US3426484A - Method of lapping and polishing - Google Patents

Description

Feb. l1, 1969 n. G. MAzuR 4 I METHOD OF LAPPING AND POLISHING Sheet of 5 original Filed July 12, 1963 om mv INYENTOR Rob'erf G. Mozur ATTORNEY Feb. 11', 1969 R. G. MAzuR 3,425,484

mamon oF LAPPINQ Annyomsnine. original Filed July 12. 1963 Sheet .2..

Feb. 11, 1969 R, G. MAZUR Y 3,426,484

METHOD OF LAPPING AND POLISHING Original Filed July 12, 1963 Sheet 3 of ."5

8O F ig,4.4 f D 86 Fig.5.

loo v 9e /84 United States Patent O 3,426,484 METHOD OF LAPPING AND POLISHING. Robert G. Mazur, Penn Hills, Pittsburgh, Pa., asslgnor to Westinghouse Electric ICorporation, Pittsburgh, Pa., a corporation of Pennsylvania Original application July 12, 1963, Ser. No. 294,716, now Patent No. 3,277,610, dated Oct. 11, 1966. Divided and this application Nov. 19, 1965, Ser. No. 508,738 U.S. Cl. 51-281 5 Claims Int. Cl. B24b 1/00 ABSTRACT OF THE DISCLOSURE A method of abrading Workpieces in which the workpieces are freely movable on a flat surface with abrasive material on it. The surface is rotated eccentrically by drive through shaft and imparts acceleration to the workpieces so that they move at random over surface.

This application is a division of Patent 3,277,610 grant* ed Oct. 1l, 1966 and assigned to Westinghouse Electric Corporation.

The present invention relates to a method or process for abrading work.

In the machining of metals, semiconductors and other materials, a subsequent operation often required is the lapping or polishing of flat, cylindrical or odd-shaped pieces or surfaces. Some of the factors that influence the lapping and polishing operation are: the materials involved, for example, workpiece, lapping or polishing compound, and the lapping or polishing plate or base; the size of the particles of which the lapping or polishing compound is composed; the pressure exerted on the Workpiece; the relative velocity of workpiece and plate; and, the arrangement provided for constraining and loading the Work during the operation. Economy urges a high rate of removal of material, leading to the use of a relatively coarse lapping or polishing compound, high pressure on the work, and rapid relative motion of work and lapping or polishing plate. In many situations, a smooth surface finish demands the use of a iine abrasive or polish, light loads and a very low rate of removal of material. The prior art solution to this problem is to use successive lapping or polishing abrasives, possibly with changes of pressure and motion so that a very high initial rate of removal and a very slow final rate lead to an overall relatively rapid rate of material removal and a line surface finish.

In addition, economy also dictates the use of simple rugged equipment that lends itself to a variety of operations and ease of maintenance, while the desired surface finish usually requires considerable care and relatively complex means to avoid contamination of liner abrasives by previously used coarser materials or air-borne particles to avoid scratches. The lapping or polishing process may be carried out by hand; but, in most cases, the time and physical labor required for a proper nish necessitates the use of various lapping and polishing machines. The utility of a particular lapping or polishing machine may be judged on the basis of its success in meeting the requirements for different jobs, as delineated above, relative to its cost, the basic criterion being the machines versatility or the ease with which it may be adapted to various work programs that arise in practice.

In accordance with the teachings of the prior art lapping or polishing is effected by combinations of opposed simple and planetary rotations of a lapping plate and work holders, or by cam operated translations and rotations combined to produce ligure eight motion of the work relative to the lapping plate or by mechanically or 3,426,484 Patented Feb. 11, 1969 ICC electrically generated repetitive translations or vibrations of the lapping plate. All of these practices provide relative motion of the work and lapping plate to ass-ure even lapping of the workpiece. However, these practices all involve the use of machines of heavy and expensive construction, with an attendant lack of liexibility; in addition these machines require excessive care and maintenance for proper operation.

An object of this invention is to overcome the diiculties and disadvantages of the prior art and to provide a method of abrading, polishing, lapping and the like for accomplishing this object.

