US2634558A - Piezoelectric crystal lapping apparatus - Google Patents

Description

April 1953 J. M. WOLFSKILL PIEZOELECTRIC CRYSTAL LAPPING APPARATUS Filed April 14, 1947 Figl,

Patented Apr. 14, 1953 UNITED STATES PATENT OFFICE PIEZOELECTRIC CRYSTAL LAPPING APPARATUS 6 Claims.

This invention relates to apparatus and method for grinding and lapping parts or pieces of material either irregular or regular in shape to relatively thin dimensions; more particularly this invention relates to the grinding and lapping of various shaped piezoelectric quartz crystals and electrodes therefor.

An object of this invention is to provide an apparatus for lapping piezoelectric crystals to extremely thin dimensions and close tolerances.

Another object of this invention is to provide an apparatus for lapping piezoelectric crystals to thicknesses of less than $5 of an inch and close tolerances.

Another object of this invention is to provide means whereby a crystal or other part or piece of material may be lapped to a thickness which is not dependent on the thickness of any carrier, holding, or driving means surrounding the crystal or other part or piece of material being lapped.

A further object of this invention is to provide an apparatus for lapping quartz crystals or similar pieces to thicknesses approaching the thickness of the abrasive particles employed.

It is another object of this invention to provide a means of accommodating all different shapes and sizes of quartz crystals or other parts to be lapped.

Still another object of this invention is to provide apparatus whereby parts may be lapped to difiicult and exacting tolerances such as are required in the grinding and lapping of piezoelectric quartz crystals to high frequencies.

A further object of this invention is to provide means for lapping piezoelectric crystals and electrodes therefor or other devices to the required degree of surface finish encountered in high frequency operation.

It is still another object of this invention to provide a means of regulating the speed of lapping the surfaces of piezoelectric crystals and/or electrodes therefor by controlling the weight thereof.

Another object of this invention is to provide a lap for piezoelectric crystal blanks in which the piezoelectric crystal blanks are placed into holes formed in a circumferentially driven circular guide member that guides the piezoelectric crystal blanks over the lap.

Still another object of this invention is to provide a lap for piezoelectric crystal blanks in which the piezoelectric crystal blanks, with weights uniformly distributed thereover, are guided by a rigid oircumferentially driven guide member over the lap.

Other and further objects of this invention will be apparent to those skilled in the art to which it relates from the following specification, drawing, and claims.

In the prior art of lapping quartz crystals or other surfaces the piece being lapped is generally handled in one of two ways. Either the piece is cemented to a fiat work holder by means of paraiiin or similar wax or other pitch, either individually or in large quantities depending on the size of the pieces, and the entire work holder is moved about over another fiat surface with an abrasive and vehicle between the aforesaid flat surface and surfaces being lapped.

Another method commonly used in the piezoelectric crystal lapping art is that in which the pieces being lapped are caused to move about between two flat surfaces by means of work holders which encircle the pieces being lapped, either closely or loosely enough to enable the pieces themselves to rotate in a space provided in the work holder.

These methods in the prior art are unduly limited insofar as the minimum thickness to which the piece or pieces may be lapped. The minimum thickness obtained by the method in which the pieces are fixed or cemented to the carrier is limited because of the distortion caused by the cementing or waxing medium on very thin pieces. The distortion takes place when the quartz crystal or other material is cemented to the work holder. The resulting lapped surfaces of the quartz or metal piece may be exceptionally flat prior to the removal thereof from the work holder but this surface may have been originally distorted in the cementing operation."

Upon removal of the quartz crystal or other material from the work holder after lapping distortions introduced therein in the course of the cementing operation are relived and the quartz crystal or other material springs back to its naturally free condition so that the surface lapped onto it, either concave or convex, may or may not be regular. In addition, the thickness of the cementing medium between the pieces being lapped and the work holder may vary to such an extent that it is impossible to lap them to any specific thickness because the fastening medium varies and furthermore there is no telling what contour the work pieces have, until after removal thereof from the work holder.

