WO1999021682A1

WO1999021682A1 - A polishing apparatus for forming aspheric surfaces

Info

Publication number: WO1999021682A1
Application number: PCT/GB1998/003182
Authority: WO
Inventors: Mark Craig Gerchman; John Raymond Hoogstrate, Jr.; Thomas Tyler Lowell; Mark Mcninch Walter, Iii; Qiang Kong; Laszlo Tamas; Robert Zimmermann
Original assignee: Precitech Inc.
Priority date: 1997-10-24
Filing date: 1998-10-23
Publication date: 1999-05-06
Also published as: EP1024925A1

Abstract

The apparatus has a frame (1) and a work-holding spindle (4) mounted to said frame. A drive assembly for moving a polishing spindle mounting element with respect to said frame is secured to the frame. A polishing spindle (57) is mounted on said polishing spindle mounting element for rotation about a spindle rotation axis. A drive assembly controller controls the drive assembly to move a polishing head mounted to the polishing spindle assembly along a theoretical profile while maintaining a contact force between said workpiece and said polishing head and controlling the movement of the polishing head using a predetermined velocity profile so that the polishing head removes material from the workpiece by abrasion in accordance with a predetermined material removal profile.

Description

A POLISHING APPARATUS FOR FORMING ASPHERIC SURFACES

The present invention relates to the high- precision polishing of aspheric surfaces. More specifically, the invention relates to the polishing of aspheric surfaces using a computer controlled machine that may be compensated using measurement data.

Optical components frequently include elements such as lenses and/or mirrors having convex or concave surfaces that define the reflection or refraction of light by the optical component. The optical surfaces are often shaped as surfaces of rotation. For example, a spherical surface has the shape of a sector of a circle that has been rotated about the central axis of the element .

In many applications, the ideal optical surface is not spherical; it is aspheric. For example, many optical applications may require parabaloid, hyperboloid or other aspheric shapes. While in certain optical components an element having a spherical surface may be used to approximate an aspheric optical element, such is not the case where extremely high precision in form is required. Some optical applications require elements having optical surfaces with form tolerances of less than one quarter wave length of light (approximately 5 millionths of an inch), and in extreme cases may require tolerances of 1/32 of a wave length (approximately 0.7 millionths of an inch) or smaller.

In general, aspheric optical surfaces are formed by first grinding the surface to the approximate shape required, using successively finer grinding compounds during successive stages of the grinding operation. The surface is then polished to its final aspheric form. Often, the shape approximating the final form is a spherical shape, which may be formed relatively inexpensively. Because a spherical surface has a constant radius of curvature throughout, the surface may be polished using a large lap that makes complete contact with the optical surface during the polishing operation. As the workpiece and the lap are repeatedly moved with respect to each other, both surfaces must take the shape of a sphere.

In one known method of polishing a spherical optical surface, the workpiece is reciprocated against a lap or form that is typically made from pitch, a material derived from pine tar. For example, a convex spherical optical surface may be polished by first forming a concave pitch lap using the convex ground surface of the workpiece as a mold. The workpiece is then lapped by rubbing the optical surface on the pitch lap using a polishing compound therebetween. The surface of the workpiece is precisely conformed to the surface of the lap, with both surfaces forming spherical shapes. In contrast to a spherical surface, an aspheric surface has a radius of curvature that changes as a function of the position on the surface. Thus, the polishing tool must contact the optical surface in a relatively small area in order to permit variation of the radius of curvature over the optical surface. To form a precise aspherical optical surface, a workpiece is typically polished by hand with the workpiece mounted on a rotary spindle. A technician applies pressure to selected regions of the surface using a felt pad or other conformable material. The felt pad and the workpiece are wetted with a slurry containing an extremely fine optical polishing media such as optical rouge. As the technician applies pressure with the polishing pad, material is removed from annular regions of the rotating workpiece at an extremely low rate, while the surface finish of the workpiece is maintained.

