US20040166770A1

US20040166770A1 - Grinding apparatus and method

Info

Publication number: US20040166770A1
Application number: US10/788,219
Authority: US
Inventors: Joseph Flasch; Frank Petrucci; Richard Amodeo
Original assignee: Novellus Systems Inc
Current assignee: Novellus Systems Inc
Priority date: 2003-02-25
Filing date: 2004-02-25
Publication date: 2004-08-26

Abstract

An apparatus for grinding a workpiece on a support surface includes a rotatable and vertically movable grinding wheel having an abrading surface and at least one cylinder coupled to the grinding wheel. The cylinder includes a piston, a stepper-motor coupled to the piston, and a converter coupled to the piston and to the stepper-motor for converting rotary movement of the stepper-motor to linear movement of the piston to vertically move the grinding wheel. A measuring device provides a representation of the distance between the support surface and the abrading surface. A controller is coupled to the measuring device and to the stepper-motor for receiving the representation and applying an activation signal to the stepper-motor to vertically move the grinding wheel to reach a predetermined distance between the support surface and the abrading surface.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/450,242, filed Feb. 25, 2003.[0001]

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus for grinding a workpiece to achieve a desired workpiece dimension, and more particularly to a thru-feed grinding apparatus utilizing an improved closed-loop, feedback control system resulting in enhanced size control.

BACKGROUND OF THE INVENTION

There are many varieties of grinding machines; for example, horizontal-spindle, reciprocating-table surface grinders; double-disc grinders; and abrasive belt grinders. A thru-feed grinder is a very efficient apparatus for the high production surface grinding of workpieces because it requires little fixturing and set-up time and provides for the continuous loading and unloading of workpieces. That is, because thru-feed grinders employ a conveyor feed assembly, workpieces are fed to the grinder on a continuous basis thus permitting virtually continuous grinding.

Certain traditional thru-feed grinders are equipped with hydraulic cylinders that support the grinding wheel. Such grinders, however exhibit certain shortcomings related to size control stability. That is, over time the hydraulic cylinders may drift resulting in changes in the distance between the chuck (i.e. the surface supporting the workpieces) and the working surface of the grinding wheel. The above described drift occurs for three primary reasons. First, it is extremely difficult to bleed all air from the hydraulic system. Second, the hydraulic cylinders are typically not completely leak-proof, and third, hoses coupled to the hydraulic cylinders are generally flexible and will expand with increasing pressure. Drift can result in dimensional variations in the processed workpieces, and if the drift exceeds a certain value, the system may lift the grinding wheel from the part in a relatively uncontrolled manner requiring a very precisely controlled subsequent downward movement of the grinding wheel to compensate for overshoot. Events such as this cannot be tolerated in the production of parts with high dimensional tolerances. Furthermore, the problem of achieving high-tolerance precision grinding is exacerbated by the wearing down of the grinding wheel with time as abrading material on the grinding wheel is consumed.

Thus, it would be desirable to provide a precision thru-feed grinding apparatus employing a closed-loop feedback control system that substantially avoids the problems associated with the above described drift. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided an apparatus for grinding a workpiece on a support surface. The apparatus comprises a rotatable and vertically movable grinding wheel having an abrading surface and at least one cylinder coupled to the grinding wheel. The cylinder includes a piston, a stepper-motor coupled to the piston, and a converter coupled to the piston and to the stepper-motor for converting a rotary movement of the stepper-motor to linear movement of the piston to vertically move the grinding wheel. A measuring device provides a representation of the distance between the support surface and the abrading surface. A controller is coupled to the measuring device and to the stepper-motor for receiving the representation and applying an activation signal to the stepper-motor to vertically move the grinding wheel to achieve a predetermined distance between the support surface and the abrading surface.

According to a further aspect of the invention there is provided an apparatus for grinding a workpiece on a support surface. The apparatus comprises a rotatable and vertically movable grinding wheel having an abrading surface, and a feedback control network for maintaining a predetermined distance between the abrading surface and support surface. The feedback control system comprises a stepper-motor-controlled hydraulic cylinder coupled to the grinding wheel for vertically moving the grinding wheel, a measuring device for indicating when the distance between the support surface and the abrading surface is different than a predetermined distance, and a controller is coupled to the measuring device and to the stepper-motor-controlled hydraulic cylinder for activating the cylinder to vertically move the grinding wheel to achieve the predetermined distance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and [0008]
FIGS. 1 and 2 are isometric views of a thru-feed grinding apparatus in accordance with one embodiment of the present invention; [0009]
FIG. 3 is a plan view of the conveyor assembly and grinding wheel utilized in the apparatus shown in FIG. 1; [0010]
FIG. 4 is an isometric view of an pneumatic gauge assembly utilized in the apparatus shown in FIG. 1; [0011]
FIG. 5 is an isometric view of the slide-posts, dampeners, and plates utilized in the apparatus shown in FIG. 1; [0012]
FIG. 6 is a schematic diagram of a grinding apparatus in accordance with the present invention; and [0013]
FIG. 7 is a schematic diagram of a pneumatic air gauge for use in conjunction with the apparatus shown in FIG. 6.[0014]