The other method in the prior art has the definite disadvantage in that the thickness to which the quartz crystals or other material being lapped may be lapped is the thickness of the carrier or work holder which must be of a thicknessand rotation of the lap. six -;to onehas been used-in this develo ment but strength to move the work pieces, that is, the quartz crystals or other material, between the laps without tearing, fracturing, or buckling. Regardless of what material such carriers or work holders are made, or regardless of the pressure on the lapping surface or between laps and work piece this disadvantage remains. There appears to be a practical limit to the thickness or thinness of the work holders of about .004" or .005" where the work holders tear, fracture, and/or buckle.

It is the purpose of this invention to disclose an apparatus for lapping piezoelectric crystals to extremely thin dimensions with the limits in minimum thickness fixed only by the precision to which the lapping plates are made. and to the maximum size of the abrasive grain used.

Briefly, this apparatus comprises a piezoelectric crystal blank guide that is rotated over a-lap such that the guide makes several rotations for each lIhe ratio of five to one or this can be varied over wide limits. The crystal blank guide is rotated by a pair of rollers-engaging the periphery thereof in such a manner that the-abrasive action on the lap remains consistent over its entire surface. One or more holes are drilled through the crystal blank guide and the crystal blanks are placed in these holes. The workpieces or crystal blanks themselves can be of any'shape laterallyand a freely floating cylinder is placed on top of each of the work pieces or piezoelectric crystals to provide pressure aga'lnsttheilap. The holes drilled in the crystal guideconflnethe work piece'and the bottomends of theifreely'floating cylinders press on the work pieces. The bottom surface of these cylinders may-be flat to produce flat work piece or crystal surfaces or they can be made convex, for instance so that most of the pressure will be in the central area of the work piece and this will produce a concave surface onithe work pieces or crystals, if it is desired. For instance, to lap a quartz crystal'concave this method would be used. If it is desired to lap one face of the crystal convex, theend of the cylinder pressing against the crystal'could be made concave in which case the pressure would be around the periphery of the crystal and this portion wouldbe lapped down faster than the center area. Inproducing extremely flat surfaces the cylinder faces themselves must .be flat so that the pressure is. uniformly distributed over the crystal or work piece surfaces. Such a. flat surface on. the cylinder face is easily established by lapping the entire crystal blank guide withits complement of cylinders without the work pieces inserted. This can becarried on prior to loading of the crystal blank guide with piezoelectric crystal blanks.

Round as well as square crystals can be lapped in this apparatus. However, if a square or rectangular shaped crystal or work piece is lapped, the sideways or lateral pressure on the corners may become excessiveif the square or rectangular crystals are positioned in the round holes of the crystal blank guide. If the crystal is extremely thin, the pressure-may be sufiicient to fracture the corners. In. such instances, the crystal blank guide may be further modified either by using a square hole originally therethrough, which is difficult .to machine, or by introducing a free cylinder with a square hole through it and in which a freely floating square or rectangular weight can be placed. In order to get around the difliculty 'of machining a square hole through a cylinder the original round cylinder may be split into two parts or may even consist of three or four parts. If it is split the two similar cylinders can be milled in the center so as to form any desired shape internally to accommodate the square or rectangular work pieces.

Further details of this invention will be apparent to those skilled in the art to which it relates from the following specification, claims and drawing in which briefly:

Figure 1 is a side view of an embodiment of this apparatus;

Figure 2 is .a fragmentary top view of Figure 1;

Figure 3 is a sectional view of the piezoelectric crystal guidingmember taken along the line 33 of Figure '2; and

Figure .4. is .a fragmentary view of a crystal guiding member adapted for lapping a plurality of crystals simultaneously, some of the crystals being round and some being rectangular.

Referring to Figure 1 of the drawing in detail reference numeral H)- designates the base plate supporting the piezoelectric crystal lapping apparatus and attachd to this base plate are the upright members i I that support the table l2. The shaft I3, the bottom end of which rests upon a thrust bearing I 4 fitted into the base 10, is provided with a worm gear l5 keyed thereto. ,This worm gear i5 is driven by the worm wheel 16 that is attached to the shaft of the motor ll resting on the aforesaid baseplate Hi. Gear l8 which forms part of the gear train including gears 18, I9, and Etis keyed to the shaft it. The lap wheel 21 having an integral extension 22 is spaced from the top of the-table by the length of the extension 22 and is'keyed to the shaft 13 by the pin 23. Roller 24' positioned above the lap wheel 25 is also keyed to the shaft I3 and this roller forms one of the peripheral driving elements for the circular piezoelectric crystal-guiding member 25. Another roller .25 keyed to the shaft 2'! to which is attached the geartt, also engages the periphery-of the. piezoelectric crystal guiding member :25 and these two rollers 24 and 26ers spaced apart a distance smaller than the diameter of the piezoelectric. crystalguiding-member 25.