Another object of the present invention is to provide a method of lapping or polishing capable of being practiced with relatively simple apparatus.

Another object of the invention is to provide a method of abrading the face of a workpiece wherein the workpiece moves freely on an eccentrically rotating abrasive surface, the motion being randomized by the essentially random forces acting between the abrasive surface and the work.

Other objects of the invention will appear hereinafter.

To more fully understand the nature and scope of the invention, reference should be had to the following detailed description and drawing in which:

FIGURE 1 is an elevation view partly in cross section of the apparatus of the invention;

FIG. 2 is a persepctive view of the support plate of the apparatus of the invention;

FIG. 3 is a perspective view of the apparatus of the invention;

FIG. 4 is a plan view of a lapping and polishing tray with Workpieces thereon;

FIG. 5 is a plan view of a lapping and polishing tray adapted to provide uniform movement of work; and

FIG. 6 is a plan view of a lapping and polishing tray adapted to simultaneously abrade both sides of the workpiece.

In accordance with the present invention and in attainment of the foregoing objects, there is provided an apparatus for abrading a workpiece. The apparatus comprises a Work supporting means, for instance, a relatively flat plate including a work supporting abrasive surface. The abrasive surface may consist of loose abrasive material or a rigid abrasive sheet or the like. The apparatus also contains a plurality of shafts each rotatable about an axis displaced from the axis of said shaft. Driving means are connected to at least one of the shafts to move the one shaft about the displaced axis. Means are provided for connecting the one shaft in driving relationship with the supporting means to move the work supporting abrasive surface as the one shaft is driven. Means are also provided for connecting the other shafts in supporting relationship with the supporting means and rotatable therewith. Finally, a counterweight is attached to each shaft and is rotatable therewith to counterbalance dynamically the masses of each shaft and the portion of the supporting means moving therewith.

Referring to FIG. l, there is shown a lapping and polishing apparatus 10. The apparatus 10 comprises a sheet metal box housing 12 containing =a variable speed AC motor 14 which is secured to the housing 12 by means of bracket 16 and hardware 18. On the upper end of the housing is disposed and attached thereto a base support portion 20 having a plurality of apertures 22 each containing a bea-ring member 24. A plurality of shafts 26 are disposed in the apertures 22 in the support portion 20 within the bearings 24 such that each shaft 26 is rotatable about its axis in a bearing 24. Above the support portion Z0 there is disposed an apertured plate 28 containing bearings 30 therein to rotatably receive shafts 32 which are displaced from the shafts 26 by portions of the counterweights 36. The displacement of axis of each shaft 32 from a axis of the shaft 26 is such that the plate 28 eccentrically rotates about the center of the base support portion in a circle ranging from about 0.5 inch to l inch in diameter.

One of shafts 26 rotatably mounted in the base support is driven by the motor 14 through a flexible coupling 34. Each shaft 26 has secured thereto a counterweight 36 to dynamically counterbalance the rotating mass and thus eliminate vibration of the support portion 20 and the plate 2'8.

The plate 28 is provided with an angle bracket 38 containing adjustable screws I40 which are capable of securing a removable lapping and polishing set 42 consisting of a tray 44, a lapping and/or polishing sheet 46 and a cover 48. However, other suitable means for securing the set may be employed. Thus it can be appreciated that a separate tray, lapping and polishing sheet and cover may be employed for each grade of lapping compound or polish employed.

As is indicated in the drawing, the tray 44 contains upstanding side walls 50 with a hooked bracket member 52 being secured by means of a bolt or rivet 54 to the inner surface 56 thereof. The hooked bracket 52 is designed to support a resilient or elastic member extending crossways from side wall to side wall as needed to provide a resilient wall to contain the work during lapping and polishing or to subdivide the lapping plate as is desired.

