,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) reducing gaseous pressure between said back side of said abrasive sheet and said flat surface of said platen to secure said sheet of abrasive material to said flat surface of said platen,e) rotating said platen at a rotational speed of at least 500 revolutions per minute and a surface speed at an outermost edge of said platen of at least 1500 surface feet per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece;Sequence of steps B comprising:a) providing a work piece to be lapped, having at least one surface to be lapped, which can be adjusted to a position parallel to said at least one surface of b) whereb) is a rotating platen having I) a back surface and ii) a flat surface said flat surface of said platen having openings therein through which air may flow,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) wherein said sheet has an outer edge and an inner edge defining an annular distribution of abrasive, said inner edge having a diameter which is greater than one-third the diameter of said outer edge,e) rotating said platen at a rotational speed of at least 500 revolutions per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece;Sequence of steps C comprising:a) providing a work piece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having a back side and a front side, said front side facing said work piece and having a flat plateau which is continuous around the perimeter of said front side of said platen and is elevated with respect to a central area on said front side, thereby forming an annular region,c) providing a sheet of abrasive material on said flat plateau, said sheet of abrasive material having a front surface with an abrasive face and a back surface, with said abrasive face facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said flat surface of said plateau, ande) rotating said platen at at least 500 revolutions per minute and contacting said abrasive material and said work piece to remove material from said work piece;Sequence of steps D comprisinga) providing a workpiece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having I) a back surface and ii) a flat surface and providing a workpiece which can be adjusted to a position parallel to said platen by rotation about a pivot joint of a workpiece holder supporting said workpiece, said flat surface of said platen having openings therein through which air may flow, and said back surface having a pivoting joint with a shaft which rotates said platen,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) reducing gaseous pressure between said back side of said abrasive sheet and said flat surface of said platen to secure said sheet of abrasive material to said flat surface of said platen, ande) rotating said platen at a rotational speed of at least 500 revolutions per minute by rotating said shaft, andf) contacting said abrasive face and said at least one surface to be lapped on said workpiece, and allowing said workpiece holder to pivot around said pivot joint so that said abrasive sheet and said at least one surface to be lapped become more parallel towards each other.Sequence of steps E comprising:a) providing a work piece with two surfaces to be lapped,b) providing two rotatable platens, each rotatable platen having I) a back surface and ii) a front surface,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said front surface of each of said two rotatable platens with the abrasive faces of each said sheet facing the other sheet,d) placing said work piece with two surfaces to be lapped between said two rotatable platens, so that each abrasive face faces only one of said two surfaces to be lapped,e) rotating said two platens at a rotational speed of at least 500 revolutions per minute,f) contacting each of said abrasive faces with said only one of said two surfaces to be lapped, andg) lapping said two surfaces of said work piece simultaneously.Sequence of steps F comprising:a) providing a work piece having two surfaces to be lapped to be lapped, having at least one surface to be lapped,b) providing two rotatable platens, each rotatable platen having a back side and a front side, said front side facing a surface to be lapped on said work piece and each of said two platens having a flat plateau which is continuous around the perimeter of said front side of each of said platens and is elevated with respect to a central area on said front side, thereby forming an annular region,c) providing a sheet of abrasive material on said flat plateau on each of said two platens, said sheet of abrasive material having a front surface with an abrasive face and a back surface, with each said abrasive face facing only one of said two surfaces on said work piece to be lapped,d) securing said sheet of abrasive material to each said flat plateau, ande) rotating said platen at at least 500 revolutions per minute and contacting said abrasive material on said two platens and said two surfaces to be lapped on said work piece simultaneously to remove material from said work piece;Sequence of steps G comprising:a) providing a work piece to be lapped, having at least one surface to be lapped which can be adjusted to a position parallel to said at least one surface of a rotating platen,b) providing a rotating platen having I) a back surface and ii) a front surface with a periphery, said front surface of said rotating platen having a raised edge symmetrically disposed about said periphery,c) providing a sheet of abrasive material having an abrasive face and a back side onto said raised edge to provide a symmetrical distribution of abrasive material on said rotating platen, said back side being on said front surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said front surface of said rotating platen, ande) rotating said rotating platen at a rotational speed of at least 500 revolutions per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece; andSequence of steps H comprising:a) providing a work piece to be lapped, having at least one surface to be lapped which can be adjusted to a position parallel to said at least one surface of a rotating platen,b) providing a rotating platen having I) a back surface, ii) a front surface, and a periphery,c) providing a sheet of abrasive material having an abrasive face and a back side onto said rotating platen, with the abrasive face of said sheet facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said front surface of said rotating platen,e) rotating said rotating platen at a rotational speed of at least 500 revolutions per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece,g) providing a first amount of liquid to assist lapping to said abrasive surface physically in front of an area where work piece contacts said abrasive face,h) providing a second amount of liquid to assist in washing solid material from said abrasive surface physically after said area, andI) directing air against said abrasive surface physically after providing said first amount of liquid to assist in removing said first and second amounts of liquid from said abrasive surface.