The roller 2 is rotated by the motor I! through worm gear [5 and 'worm wheel 16 and the shaft l3 whereas the roller 26 is rotated by the motor I! through the aforesaid worm gear and worm wheel, shaft l3'and the gear train consisting of the gears l8, l9, and 20 that rotate the shaft '21 to which the roller 28 is attached. In this way the rollers 24' and 23 both rotate'in the same direction and cause the circular piezoelectric crystal guiding member 25 to rotate over the top face of the lap 2| as this lap is itself rotated by shaft 1 3. The guide member 25 is made of smaller diameter than the distance between the rollers 24 and 26 so that the memberit will be thrust against these rollers 24' and 26 as the lap 2| is rotated in the direction indicated by the arrow 28, but will not be permitted to pass between these rollers 24 and 25 because of its greater diameter.

The guide member 25'is provided with a hole 29 which may be positioned in the center or it may be displaced from the center thereof. A plurality of holes such as hole 29 may be provided in the guiding member 25 as will be described. Weight 30 fitting into the hole 29 is positioned therein and rests evenly upon the piezoelectric crystal blank 3| as shown in Figure 3. A coil spring 32 surrounding the stud 33 of...the .weight 30 3 and positioned between the weight 3|] and the clamping strip, is employed forthe purpose of providing additional pressure uniformly tov the piezoelectric crystal blank. The clamping strip 34 is. fastened to the grinding member.25 by themachine screws 35, and suit able. slots 36 as shown .in Figure 2 may becut intothe strip 34 sov that this strip may be slightly rotated and released from the grip of the machine screws 35 when desired.

The abrasive with a suitable vehicle, such as, kerosene, oil, water, or the like, may be fed to the piezoelectric crystal blank 3! by placing some of the abrasive and vehicle into the hole'2ii above the weight 30; This abrasive and vehicle will work down to the piezoelectric crystal blank between cylindrical surface of the weight 30 and the walls of the hole 29 during the lapping operation. Of course the abrasive and vehicle may also be placed on the top'surface of the lap 2i, and. allowed to work-to the piezoelectric crystal blank during rotation of thelap.

In Figure 4-is shown 'a' fragmentary view of a piezoelectric crystal guide member 25a; corresponding to 'theguidemember 25 shown in Figures 1, 2, and 3; havinga plurality of holes 29a for receiving'a plurality of piezoelectric crystal blanks simultaneously. In the form shown in Figure 4 a plate34a having a plurality of ears 38 spaced around the periphery thereof, is employed for applying-pressure to the weight members 30a position in the holes 29a. Each of these weight members is provided with a stem 33a which is surrounded by a spring corresponding to the spring 32 as shown in Figure 3 so that additional pressure may be provided to the weights when the plate 34a is assembled to the guide 25a. This plate 34a is attached to the guide member 25a by suitable machine screws 35a engaging the cars 38.

A portion of the plate 34a is shown cut away for the purpose of showing the cylindrical plugs 31 positioned in two of the holes 29a. These plugs 31 are split through the center and are provided with rectangular holes therethrough into which rectangular crystal blanks may be inserted whereby this apparatus may be adapted for lapping of rectangular piezoelectric crystals. In this case the weight fitting into the rectangular hole is also of rectangular configuration and is freely floating in the hole so as to press on the crystal blank uniformly over the face thereof, for the purpose of lapping a crystal blank of uniform thickness. Where it is desired to get convex crystal blanks the face of the weight is concave as pointed out above or if it is desired to obtain concave crystal blanks the face of the weight is convex as also is pointed out above.