Referring to FIG. 2, there is shown the bottom side of the plate 28 of FIG. 1 to more fully iappreicate the spaced relationship of the shafts 26 and counterweights 36. The disposition of the shafts 26 and counterweight 36 at each corner of the plate 28 in a substantially equal spaced relationship has been considered particularly satisfactory for the size and shape of the counterweights employed to counterbalance dynamically the masses of the shafts 26 and the portion of the plate 28 moving therewith. However, it should be `understood that counterweights of different design and a different spaced relationship of a number more or less than the number of shafts and counterbalances shown in the drawing is within the scope of the invention so long as the plate 28 is counterbalanced.

Referring to FIG. 3, the lapping and polishing apparatus 150 is shown in perspective with a slight modification in the lapping and polishing set. With respect to the cover 154, the drawing shows that the cover having relatively long downstanding side `walls 156 tits on top of the upstanding side walls 158 of the lapping and polishing tray 60. A plurality of hooked members 62 are secured to the inner surfaces 64 of the upwardly extending side walls 158. Rubber bands 66 are then easily disposed on the hooked members on one wall and are stretched across to the -opposing wall and are disposed on a hooked member thereat to subdivide the tray 60. A workpiece 68 is shown in position for the lapping and/ or polishing operation within the subdivision delineated by the rubber bands 66. The apparatus 150 also includes a motor speed control 70 and a timer 72.

A particular feature of the lapping and polishing apparatus described herein is its versatility. The basic machine or apparatus may be easily and inexpensively adapted to any type of work desired. Specifically, the use of a removable lapping and polishing set consisting of a tray and a lapping and polishing sheet with `an abrasive surface and cover allows a change of abrasive from the coarsest lapping compound to the finest polishes in seconds with a minimal possibility of contamination. The lapping and/or polishing plates may be easily subdivided as indicated in FIG. 3 to allow independent motion of the separate workpieces or jigs or the workpieces may be placed loosely together on the plate to completely randomize the motion.

With reference to FIG. 4, an application of the randomized motion principle is shown. The workpieces 74 and 76 are disposed on the polishing or lapping tray 78 which contains la loose abrasive surface. When the eccentric motion is imparted to the tray, the regions of the tray on which the workpieces are disposed undergo acceleration so that the workpieces are moved by inertial forces relative to the tray surface. The spacing of the workpieces is such in relation to the frictional forces on the workpieces exerted by the tray surface that as the workpieces 74 and 76 are moved by the inertial forces they contact each other so that the workpieces are displaced over a substantial area of the surface during their movement. A resilient member 80 is disposed along the outer periphery of the tray and is suspended a slight distance above the surface of the tray 78 by means of springs 82 to facilitate continuous inertial movement without damage to the workpieces.

Referring to FIG. 5, another application of the above principle is shown fbut with limited and more uniform movement of the work over the lapping or polishing surface. In FIG. 5, there is shown a lapping and polishing tray `84 with a resilient member 86 disposed within the periphery of the tray and suspended therein by a spring member 88 attached to each corner 90 of the tnay '84 and a portion of the resilient member 86 similar to that shown in FIG. 4. Merely as an example, the drawing shows three cylindrical metal members 92, 94 and 96, the ring 94 having a workpiece 98 disposed Within its periphery on the surface of the tray 84 and `a workpiece 100 disposed on the lapping or polishing tray 84 in closely spaced relationship. The closely spaced relationship is provided so that the workpiece moves uniformly over the lapping or polishing plate with the members 92, 94 and 96 so that when the tray `84 is eccentrically rotating in a counterclockwise direction, the cylindrical members and workpiece will move in a clockwise direction each maintaining the same relative position with respect to the other moving members.

It should be appreciated that cylindrical members are employed since their size and shape are readily controllable thus facilitating a desired spaced relationship on the tray. This is important since all of the workpieces desired to be polished may not be of a uniform size and shape to provide a desired closely spaced relationship and thus detract from the uniform movement of the workpieces. However, since the cylindrical members have a hollow center, a workpiece of any reasonable size and shape may be disposed within each of the members so that a plurality of workpieces may be lapped or polished simultaneously and the objective of uniformity may be fully met.