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. [0015]
FIG. 1 and FIG. 2 are isometric views of a thru-[0016] feed grinding apparatus 10 in accordance with an embodiment of the present invention. The apparatus comprises a lower housing 12, an upper housing 14, a continuous conveyor assembly 16 powered by a motor (not shown) within lower housing 12, guide rails 18 and 20, and a grinding wheel 22 mounted for both vertical and rotational movement to an upper carriage assembly (not shown) within upper housing 14. Grinding wheel 22 may be a standard, inserted-nut, disc wheel mounted on a vertical spindle (65 in FIG. 6) over conveyor assembly 16. Conveyor assembly 16 includes a motor-driven, continuous conveyor belt 24 (preferably an abrasive belt) which passes over a magnetic table 26 in a direction indicated by arrow 28. Magnetic table 26 may comprise a variable-power, electromagnetic table that serves as a locating table and magnetic chuck.
[0017] Conveyor assembly 16 is preferably a variable speed system and imparts translational movement to conveyor belt 24 that is of sufficient width (e.g. six inches) to carry workpieces 30, placed on the belt by an operator 36, beneath grinding wheel 22 and through the grinding operation until the workpiece is off-loaded at the downstream end of the belt. As can be seen in FIG. 2, grinding wheel 22 is configured for rotation as indicated by arrow 32 and for vertical translation as indicated by arrow 34 in a manner to be described herein below. In this manner, a workpiece 30 passes under grinding wheel 22, and a surface of the workpiece is ground to a desired dimension. Guide rails 18 and 20 are provided to position the workpiece 30 on conveyor belt 24 and absorb the side thrust of grinding wheel 22. The guide rails are adjustable and assist in orienting and directing the workpieces by allowing the torsional thrust of the grinding wheel to automatically position the workpieces against the rails while at the same time prevent the workpieces from being swept off the conveyor. The guide rails also prevent tipping of the workpieces as they pass under the grinding wheel thus permitting the operator to simply and continuously place workpieces on the conveyor.
Referring to FIG. 3, [0018] workpieces 30 loaded onto conveyor belt 16 are carried under the working surface of grinding wheel 22 and ground as they are held in position by the magnetic table 26 and adjustable guide rails 18 and 20. Finished parts 48 are discharged at the opposite end 40 of conveyor 24 into a suitable container or, if desired, to subsequent handling equipment. A longitudinal inclination of the magnetic table (e.g. 0.0015 inches per inch) permits the workpieces 30 to be ground only when entering the grinding wheel area and pass through the center 42 and trailing section 44 of the grinding wheel 22 untouched. This causes a slight taper 46 on the face of the grinding wheel proportional to the inclination of the magnetic table 26. In this manner, grinding wheel 22 is continuously dressed by the workpieces. Workpiece dimension is determined by the distance 50 between the abrading surface of wheel 22 and the surface 52 upon which the workpiece is supported.
In short, the grinding process is accomplished by four elements; (1) magnetic table [0019] 26 that serves as a locating surface and magnetic chuck, (2) conveyor belt 24 that moves workpieces 30 through the grinding operation, (3) rotating grinding wheel 22 that imparts a vertical force on workpieces 30 to keep them securely position on the locating surface or conveyor belt; and (4) adjustable guide rails 18 and 20 against which the workpieces are firmly held and which absorb the grinding torsional force of the grinding wheel.
[0020] Housings 12 and 14 are made of any material (e.g. steel) of sufficient strength to withstand the strain of heavy stock material while at the same time provide for smooth operation. Referring to FIG. 5, a plurality (e.g. four) vertical slide posts 54 support grinding wheel 22, motorized spindle 65 (FIG. 6) and a hydraulic feed mechanism (74 and 76 in FIG. 6). All sub-assemblies are mounted on heavy support plates (e.g. 56) which form an integral structural unit with posts 54. Dampeners 58 under support plates 56 reduce machine-to-workhead vibrations
FIG. 6 is a schematic diagram of a grinder apparatus in accordance with the present invention wherein like elements are denoted by like reference numerals. An operator places workpieces on [0021] conveyor 24 which moves in the directions indicated by arrows 60 to bring the workpieces beneath grinding wheel 22 as described above. Conveyor 24 is driven by a conveyor motor 62, and grinding wheel 22 is rotated by motor 64 as indicated by arrow 66. Grinding wheel 22 is coupled to a support plate 68 which is configured to slide vertically on posts 54 as is indicated by arrow 70. Posts 54 are coupled at their upper ends to a top plate 72.
Mounted above [0022] support plate 68 are a plurality (e.g. two) of stepper-motor-controlled high-precision hydraulic cylinders 74 and 76. Each precision hydraulic cylinder includes a cylinder shaft 78 housing a piston 80 coupled to a piston rod 82 that extends through openings 84 in top plate 72 so as to move wheel support plate 68 vertically on posts 54. As can be seen, each cylinder includes a stepper-motor 86, a spool and valve assembly 88 and a rotation-to-translation converter 90. Spool and valve assemblies 88 are coupled to a source of pressure 92 and, via conduits 94 and 96, to the inner regions of shaft 78 on opposite sides of piston 80.