In order to determine exactly where and how much material must be removed in order to attain the desired aspheric shape, the workpiece must be tested or measured. In one technique used for lenses and mirrors having a single focal point, a point of light is projected onto the optical surface, and a reflected or refracted image is observed as a knife edge is passed through the focal point. An experienced technician observing the resulting pattern is able to determine where and how much material must be removed in order to arrive at the desired shape. Typically several iterations of polishing and measuring are necessary before the form of the optical surface is within tolerance. Manual techniques for polishing aspheric lenses depend heavily on the experience and skill of the technician in correctly interpreting the test results and in subsequently applying the correct pressure to correct regions of the optical surface for the correct duration of time. Errors in that procedure result in the shape diverging from the desired shape instead of converging during subsequent iterations of the measure/polish sequence. The manual nature of the process necessitates investing a large amount of time in each optical element, and imposes a practical limit on the precision of the optical surfaces .

One aspect of the present invention provides a method for polishing an existing profile on the surface of a workpiece by determining the difference between the existing and predetermined profiles and controlling a polishing machine to produce the desired profile in accordance with the determined difference. In an embodiment, a method according to this aspect of the invention includes the steps of measuring the existing profile of the workpiece and then comparing that measured profile with the predetermined profile to be formed on the workpiece. A material removal profile is calculated by determining the difference between the existing profile and the target predetermined profile. In an embodiment, the workpiece surface is then contacted with the polishing surface of a polishing head _,at a contact point as the polishing head is rotated. A contact force is applied to the polishing surface and the workpiece surface. The rotating polishing head is then translated so as to traverse the contact point to form a path duplicating the predetermined profile superimposed on the workpiece. The contact point is traversed along the profile at a traverse velocity. The polishing head is translated and rotated so as to maintain the polishing surface substantially tangent to the predetermined profile while maintaining a contact force between the polishing surface and the workpiece surface. The traverse velocity is varied during the translating and rotating step so as to remove material according to the material removal profile.

According to a further aspect of the invention, an apparatus is provided for polishing a surface of a workpiece having means for removing material according to a predetermined material removal profile. In an embodiment, the apparatus includes a frame and a work holding element mounted to the frame for mounting the workpiece. A drive assembly is secured to the frame and has a polishing spindle mounting element. The drive assembly comprises at least one drive for moving the polishing spindle mounting element with respect to the frame.

In an embodiment, a polishing spindle assembly is mounted on the polishing spindle mounting element for rotation about a spindle rotation axis . The spindle assembly includes the polishing head for contacting and abrading the surface of the workpiece near a theoretical contact point between the polishing head and the workpiece. In this embodiment, the drive assembly further comprises means for maintaining a contact force between the workpiece and the polishing head. A drive assembly control means is provided for controlling the drive assembly to produce movement of the polishing head along a theoretical profile while maintaining the contact force between the workpiece and the polishing head. The drive assembly control means controls the movement at a predetermined velocity profile so that the polishing head moves material at the predetermined material removal profile.

In another aspect of the invention, a polishing head is provided for removing material in conjunction with an abrasive slurry in a polishing machine. In an embodiment the polishing head includes a felt or like matrix having a network of fibers, and a pitch or like material binder substantially impregnating a network of fibers .

In yet a further aspect of the invention, an apparatus is provided for polishing a profile on an optical surface of a workpiece. In an embodiment the apparatus includes a frame and a work holding spindle rotatably mounted to the frame. The work holding spindle has a means for mounting the workpiece. A first driveable linear slide assembly is mounted to the frame and has a first slide moveable relative to the frame in a first direction. A second driveable .linear slide assembly is mounted to the first slide and has a second slide moveable relative to the first slide in the second direction. A polishing force slide assembly has a fixed element mounted to the second slide, and a moveable element slideably mounted to the fixed element for linear movement in a substantially vertical direction. The polishing force slide assembly also includes a spring interconnecting the fixed and moveable elements for urging the moveable element upward with respect to the fixed element with a spring force proportional to a relative position between the fixed and the moveable elements. A driveable rotary stage assembly is mounted to the moveable element of the polishing force slide assembly. The rotary stage assembly has a rotary stage rotatable about a horizontal rotary stage axis . A polishing spindle assembly is rotatably mounted on the rotary stage for rotation about a spindle rotation axis. The spindle assembly includes a polishing head for contacting the optical surface of the workpiece. A contact force between the polishing head and the workpiece may be reduced or increased by changing the spring force by driving the first and second slide assemblies so as to change the relative position between the fixed and moveable elements of the polishing force slide assembly.