- The process of claim 3 wherein said sheet of abrasive material comprises a circular or annular sheet of material which is sufficiently non-porous as to be secured to a surface by reduced gas pressure with a differential between a front side of said sheet and a back side of said sheet of 600 mm Hg.
- A lapper system according to claim 2 comprising:a) a shaft which is connected to a rotatable platen having on a front surface of said platen vents for air, said rotatable platen having a back side to which said shaft is connected and a flat front side on said rotatable platen to which can be secured an abrasive sheet by reduced air pressure conveyed through said vents;b) a frame having a total weight of at least 200 kg supporting a work piece holder and said shaft connected to a rotatable platen;c) a work piece holder which is movable on said frame;d) said work piece holder is attached to a movable element on said frame which is capable of moving along said frame in a direction towards and away from said platen to perform lapping of a work piece held on said work piece holder;e) said work piece holder having control element thereon which allow for independent movement and alignment of said work piece holder along three perpendicular axes so that a work piece on said work piece holder can be adjusted and oriented towards parallelity with said rotatable platen so that a work piece can be lapped; andf) said control elements having at least 100 settings per rotation, each setting moving said shaft along one of said three axes by a dimension less than 0.005 mm, andg) wherein said shaft which is connected to said rotatable platen has a back side to which said shaft is connected and a front side on said rotatable platen to which can be secured an abrasive sheet, said rotatable platen having i) a back surface, ii) a front surface, and iii) a raised edge forming an abrading plateau on said front surface of said rotatable platen, with an abrasive sheet secured to said raised edge to form an annular abrading area.
- A lapper platen system comprising:a) a rotatable platen having I) a back surface and ii) a front surface, wherein said front surface of said rotating platen facing a work piece and said front surface has a flat plateau which is continuous around a perimeter of said front side of said platen and is elevated with respect to a central area on said front surface,b) said front surface also having vents for air,c) said platen having a back side to which a shaft is connected and a front side on said platen to which is secured an abrasive sheet by reduced air pressure conveyed through said vents,d) said back side also being pivotally connected to a rotating joint which is in turn connected to said shaft which rotates said workpiece;e) a frame having a total weight of at least 200 kg supporting a work piece holder and said shaft connected to a rotatable platen;f) a work piece holder which is movable on said frame;g) said work piece holder is attached to a movable element on said frame which is capable of moving along said frame in a direction towards and away from said platen to perform lapping of a work piece held on said work piece holder;h) said work piece holder having control element thereon which allow for independent movement and alignment of said work piece holder along three perpendicular axes so that a work piece on said work piece holder can be adjusted and oriented towards parallelity with said platen so that a work piece can be lapped;i) said control elements having at least 50 settings per rotation, each setting moving said shaft along one of said three axes by a dimension less than 0.005 mm;j) a first liquid supply means upstream from said work piece holder with respect to a direction of rotation of said platen;k) a second liquid supply means downstream from said work piece holder with respect to a direction of rotation of said platen; andl) an air blowing means located downstream of said first liquid supply means.
- A process according to claims 1, 3 or 4 including a pivoting workpiece system comprising:a) a shaft which is connected to a platen, said platen having a back side to which said shaft is connected and a front side on said platen to which can be secured an abrasive sheet;b) a pivoting joint comprising a gimbal joint, andc) said shaft being able to pivot about said pivoting joint relative to said platen.