If desired, continuous conditioning of the lapping or polishing plate may be achieved by disposing a suitably shaped relatively large plate on the lapping plate. Such a plate will in general move randomly with respect t-o the basic lapping or polishing plate, and if provided with suitable openings for workpieces or jigs, will allow lapping or polishing of the work while keeping the lapping or polishing plate flat. With reference to FIG. 6, there is shown an application of the above principle. The lapping or polishing tray 102 containing a lapping or polishing sheet 103 has disposed on its upper surface a fiat circular apertured plate 104 in which apertures a plurality of workpieces 106 are disposed. In addition, another suitably shaped relatively large plate 108 with or without apertures 110 is disposed on the exposed surfaces of the workpieces 106. Accordingly, when eccentric motion is imparted to the lapping or polishing sheet 103 both the lower plate 104 containing the workpieces 106 and the upper plate 108 will move in a random fashion thus lapping or polishing both sides of the workpiece 106 simultaneously. The abrasive may be applied by immersing both plates and workpieces in a liquid abrasive material 112 or alternatively the lower end of the workpieces may be abraded by applying a loose abrasive or a rigid abrasive sheet to the lapping or polishing sheet 103 and the upper ends may Ibe abraded by feeding a loose abrasive through the apertures 110 in the upper plate.

In order to show with more particularity the versatility of the lapping and polishing apparatus, the following examples are considered to be indicative.

Example l An optical at was lapped using an alumina abrasive with a water vehicle on the lapping tray having a glass lapping sheet thereon. The optical flat was 1.5 inches in diameter and was originally flat to only one fringe (using helium light, this is 0.295 micron). The average particle size of the alumina abrasive was about 5 microns. The optical flat was lapped for a period of the order of 15 minutes and the fiat was rechecked by interference techniques against a master at and was found to be still flat to one fringe. This illustrates the ability of the apparatus to maintain a degree of atness within close parameters.

Example II A plurality of silicon slices having a thickness -of about 0.004 inch were polished in the apparatus of the invention using a pitch lap base with a cerium oxide vehicle. The surface finish achieved was regarded as being as good as or better than the finish obtained with standard methods heretofore used in polishing silicon.

Example Ill Silicon carbide slices were polished with both 6 micron and 1A micron diamond on a cloth lap axed to a glass plate using kerosene as a vehicle. The polishing times for silicon carbide on the diamond laps ranged from onewith said workpiece, imparting to said surface motion such that the points of said surface undergoing acceleration, whereby said workpiece is moved by inertial forces relative to said surface, the said method vbeing characterized by the placing on said surface of at least one member movable by said inertial forces, the dimensions of said member relative to said surface and to said workpiece being such relative to the frictional forces on said workpiece and on said member exerted by said surface that the workpiece and member contact each other and that as a result `of said contact said workpiece is moved over a substantial area of said surface.

2. The method of claim 1 wherein the member is another workpiece having the dimensions of said member as recited in claim 1.

3. A method of abrading the face of a workpiece comprising placing the workpiece on a relatively fiat plate having a vertical axis and also having an abrasive surface, leaving the workpiece free to move on the abrasive surface, eccentrically rotating the plate to move its periphery in a horizontal plane with respect to said vertical axis, each point of said plate moving in a relatively small circular path, thereby causing the workpiece to move randomly on the abrasive surface by virtue of its -own inertia.

4. The method of claim 3 wherein the circular path has a diameter of about .5 to l inch.

5. The method of claim 3 wherein the plate is rendered abrasive by containing a mass of loose abrasive material.

References Cited UNITED STATES PATENTS 622,554 4/ 1899 Smith 51--6 2,337,453 12/1943 Corey et al. 51--6 2,412,141 12/1946 Ford 51--119 2,700,259 1/1955 Dreyfus 51-119 X 2,922,264 1/ 1960 Mushrush 51-3 X 3,061,981 11/1962 Banta 51--119 3,088,247 5/1963 Mushrush 51-3 3,120,087 2/ 1964 Holloway 51-281 3,137,977 6/1964 Graves 51-119 LESTER M. SWINGLE, Primary Examiner.

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