These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings . BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aspheric polishing machine according to an embodiment of the invention;

FIG. 2 is a schematic diagram of the slide arrangement of the aspheric polishing machine shown in FIG. 1;

FIG. 3 is a perspective view of an aspheric polishing machine according to another embodiment of the invention;

FIG. 4 is a schematic diagram of the slide arrangement of the aspheric polishing machine shown in

FIG. 3;

FIG. 5 is a cross-sectional view of a polishing tool according to one embodiment of the invention;

FIG. 6 is a schematic, cross-sectional view of a polishing tool in contact with an aspheric workpiece according to a method embodying the invention; and

FIG. 7 is a block diagram showing a method for polishing an aspheric optical element according to an embodiment of the invention. FIGS. 8 to 11 show aspects of aspheric polishing machines embodying the invention.

Figure 1 shows an aspheric polishing machine

10 according to one embodiment of the invention. The machine includes a frame 1 supporting a cabinet 11 upon which is a tabletop 2. Cabinet 11 houses wiring and small components that support the operation of the machine. The tabletop 2 also has shelf 21_, for support and protection of a computer 3. The computer is a commercially-available desk top computer, and is provided in the preferred embodiment with appropriate data storage and communication hardware, and an axis control board for controlling the axes of the machine.

A work-holding spindle 4 is mounted beneath the tabletop 2 , with the upper end of the spindle protruding through a hole in the tabletop provided therefor. The spindle is mounted for rotation about a vertical axis in precision bearings (not shown) rigidly fixed to the tabletop. The spindle is driven by a spindle motor (not shown) mounted below the tabletop. The upper end of the working-holding spindle 4 is adapted to hold the workpiece 44. A precision mechanism may be provided on the spindle for securing the workpiece and/or a blocked workpiece (FIG. 6). As used herein, the term "blocked workpiece" means a workpiece that has been temporarily attached to a work holding means for securing in the polishing machine. For example, workpiece 44 (FIG. 6) is attached to work holding means 45 using wax (not shown) . The workholding spindle 4 is surrounded by a tray 41 (FIG. 1) or sink for containing the polishing slurry used during the polishing operation. Polishing slurry is stored in a slurry reservoir 42, and is pumped by a slurry pump 43 through a tube (not shown) to a nozzle (not shown) or other outlet near the workpiece for delivery of the slurry onto the workpiece 44 and polishing tool 56. The slurry comprises a suspension of a polishing compound in an appropriate fluid. A bubbler or other mixing apparatus (not shown) is provided in the reservoir to constantly agitate the slurry to keep the polishing compound in suspension.

Also mounted on the tabletop 2 is a slide support 22, which supports slide assembly 5. The slide assembly 5 includes the necessary drive motors and slides for translating and rotating the polishing tool 56, according to instructions received from the computer. In this embodiment, the slide assembly includes two translational axes (X, Z) and one rotational axis (B). Instructions are sent from the computer to drive motors for each of those axes. The slide assembly also includes a free-falling polishing force control slide, which is not driven.

An X-axis motor/slide 51 is mounted directly to the support 22 and permits translation of the polishing tool 56 across the workpiece 44 (FIGS. 1, 2). In this embodiment, the motion of the tool 56 is in a left-right or right-left direction, as the workpiece is viewed from the front of the machine 10. Thus, the X- axis slide extends left-right as it is viewed from the front of the machine.

The X-axis slide is preferably of the precision bearing type. While, the method described more fully below reduces the criticality of straightness and positioning accuracy of the axes, it remains important that the axes provide smooth and continuous motion.

The drive motors are preferably servomotors capable of continuously positioning the slides. The method described below utilizes extremely slow, smooth speeds in order to selectively remove material during polishing. The inventors have found that servomotors are capable of providing such movement . Stepper motors , which may alternately be used to drive the axes of the machine, move the axes in finite discrete steps such as 30 micron steps. Such steps may cause the axes to stop and start movement when driven at extremely slow speeds. Such stop/start movement of the axes may affect the smoothness of the resulting workpiece surface. Servomotors are therefore preferred over stepper motors for driving the axes of the machine.