- The process of claims 1, 3 or 4 wherein said sheet of abrasive material comprises a surface having abrasive particles with an average diameter of from 0.1 to 100 micrometers and said platen is rotated at a speed of at least 2,000 rpm.
- The process of claims 1, 3 or 4 wherein during rotation of said platen a liquid is placed between said sheet and said work piece, said liquid forms a boundary layer as it moves from an inner portion of said sheet to an outer portion of said sheet, said sheet comprising abrasive particles which protrude by an average height on said surface of said sheet, and said boundary layer is less than 50% of the average height of abrasive particles protruding from said sheet, or wherein a liquid is placed between said sheet and said work piece, said liquid forms a boundary layer as it moves from an inner portion of said sheet to an outer portion of said sheet, said sheet has abrasive particles which protrude by an average height on said surface of said sheet, and said boundary layer thickness is less than ±50% the average height of abrasive particles protruding from said sheet.
- The lapper platen system of claims 2 or 6 wherein a protective structure is present on said lapper system, said protective structure being selected from the group consisting of a) an upright frame extends around and above an uppermost surface of said rotatable platen to intercept material projected outwardly from said platen and b) an upright frame extends around and above an uppermost surface of said rotatable platen to intercept material projected outwardly from said platen, with an extension on the top of said upright frame which is angled or curved towards a central area over said platen.
- A process according to claims 1, 3 or 4 for lapping a surface comprising:a) providing a work piece to be lapped, said work piece having a first surface and a second surface which are parallel to each other, and at least one of said first and second surface is a surface to be lapped,b) providing a first and second rotating platen, each of said first and rotating platen having I) a back surface and ii) a flat front surface which can be adjusted so that said first platen is facing and parallel to said first surface of said work piece and said second platen is facing and parallel to said second surface of said work piece,c) providing a sheet of abrasive material on at least said flat surface of said first platen with an abrasive face of said sheet facing said first surface of said work piece which is said at least one surface to be lapped,d) securing said sheet of abrasive material to said flat surface of said first platen, ande) putting a liquid between both I) said first platen and said first surface of said work piece and ii) said second platen and said second surface of said work piece,f) rotating both of said platen at at least 500 revolutions per minute and contacting said abrasive material and said work piece,g) wherein contact pressure between said both I) said first platen and said first surface of said work piece and ii) said second platen and said second surface of said work piece are sufficiently similar that said work piece does not flex more than 0.1mm at its exterior regions between said two platens.
- A process for initiating contact between a workpiece to be ground and an abrasive surface comprising abrasive sheeting on a rotatable platen, said process comprising:a) supporting a workpiece on a workpiece holder,b) supporting said workpiece holder on a linearly movable support,c) advancing the workpiece into contact with an abrasive surface comprising abrasive sheeting on a rotatable platen,
said process being further characterized byd) determining a position at least approximating the position of contact between a surface of said workpiece to be ground and said abrasive surface,e) removing said workpiece from said position approximating the position of contact,f) advancing the workpiece towards said abrasive surface while said rotatable platen is rotating, andg) controlling forces which advance said workpiece towards said abrasive surface and into contact with said abrasive surface. - The process of claim 12 wherein mechanical alignment of said workpiece holder is effected to promote parallelity between a surface of said workpiece to be ground and said abrasive surface after step c) but before step e).
- The process of claim 13 wherein said controlling forces provides a contact force between 0.1 and 10 pounds per square inch between a surface of said workpiece to be ground and said rotating platen during lapping of said workpiece while said abrasive sheet is moving with at least 1,500 surface feet per minute while in contact with said workpiece.
- The process of claim 14 wherein said workpiece holder is supported by a pivot joint and said workpiece holder pivots upon contact between said workpiece and said abrasive surface to hold a surface of said workpiece to be lapped in a more parallel orientation with said abrasive surface.