Stepper-[0023] motors 86 are electrically coupled to a programmable logic controller 98 which is in turn configured to receive a measurement signal from pneumatic gauge 100, to be more fully described below. Simply stated, as grinding wheel 22 grinds workpieces on conveyor 24, a small amount of abrading material is lost on the abrading surface of grinding wheel 22. Pneumatic gauge 100 monitors the distance between the abrading surface of grinding wheel 22 and the surface upon which the workpieces are resting (i.e. conveyor 24), and when this distance exceeds a predetermined value due to the loss of abrading material on grinding wheel 22, controller 98 activates stepper-motors 86 which in turn causes pistons 80 to move downward and, via piston rods 82, to move wheel support plate 68 downward so as to achieve a desired spatial relationship between the abrading surface of grinding wheel 22 and the upper portion of conveyor 24 upon which the workpieces are loaded. That is, the activation signal provided by controller 98 to stepper-motors 86 will cause the stepper-motor shafts to rotate in very precise increments. The stepper-motor shafts operate on an internal spool and valve assemblies 88 imparting rotary and linear movement to the spool and valve assemblies 88 and the appropriate closure and opening of valves to provide fluid pressure to cylinder shafts 78. Rotation of the spool is translated to linear movement in rotary-to-translation converters 90 to move pistons 80 vertically in an appropriate direction. This may be accomplished by a ball nut attached to each piston 80 that rotates a ball screw directly coupled to the valve spool. In this manner, the speed of pistons 80 is positively synchronized to the rotational speed of the stepping motor. The piston continues rotating the spool until a shut-off position is reached (i.e. when the predetermined spacing between the abrading surface of grinding wheel 22 and the upper portion of conveyor 24 has been reached). Thus, the digitally operated rotating stepping motors 86, cylinders 74 and 76, grinding wheel 22, pneumatic gauge 100 and controller 98 form a closed-loop feedback control system. Each step of the stepping motors 86 is very precise and therefore very accurate positioning of grinding wheel 22 over conveyor 24 results. Stepping motors 86 may be incrementally rotated in either direction depending upon the manner in which the signal from controller 98 is applied. In the absence of any activation signal from controller 98, the cylinder is inherently braked and maintained stationary. A more detailed discussion of high-precision digitally, controlled hydraulic cylinders may be found in U.S. Pat. No. 3,457,836 issued Jul. 29, 1969 and entitled “DIGITALLY OPERATED ELECTROHYDRAULIC POWER SYSTEM” assigned to The Superior Electric Company, Bristol, Conn. Such devices are also commercially available from Fluid Power Technology, located in Charlotte, N.C.
FIG. 7 is a schematic diagram of a pneumatic gauge suitable for use at [0024] air gauge 100 in FIG. 6. The pneumatic (air) gauge provides for continuous and automatic compensation for wear on grinding wheel 22. It maintains a substantially constant dimension between the surface on which workpieces 30 rest (i.e. conveyor belt 24) and the abrading surface of the grinding wheel. This distance corresponds to the ground dimension of the finished part. Once established, workpiece thickness is maintained until all usable abrasive in the grinding wheel is consumed. At this point, a controller coupled to the air gauge automatically shuts the system down and generates an alert or warning (e.g. illuminates a light on a control panel). It operates on the principals of air flow at constant velocity and a pneumatic wheatstone bridge. The device may be considered an air to electric converter and is fed by a single air supply line 102 which is divided into two parallel paths 104 and 106. Air lines 104 and 106 are separated by an extremely flexible, air-tight diaphragm 108. Lower air line 106 is referred to as a measurement line and comprises a calibrated opening or measurement nozzle 110 and a measurement opening 112 which is the resultant opening produced when the calibrated air jets (or variable restricting device of a gauge) is combined with the workpiece or master. The upper airline 104 is an adjustment or balance line and comprises a calibrated opening or balance nozzle 106 and an opening 114 (e.g. an annular orifice) which permits air to escape to the atmosphere at a rate dependant upon the position of a tapered needle 116 with respect to outlet 114. A balance or equilibrium condition exists when there is substantially equal pressure in both the balance line 104 and the measurement line 106.
Any increase in pressure in the [0025] measurement line 106 will propel the diaphragm 108 upward thus moving needle 116 upward until the annular outlet 114 around needle 116 is such that the pressure in both upper line 104 and lower line 106 is substantially equal. An opposite effect would occur if pressure were to drop in measurement line 106. A distance measurement between the lower surface of the grinding wheel and the upper surface of the conveyor is related to the displacement of needle 116 acting on plunger 118 and indicator 120 in relation to the original position which was determined when the instrument was calibrated against a known dimension, part, or master. This measurement may be read by controller 98 (FIG. 6) via an electric probe. The displacement measurement provided by air gauge 100 to controller 98 is then converted in controller 98 to energize stepper-motors 86 to increment or decrement. As a result, cylinder 74 and 76 vertically move wheel plate 68 until the appropriate dimension has been reached at which point controller 98 terminates activation of stepper-motors 86.
Thus, there has been provided a precision thru-feed grinding apparatus that avoids the problems associated with drift and compensates for loss of abrading material on the grinding wheel through the use of high precision digital cylinders and a closed look feedback system. [0026]
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims. [0027]