Mounted directly to the X-axis slide is a Z- axis motor/slide 52 for moving the polishing assembly in an upward and a downward direction. The Z-axis motor/slide is used to lower the polishing tool 56 onto the workpiece at the beginning of a cycle, to follow the contour of the described aspheric surface during polishing, and to raise the tool at the end of the cycle. The Z-axis motor/slide may, as described below, also be used in adjusting the force between the tool and the workpiece. As with the X-axis, it is preferred to utilize a servomotor drive and precision linear bearings for the Z-axis slide.

Preferably, a polishing force control slide 53 is mounted to the Z-axis slide. The force control slide, in conjunction with the Z-axis, controls the amount of force between the polishing tool 56 and the workpiece 44 by regulating the percentage of the free-floating weight of the polishing spindle 55 and the B-axis motor/rotary stage 54 that is applied between the polishing tool 56 and the workpiece 44.

As best shown in FIG. 2, the polishing force control slide 53 comprises an inboard element 80, an outboard element 81 and a compression spring 82 biasing the inboard and outboard elements away from each other in a vertical direction. The term "outboard" as used herein with respect to the drive assembly 5 means in a direction away from the slide support 22, and toward the spindle 57. The inboard and outboard elements 80, 81 are also connected by a linear, precision bearing slide (not shown) permitting only linear vertical motion between the two elements .

To use the polishing force control slide 53, the polishing tool 56 is brought into contact with the workpiece 44 by advancing the Z-axis motor/slide 52 in order to lower that portion of the drive assembly 5 outboard of the Z-drive. Before the polishing tool 56 contacts the workpiece 44, the total weight of the outboard element 81 of the control slide 53, the B-axis motor/rotary stage 54, the polishing spindle assembly 55 and the polishing tool 56 (approximately 10-15 pounds) is supported by the compression spring 82. Once contact is made between the polishing tool 56 and the workpiece 44, a portion of that total weight is applied between the workpiece 44 and the polishing tool 56, while the remaining portion of that weight remains supported by the compression spring 82. The position of the Z-axis slide determines the portion of the total weight that is applied to the tool, according to spring constant of the compression spring 82. Thus, polishing force between the polishing tool 56 and the workpiece 44 may be automatically adjusted during the polishing operation. The polishing force control slide 53 may also include an adjustment screw 83 for adjusting the zero position of the slide with respect to the spring.

Further, the slide may have a load cell 84 in series with the compression spring for providing feedback for control of the portion of the weight force taken up by the spring .

Outboard of the polishing force control slide

53 is mounted a B-axis motor/rotary stage 54. The rotary stage permits rotation of the spindle in the X-Z plane, about a horizontal axis extending in a Y-direction (FIG.

1 ) . As with the other axes , the motor driving the rotary axis is preferably a servomotor.

Preferably, the B-axis motor/rotary stage 54 is mounted outboard from the polishing force control slide 53. In that way, the polishing force control slide remains vertical regardless of the position of the rotary stage, and the weight of the outboard components of the slide assembly always acts along the line of force of the compression spring 82. Thus, the weight of the outboard components may be directly compensated by using the Z- axis 52 to compress or decompress the compression spring

82.

The force F_τ exerted by the polishing tool on the workpiece in a direction normal to the surface of the workpiece deviates slightly from the vertical spring- compensated weight force F_w. Specifically, the tool force F_τ is a function of the angle θ (FIG. 6) of the B- axis rotary stage from vertical as follows: F_τ = F„/cosθ

Such deviation in polishing force during a polishing cycle may be compensated by adjusting the compression of the spring through the Z-axis motor/slide, or by adjusting the transverse speed to increase or decrease polishing time in a given region.

Outboard of the B-axis motor/rotary stage 54 is a polishing spindle assembly 55 including a polishing spindle 57 on which the polishing tool 56 is mounted.

The polishing spindle 57 is mounted in precision rotary bearings for rotation about a spindle axis 58.