- A process for lapping a surface according to claim 15 also comprising at least one of the following sequence of steps:Sequence of steps A comprising:a) providing a work piece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having I) a back surface and ii) a flat surface providing a rotating platen having I) a back surface and ii) a flat surface and providing a workpiece which can be adjusted to a position parallel to said platen, said flat surface of said platen having openings therein through which air may flow,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) reducing gaseous pressure between said back side of said abrasive sheet and said flat surface of said platen to secure said sheet of abrasive material to said flat surface of said platen,e) rotating said platen at a rotational speed of at least 500 revolutions per minute and a surface speed at an outermost edge of said platen of at least 1500 surface feet per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece;Sequence of steps B comprising:a) providing a work piece to be lapped, having at least one surface to be lapped, which can be adjusted to a position parallel to said at least one surface of b) whereb) is a rotating platen having I) a back surface and ii) a flat surface said flat surface of said platen having openings therein through which air may flow,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) wherein said sheet has an outer edge and an inner edge defining an annular distribution of abrasive, said inner edge having a diameter which is greater than one-third the diameter of said outer edge,rotating said platen at a rotational speed of at least 500 revolutions per minute, andcontacting said abrasive face and said at least one surface to be lapped on said work piece;Sequence of steps C comprising:a) providing a work piece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having a back side and a front side, said front side facing said work piece and having a flat plateau which is continuous around the perimeter of said front side of said platen and is elevated with respect to a central area on said front side, thereby forming an annular region,c) providing a sheet of abrasive material on said flat plateau, said sheet of abrasive material having a front surface with an abrasive face and a back surface, with said abrasive face facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said flat surface of said plateau, ande) rotating said platen at at least 500 revolutions per minute and contacting said abrasive material and said work piece to remove material from said work piece;Sequence of steps D comprisinga) providing a workpiece to be lapped, having at least one surface to be lapped,b) providing a rotating platen having I) a back surface and ii) a flat surface providing a rotating platen having I) a back surface and ii) a flat surface and providing a workpiece which can be adjusted to a position parallel to said platen by rotation about a pivot joint of a workpiece holder supporting said workpiece, said flat surface of said platen having openings therein through which air may flow, and said back surface having a pivoting joint with a shaft which rotates said platen,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said flat surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) reducing gaseous pressure between said back side of said abrasive sheet and said flat surface of said platen to secure said sheet of abrasive material to said flat surface of said platen, androtating said platen at a rotational speed of at least 500 revolutions per minute by rotating said shaft, andcontacting said abrasive face and said at least one surface to be lapped on said workpiece, and allowing said workpiece holder to pivot around said pivot joint so that said abrasive sheet and said at least one surface to be lapped become more parallel towards each other.Sequence of steps E comprising:a) providing a work piece with two surfaces to be lapped,b) providing two rotatable platens, each rotatable platen having I) a back surface and ii) a front surface,c) providing a sheet of abrasive material having an abrasive face and a back side, said back side being on said front surface of each of said two rotatable platens with the abrasive faces of each said sheet facing the other sheet,d) placing said work piece with two surfaces to be lapped between said two rotatable platens, so that each abrasive face faces only one of said two surfaces to be lapped,e) rotating said two platens at a rotational speed of at least 500 revolutions per minute,f) contacting each of said abrasive faces with said only one of said two surfaces to be lapped, andg) lapping said