Claims

What is claimed is:

1. An apparatus for grinding a workpiece on a support surface, the apparatus comprising:

a rotatable and vertically movable grinding wheel having an abrading surface;

at least one cylinder coupled to said grinding wheel, said cylinder comprising:

a piston;

a stepper-motor coupled to said piston; and

a converter coupled to said piston and to said stepper-motor for converting rotary movement of said stepper-motor to linear movement of said piston to vertically move said grinding wheel;

a measuring device for providing a representation of the distance between said support surface and said abrading surface; and

a controller coupled to said measuring device and to said stepper-motor for receiving said representation and applying an activation signal to said stepper-motor to vertically move said grinding wheel to achieve a predetermined distance between said support surface and said abrading surface.

2. An apparatus according to claim 1 wherein said support surface is continuously movable beneath said grinding wheel.

3. An apparatus according to claim 2 wherein said support surface is a conveyor belt.

4. An apparatus according to claim 3 wherein said conveyor belt is inclined downward in the direction of movement of said support surface beneath said grinding wheel.

5. An apparatus according to claim 3 wherein said measuring device is a pneumatic gauge.

6. An apparatus according to claim 5 wherein said pneumatic gauge is positioned between said abrading surface and said support surface.

7. An apparatus according to claim 6 wherein said pneumatic gauge is positioned proximate an outer periphery of said grinding wheel.

8. An apparatus according to claim 7 wherein said controller is a programmable logic controller.

9. An apparatus according to claim 8 wherein said controller terminates activation of said stepper-motor when the distance between said support surface and said abrading surface reaches a predetermined distance.

10. An apparatus for grinding a workpiece on a support surface, the apparatus comprising:

a rotatable and vertically movable grinding wheel having an abrading surface; and

a feedback control network for maintaining a predetermined distance between said abrading surface and said support surface, said feedback control system comprising:

a stepper-motor controlled hydraulic cylinder coupled to said grinding wheel for vertically moving said grinding wheel;

a measuring device for indicating when the distance between said support surface and said abrading surface is different than a predetermined distance; and

a controller coupled to said measuring device and to said stepper-motor controlled hydraulic cylinder for activating said cylinder to vertically move said grinding wheel to reach said predetermined distance.

11. An apparatus according to claim 10 wherein said controller activates said cylinder to lower said grinding wheel as it becomes thinner due to wear.

12. An apparatus according to claim 10 wherein said measuring device is a pneumatic gauge.

13. An apparatus according to claim 12 wherein said pneumatic gauge is positioned between said abrading surface and said support surface.

14. An apparatus according to claim 13 wherein said pneumatic gauge is positioned proximate an outer periphery of said grinding wheel.

15. An apparatus according to claim 14 wherein said controller is a programmable logic controller.

16. An apparatus according to claim 13 wherein said support surface is continuously movable beneath said grinding wheel.

17. An apparatus according to claim 16 wherein said support surface is a conveyor belt.

18. An apparatus according to claim 13 wherein said stepper-motor controlled hydraulic cylinder comprises:

a piston;

a stepper-motor coupled to said piston; and

a converter coupled to said piston and to said stepper-motor for converting rotary movement of said stepper-motor to linear movement of said piston to vertically move said grinding wheel.