Contact between the polishing tool 56 and the workpiece 44 is maintained normal to the curved surface of the workpiece as the polishing tool is traversed across the workpiece by the X-axis motor/slide 51. To maintain such normal contact, the polishing spindle 57 is rotated in the X-Z plane by the B-axis motor/rotary stage 54 to align the spindle axis 58 parallel to a vector 59 (FIG. 6) normal to the workpiece surface at the center of the active polishing region. In sum, the three driven axes, X, Z and B, are controlled to maintain a predetermined contact force between the polishing tool 56 and the workpiece 44, while maintaining normal contact between the polishing tool and the workpiece. Additionally, as described below, the axes are driven at predetermined speeds such that the polishing tool 56 contacts certain areas of the surface of the workpiece 44 longer than other areas, according to a calculated profile. Material removal may thereby be precisely controlled by controlling the polishing force and polishing time for each annular portion of the workpiece .

In another embodiment of the aspheric polishing machine according to the invention, shown in FIGS. 3 and 4, a free-floating slide 153 is mounted outboard from the

B-axis rotary stage 154. The floating slide is a precision bearing slide nominally oriented in the vertical direction to permit the spindle assembly 57 to move freely in a vertical direction. A positive stop 155 provides a downward limit to movement of the spindle assembly. In operation, the Z-drive 152 is moved downward until the tool 56 comes into contact with the workpiece 44. A small amount of additional movement of the Z-axis lifts the spindle assembly off the positive stop and assures that the full weight of the spindle assembly 57 is applied between the tool 56 and the workpiece 44. No means is provided for independently controlling the force between the tool and the workpiece during a polishing operation. Because the floating slide 153 is mounted outboard of the B-axis rotary stage 154, the weight of the spindle does not act in line with the slide 153. The actual force F_τ applied between the polishing tool 56 and the workpiece 44 is therefore a function of the weight force F_w and the cosine of the index angle θ of the B- axis as follows:

F_τ = F_Hcosθ .

The traverse speed of the tool across the surface may be compensated to provide additional worktime on steeper portions of the workpiece surface in order to compensate for the decreased pressure between the tool and the workpiece .

The polishing tool 56 (FIG. 5) comprises a polishing head 90 bonded to a mounting flange 93. The mounting flange 93 may be aluminum, steel or other rigid material. A centering surface 96 is provided for centering the tool in a corresponding datum hole (not shown) in the spindle 57. The spindle is provided with a set screw, split bushing or other means (not shown) for retaining the polishing tool.

The polishing head 90 contacts the workpiece and provides a surface 91 for abrading the workpiece. The polishing head 90 is formed from a felt pad that is impregnated with optical pitch. In a fabrication method embodying the invention, a cylindrical pad of felt having dimensions somewhat larger than the finished head 90 is thoroughly impregnated with optical pitch.

The felt is impregnated with the pitch in any suitable manner. For instance, the pitch can be heated to a liquid state, and the pad is immersed in the molten pitch and while submerged is compressed, squashed or wrung or otherwise manipulated to eliminate as much air as possible, drawing molten pitch into interstices of the felt between fibers. Preferably, a high density felt is used in conjunction with a low melting temperature pitch to fabricate the polishing head 90.

After impregnating the felt with pitch, the resulting polishing head blank is bonded to the flange 93 using an adhesive or by heating the pitch at the interface, applying the flange and cooling. After the head 90 is bonded to the flange 93, the flange 93 is mounted in a lathe and the head 90 is machined to a predetermined external diameter and length. Any desired shape and size can be developed from such machining. For example, a relief 92 is machined on the end of the head 90 in order to form an annular, external lip 95. A polishing surface 91 comprises a lower edge of the lip 95. The width W of the surface 91 is chosen to be small enough to conform to the lens curvature and to permit localized polishing of the surface, while being wide enough to maintain reasonable dimensional stability during the polishing process. The pitch and the felt in the polishing head function cooperatively to provide a material having characteristics needed for use with the aspheric polisher. The fibrous structure of the felt provides dimensional stability to the polishing head by interconnecting portions of the pitch, preventing it from deforming. The pitch provides a semi-rigid surface that precisely conforms to the work surface during the polishing operation. As the polishing head traverses the workpiece surface across changing radii of curvature, the

17 SUBSTITUTE SHEET (RUL£ 26) shape of the polishing surface 91, which becomes warm due to friction, conforms precisely to the work surface. At the same time, the lip 95 remains substantially intact.