two surfaces of said work piece simultaneously.Sequence of steps F comprising:a) providing a work piece having two surfaces to be lapped to be lapped, having at least one surface to be lapped,b) providing two rotatable platens, each rotatable platen having a back side and a front side, said front side facing a surface to be lapped on said work piece and each of said two platens having a flat plateau which is continuous around the perimeter of said front side of each of said platens and is elevated with respect to a central area on said front side, thereby forming an annular region,c) providing a sheet of abrasive material on said flat plateau on each of said two platens, said sheet of abrasive material having a front surface with an abrasive face and a back surface, with each said abrasive face facing only one of said two surfaces on said work piece to be lapped,d) securing said sheet of abrasive material to each said flat plateau, ande) rotating said platen at at least 500 revolutions per minute and contacting said abrasive material on said two platens and said two surfaces to be lapped on said work piece simultaneously to remove material from said work piece;Sequence of steps G comprising:a) providing a work piece to be lapped, having at least one surface to be lapped which can be adjusted to a position parallel to said at least one surface of a rotating platen,b) providing a rotating platen having I) a back surface and ii) a front surface with a periphery, said front surface of said rotating platen having a raised edge symmetrically disposed about said periphery,c) providing a sheet of abrasive material having an abrasive face and a back side onto said raised edge to provide a symmetrical distribution of abrasive material on said rotating platen, said back side being on said front surface of said platen with the abrasive face of said sheet facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said front surface of said rotating platen, ande) rotating said rotating platen at a rotational speed of at least 500 revolutions per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece; andSequence of steps H comprising:a) providing a work piece to be lapped, having at least one surface to be lapped which can be adjusted to a position parallel to said at least one surface of a rotating platen,b) providing a rotating platen having I) a back surface, ii) a front surface, and a periphery,c) providing a sheet of abrasive material having an abrasive face and a back side onto said rotating platen, with the abrasive face of said sheet facing said at least one surface to be lapped,d) securing said sheet of abrasive material to said front surface of said rotating platen,e) rotating said rotating platen at a rotational speed of at least 500 revolutions per minute, andf) contacting said abrasive face and said at least one surface to be lapped on said work piece,g) providing a first amount of liquid to assist lapping to said abrasive surface physically in front of an area where work piece contacts said abrasive face,h) providing a second amount of liquid to assist in washing solid material from said abrasive surface physically after said area, andI) directing air against said abrasive surface physically after providing said first amount of liquid to assist in removing said first and second amounts of liquid from said abrasive surface.
- The process of claims 1, 3, 12, 13, 14, 15 or 16 wherein contacting said abrasive face and said at least one surface to be lapped on said work piece is performed by a combination of workpiece holder supporting devices and speed dampening devices, said speed dampening devices acting so that the momentum of the workpiece is moderated when it first contacts a rotating platen with an abrasive sheet thereon.
- The process of claims 1, 3, 12, 13, 14, 15 or 16 wherein said workpiece is rotated at a rate of at least 2 revolutions per minute while said workpiece is in contact with abrasive sheet rotating at at least 500 revolutions per minute.
- The process of claims 1, 3, 12, 13, 14, 15 or 16 wherein said workpiece has an outside circumference on a surface facing said rotatable platen, and a sacrificial material of a composition other than said workpiece is located on at least a portion of said circumference.
- The process of claims 1, 3, 12, 13, 14, 15 or 16 wherein a vibration damping element is connected to said workpiece holder to reduce vibration during lapping by said abrasive sheet.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US81274897A | 1997-03-06 | 1997-03-06 | |
| US812748 | 1997-03-06 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| EP0868976A2 true EP0868976A2 (en) | 1998-10-07 |