During a single cycle of a polishing process, the axes of the machine are controlled to perform several tasks simultaneously. The polishing tool 56 is traversed across the workpiece 44 (FIG. 6). During the traverse, the angle θ between the vertical axis 100 of the workpiece 44 and the axis 58 of the polishing spindle is adjusted so that normal contact between the polishing surface 91 of the polishing head 90 and the surface of the workpiece 44 is maintained. In the embodiment including a compression spring 82 (FIG. 3) to compensate the polishing force using the weight of the polishing spindle, the Z-axis is also controlled to apply a predetermined polishing force between the tool and the workpiece .

The X and B axes are also controlled so as to traverse the surface of the workpiece at a predetermined velocity profile. Thus, material removal is determined by the amount of time that the polishing tool 56 is in contact with a given region of the workpiece 44. Because material removal is dependent on polishing time rather than on the precise path of the tool, the resulting form and texture of the workpiece 44 may be controlled to extremely precise tolerances without requiring comparable precision in the drives and axes of the polishing machine. Instead, those drives and axes must be sufficiently precise to translate and rotate the polishing surface 91 smoothly through its path while maintaining it in substantially tangential contact with the surface of the workpiece 44. Thus, the drives and axes of the machine may have dimensional variations on the order of thousandths of an inch, while providing the capability of polishing an optical element having a form tolerance of 10 millionths of an inch or less.

In a process 210 (FIG. 7) for polishing an aspheric element embodying the invention, the blocked workpiece is first measured on a workpiece measurement machine, represented by block 200. The workpiece measurement machine can be any appropriate machine or device but is preferably a Form Talysurf Series contact profilometer marketed by Taylor Hobson Limited of Leicester, England, UK, and Precitch Inc. of Keene, New

Hampshire, USA. A contact profilometer utilizes a stylus to measure a two-dimensional representation of the form and texture of the workpiece surface. The form is measured to accuracies approaching millionths of an inch. The measurement machine then compares the measured form with a theoretical perfect form for the surface of the workpiece, and form errors are calculated.

If the form and texture errors are within a predetermined tolerance envelope, the part is completed. If the form and/or texture errors exceed the predetermined tolerance, the form error data is transferred to the computer 3 of the aspheric polisher for compensation. The data is transferred using a diskette or over a network.

A predetermined path and velocity profile of the tool as it traverses the face of the workpiece is next determined using compensation software, represented by block 201 (FIG. 7). The compensation software resides in the computer 3 (FIG. 1). The compensation software utilizes form errors calculated by the workpiece measurement machine. The software determines which annular regions of the workpiece surface require additional material removal, determines the degree of such removal, and determines a velocity profile for the tool along the theoretical form of the part in order to remove the determined amount of material from the determined areas. Thus, while the actual path of the tool is determined by the theoretical form of the finished part, the speed of traverse of the tool is determined by the need to remove material in specific regions . In those portions of the theoretical form where little or no material is to be removed, the traverse is relatively rapid, while in those portions where a significant amount of material must be removed, the traverse is slowed or even stopped.

In addition, in those embodiments where the Z- axis position may be used to adjust polishing force, the compensation software may adjust that parameter to increase or decrease material removal during a traverse. After the tool path and velocity profile are calculated, the blocked workpiece 44 is mounted on the polishing machine spindle, and a polishing cycle, represented by block 202, is executed, driving the axis motor controls according to the schedule calculated by the compensation software. As described above, polishing media in the form of a slurry is introduced between the polishing tool and the workpiece, and the polishing tool traverses the workpiece in the predetermined velocity profile and in such a way that the polishing surface 91 of the polishing tool maintains tangential contact with the workpiece surface.

The blocked workpiece is then removed from the machine, cleaned and transferred to the form measurement machine. The workpiece is preferably left bonded to the workpiece flange 45 (FIG. 6) in order to maintain the location of the center of the workpiece.

The process continues with subsequent iterations of the above steps, using the form errors from the previous measurement to determine the amount of material to be removed , and to compensate the traverse speed and possibly the tool force across various regions of the workpiece. The process ceases when the workpiece as measured is within the desired tolerance, and is ready for commercial sale or use.

The traverse must be sufficiently slow to permit significantly different rates of traverse in different areas of the part, in order to change the profile according to the compensation data. Furthermore, empirical compensation may additionally be required in order to compensate for other factors influencing material removal .

Figure 8 shows various components and stages of completion of polishing tools embodying the invention, including, from left to right, two polishing tools shown in various stages of completion; a completed polishing tool including a metal flange and a turned block of pitch-impregnated felt; a block of pitch-impregnated felt; and a block of felt before impregnation with pitch.

Figure 9 shows a complete front view of an aspheric polishing machine according to the embodiment of FIG. 3, wherein the B-axis rotary stage has been indexed approximately 30 degrees from vertical. Figure 10 shows the drive assembly, polishing spindle assembly and workholding spindle of the machine shown in Figure 9 , including a workpiece mounted on the workholding spindle.

Figure 11 shows a close-up of the polishing spindle of the machine shown in Figure 9.

These and other combinations and variations of the features discussed above can be utilized without departing from the present invention. For example, the slide arrangement shown in FIG. 2 may be used without a compensating spring, and the compensating spring of

FIG. 2 may be used in the slide arrangement of FIG. 3.

The foregoing description of the embodiments should therefore be taken as illustrative, rather than limiting the invention . As will be understood from the above, a polishing apparatus embodying the present invention can be controlled or a method embodying the present invention can be implemented by a computer program operating on a standard desk top computer. The computer program may be supplied by a removable disc insertable into the disc drive unit shown on the front panel of the computer 2. An aspect of the present invention thus provides a storage medium storing processor implementable instructions for controlling a processor to carry out the method as hereinabove described.

Further, the computer program can be obtained in electronic form for example by downloading the code over a network such as the Internet. Thus in accordance with another aspect of the present invention there is provided an electrical signal carrying processor implementable instructions for controlling a processor to carry out the method as hereinbefore described.

Claims

CLAIMS :

1. A method for polishing an existing profile on a surface of a workpiece to form a predetermined profile, comprising: (a) measuring the existing profile of the workpiece ;

(b) comparing the measured existing profile with the predetermined profile, and calculating a material removal profile; (c) contacting the workpiece surface with a polishing surface of a polishing head at a contact point, and so that a contact force is applied between the polishing surface and the workpiece surface;

(e) translating and rotating said polishing head so as to traverse said contact point along a path duplicating said predetermined profile superimposed on said workpiece, said contact point traversing said profile at a traverse velocity, said head being translated and rotated so as to maintain said polishing surface substantially tangent to said predetermined profile and to maintain a contact force between the polishing surface and the workpiece surface; and

( f ) varying said traverse velocity during said translating and rotating step so as to remove material according to said material removal profile.

2. An apparatus for polishing a surface of a workpiece by removing material according to a predetermined material removal profile, comprising:

(a) a frame

(b) a workholding element mounted to said frame for mounting the workpiece; (c) a drive assembly secured to said frame and having a polishing spindle mounting element; said drive assembly comprising at least one drive for moving said polishing spindle mounting element with respect to said frame; (d) a polishing spindle assembly mounted on said polishing spindle mounting element for rotation about a spindle rotation axis, said spindle assembly including a polishing head for contacting and abrading the surface of the workpiece near a theoretical contact point, said drive assembly further comprising means for maintaining a contact force between said workpiece and said polishing head; and

(e) drive assembly controller for controlling said drive assembly to produce movement of said polishing head along a theoretical profile while maintaining said contact force between said workpiece and said polishing head, said drive assembly controller further controlling said movement at a predetermined velocity profile so that said abrading by said polishing head removes material at the predetermined material removal profile.

3. A polishing head for removing material in conjunction with an abrasive slurry, said polishing head comprising:

(a) a felt matrix having a network of fibers; and

(b) a pitch binder substantially impregnating said network of fibers.