| EP0868976A3 EP0868976A3 (en) | 2000-08-23 |
Family
ID=25210509
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| EP98301690A Withdrawn EP0868976A3 (en) | 1997-03-06 | 1998-03-06 | Lapping apparatus and method for high speed lapping with a rotatable abrasive platen |
Country Status (1)
| Country | Link |
|---|---|
| EP (1) | EP0868976A3 (en) |
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| WO1999050022A2 (en) * | 1998-03-31 | 1999-10-07 | Applied Materials, Inc. | Chemical mechanical polishing conditioner |
| US7367872B2 (en) | 2003-04-08 | 2008-05-06 | Applied Materials, Inc. | Conditioner disk for use in chemical mechanical polishing |
| WO2009137685A2 (en) * | 2008-05-07 | 2009-11-12 | Zygo Corporation | Configuring of lapping and polishing machines |
| US9352442B2 (en) | 2013-07-02 | 2016-05-31 | Seagate Technology Llc | Lapping carrier |
| CN106475914A (en) * | 2016-10-31 | 2017-03-08 | 南京航空航天大学 | A kind of titanium alloy surface amino film plastics abrasive air Jet Polishing method |
| WO2018114488A1 (en) * | 2016-12-23 | 2018-06-28 | Scanlab Gmbh | Scanning mirror having balancing body, and method for balancing a scanning mirror |
| CN109304817A (en) * | 2017-07-26 | 2019-02-05 | 株式会社迪思科 | Tool-holding device |
| CN111403885A (en) * | 2020-03-31 | 2020-07-10 | 重庆思睿创瓷电科技有限公司 | Alignment and pressing process and tool for filter |
| CN112197754A (en) * | 2020-09-17 | 2021-01-08 | 倪可 | Tripod for architectural survey |
| CN112911850A (en) * | 2020-12-30 | 2021-06-04 | 西安熊福芳电子科技有限公司 | Intelligent monitoring equipment control operation panel convenient to remove |
| CN112935963A (en) * | 2021-02-05 | 2021-06-11 | 许昌学院 | Decorative board grinding device for furniture design |
| CN113267425A (en) * | 2021-06-25 | 2021-08-17 | 郑州磨料磨具磨削研究所有限公司 | Device and method for testing hardness of semi-solid extrusion abrasive material flow |
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| CN115847225A (en) * | 2023-03-02 | 2023-03-28 | 成都裕鸢航空智能制造股份有限公司 | Metal plate surface grinding and polishing device and method |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| WO1999050022A2 (en) * | 1998-03-31 | 1999-10-07 | Applied Materials, Inc. | Chemical mechanical polishing conditioner |
| WO1999050022A3 (en) * | 1998-03-31 | 2000-03-09 | Applied Materials Inc | Chemical mechanical polishing conditioner |
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| WO2009137685A2 (en) * | 2008-05-07 | 2009-11-12 | Zygo Corporation | Configuring of lapping and polishing machines |
| WO2009137685A3 (en) * | 2008-05-07 | 2010-02-25 | Zygo Corporation | Configuring of lapping and polishing machines |
| US8123593B2 (en) | 2008-05-07 | 2012-02-28 | Zygo Corporation | Configuring of lapping and polishing machines |
| US9352442B2 (en) | 2013-07-02 | 2016-05-31 | Seagate Technology Llc | Lapping carrier |
| CN106475914A (en) * | 2016-10-31 | 2017-03-08 | 南京航空航天大学 | A kind of titanium alloy surface amino film plastics abrasive air Jet Polishing method |
| WO2018114488A1 (en) * | 2016-12-23 | 2018-06-28 | Scanlab Gmbh | Scanning mirror having balancing body, and method for balancing a scanning mirror |
| US11402605B2 (en) | 2016-12-23 | 2022-08-02 | Scanlab Gmbh | Scanning mirror having balancing body, and method for balancing a scanning mirror |
| CN109304817A (en) * | 2017-07-26 | 2019-02-05 | 株式会社迪思科 | Tool-holding device |
| CN111403885A (en) * | 2020-03-31 | 2020-07-10 | 重庆思睿创瓷电科技有限公司 | Alignment and pressing process and tool for filter |
| CN112197754A (en) * | 2020-09-17 | 2021-01-08 | 倪可 | Tripod for architectural survey |
| CN112197754B (en) * | 2020-09-17 | 2022-03-15 | 浙江海致建设有限公司 | Tripod for architectural survey |
| CN112911850A (en) * | 2020-12-30 | 2021-06-04 | 西安熊福芳电子科技有限公司 | Intelligent monitoring equipment control operation panel convenient to remove |
| CN112935963A (en) * | 2021-02-05 | 2021-06-11 | 许昌学院 | Decorative board grinding device for furniture design |
| CN112935963B (en) * | 2021-02-05 | 2022-12-02 | 许昌学院 | Decorative board grinding device for furniture design |
| CN113267425A (en) * | 2021-06-25 | 2021-08-17 | 郑州磨料磨具磨削研究所有限公司 | Device and method for testing hardness of semi-solid extrusion abrasive material flow |
| CN114952554A (en) * | 2022-06-20 | 2022-08-30 | 武汉船用机械有限责任公司 | Automatic grinding device |
| CN115847225A (en) * | 2023-03-02 | 2023-03-28 | 成都裕鸢航空智能制造股份有限公司 | Metal plate surface grinding and polishing device and method |
| CN115847225B (en) * | 2023-03-02 | 2023-04-28 | 成都裕鸢航空智能制造股份有限公司 | Metal plate surface polishing device and polishing method |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0868976A3 (en) | 2000-08-23 |
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