4. An apparatus for polishing a profile on an optical surface of a workpiece, comprising:

(a) a frame

(b) a workholding spindle rotatably mounted to said frame, said workholding spindle having means for mounting the workpiece;

(c) a first driveable linear slide assembly mounted on said frame and having a first slide moveable relative to said frame in a first direction; (d) a second driveable linear slide assembly mounted to said first slide and having a second slide moveable relative to said first slide in a second direction;

(e) a polishing force slide assembly comprising a fixed element mounted to said second slide, and a moveable element slideably mounted to said fixed element for linear movement in a substantially vertical direction, said polishing force slide assembly further comprising a spring interconnecting said fixed and moveable elements for urging said moveable element upward with respect to said fixed element with a spring force proportional to a relative position between said fixed and moveable elements ;

(f ) a driveable rotary stage assembly mounted to said moveable element of said polishing force slide assembly, said rotary stage assembly having a rotary stage rotatable about a horizontal rotary stage axis; and

(g) a polishing spindle assembly rotatably mounted on said rotary stage for rotation about a spindle rotation axis, said spindle assembly including a polishing head for contacting the optical surface of the workpiece; whereby a contact force between said polishing head and said workpiece may be reduced or increased by changing said spring force by driving said first and said second slide assemblies so as to change said relative position between said fixed and moveable elements.

5. A method for polishing a workpiece, for example an optical surface such as, for example, a lens or a mirror, to achieve a desired profile, which comprises determining the difference between the existing profile of the workpiece and the desired profile to obtain a profile of the material to be removed from the workpiece to achieve the desired profile and controlling a polishing means in accordance with the material removal profile.

6. A method according to claim 5 which comprises controlling a velocity of the polishing means across the workpiece in accordance with the material removal profile.

7. Apparatus for polishing a workpiece, for example an optical surface such as, for example, a lens or a mirror, to achieve a desired profile, which comprises means for determining the difference between the existing profile of the workpiece and the desired profile to obtain a profile of the material to be removed from the workpiece to achieve the desired profile and means for controlling a polishing means in accordance with the material removal profile .

8. Apparatus according to claim 7 wherein the controlling means is arranged to control a velocity of the polishing means across the workpiece in accordance with the material removal profile.

9. Apparatus for polishing a workpiece surface, for example an optical surface such as, for example, a lens or mirror surface, wherein means are provided for regulating or adjusting a polishing force applied by polishing means to a workpiece during a polishing operation .

10. Apparatus according to claim 9 whereinthe polishing force adjusting or regulating means comprises a spring biassing means exerting on the polishing means a spring force which is dependent on the relative positions of a fixed and a movable member interconnected by the spring biassing means one of the fixed and movable members being coupled to the polishing means and the other to a support of the apparatus which support may be, for example, a further movable member.

11. A method according to claim 1 which further comprises repeating steps a) to f) until the predetermined profile is formed within acceptable tolerance limits .

12. A method according to claim 1, 5, 6 or 11, which further comprises using as the polishing head or polishing means a felt matrix impregnated with a pitch binder.

13. An apparatus according to claim 2, 4, 7, 8, 9 or 10, which comprises a polishing head having a felt matrix impregnated with a pitch binder.

14. An apparatus according to claim 2, 4, 7, 8, 9, 10 or 13, which comprises a drive assembly having servomotors for effecting relative movement between a workpiece and the polishing means or head..

15. A method of manufacturing a polishing head suitable for use in a method in accordance with any one of claims 1, 5, 6 and 11 or an apparatus in accordance with any one of claims 2, 4, 7, 8, 9 and 10, which comprises heating pitch to a liquid or molten state, and immersing a pad in the liquid or molten pitch.

16. A method according to claim 15, which comprises using a felt pad and, while the pad is immersed in the liquid or molten pitch, manipulating the pad to expel air from the pad and draw pitch into interstices between the fibres of the felt pad.

17. A storage medium storing processor implementable instructions for controlling a processor to carry out a method according to any one of claims 1, 5, 6, 11 or 12.

18. An electrical signal carrying processor implementable instructions for controlling a processor to carry out a method according to any one of claims 1, 5, 6, 11 or 12.