While I have described certain embodiments of this invention in detail it is of course understood that modifications of this invention may be made without departing from the spirit and scope thereof and I do not desire to limit this invention to the exact embodiments described and i1- lustrated except insofar as these are defined by the appended claims.

I claim:

1. Apparatus for lapping piezoelectric crystal blanks to a thinness corresponding substantially to the size of the abrasive particles used in the lapping process comprising a lap plate, means for rotating said lap plate, a rigid substantially circular piezoelectric-crystal-guiding member freely resting on said lap plate, said guiding member having a thickness equal to several times the thickness of the crystalblenkiseid guidingmemher having a holeformed thereinforreceivinga piezoelectric. crystal, of ,thin wafer-like. configue ration, said hole having adiameter correspond-. ing substantially to .the, diameter of. said piezoe electric crystal, means for frictionally engaging the periphery of said guiding. member. for.ro-. tating said .piezoelectric:crystal-guiding. member over the lapping surface .of said lap plate, and means fitting into said. holefor uniformly pressing a surface ofsaidpiezoelectric crystal against said lap plate without distorting the shape of .said

piezoelectric crystal.

2. Apparatus .for lapping, piezoelectric crystal blanks to a thinness. corresponding substantially to the size of theabrasive particles usedinthe lapping process comprising .a lap plate, means for rotating said'lap plate on its axis, a piezoelectric-crystal-guidingv member freely resting on said lap, said guiding member having a hole formed therein for receiving a piezoelectric crystalof thin wafer-like configuration, saidguiding member havinga thickness equal to several times the thicknessof the crystal blank, said hole hav-' ing a diameter corresponding substantially to the diameter of said piezoelectric crystal, means for engaging the periphery of said piezoelectric-crystal-guiding member for rotating said member overthe lapping surface of said lap plate, and means fitting into saidhole'for uniformly pressing a surface of saidpiez'oelectric crystal against said lap plate without distorting the shape of said piezoelectric crystal.

3. Apparatus for lapping piezoelectric crystal blanks to a thinness corresponding substantially to the size of the abrasive particles used comprising a lap plate, means for rotating said lap plate, a piezoelectric crystal guiding member freely resting on said lap plate, said guiding member having a thickness equal to several times the thickness of the crystal blank, said guiding member having a hole formed therein for receiving a piezoelectric crystal of thin wafer-like configuration, said hole having a diameter corresponding substantially to the diameter of said piezoelectric crystal, means for frictionally engaging the periphery of said guiding member for rotating said piezoelectric crystal guiding member over the lapping surface of said lap plate, a weight fitting into said hole I for uniformly pressing a surface of said piezoelectric crystal against said lap plate without distorting the shape of said piezoelectric crystal, and a spring member for pressing said weight against said piezoelectric crystal.

4. Apparatus for lapping piezoelectric crystal blanks to a thinness corresponding substantially to the size of the abrasive particles used comprising a lap plate, means for rotating said lap plate, a piezoelectric crystal guiding member freely resting on said lap plate, said guiding member having a plurality of holes formed therein for receiving a piezoelectric crystal of thin waferlike configuration, said hole having a diameter corresponding substantially to the diameter of said piezoelectric crystal, means for frictionally engaging the periphery of said guiding member for rotating said piezoelectric crystal guiding member over the lapping surface of said lap plate, a weight fitting into said hole for uniformly pressing a surface of said piezoelectric crystal against said lap plate without distorting the shape of said piezoelectric crystal, a spring for pressing said weight against said piezoelectric crystal and a bar-shaped member for retaining said spring in place.

52 Apnaratusi fc blanks p th dimensionsand close tolerazices; comprising the cbmbination' ofi a lap; a 1- igid eu'ide memberifreelyz resting: on said lap; said guide member having; a thickness equalto several times:- the thickness' of the crystal blank; said rigid guide member having) a hole formed therethrough fon'receivingzapiezoelectric-crystal blame-(weight nieans at lea'st part1 'allys positioned said holefor unifdrmly pressingasaid piezo= electric crystal-"blankagainst said lap; means for rotating said lap and 'meaiis' for engagingthe circumference of said rigid'guide niemb'erand for rotating said r-igid guide member and the piezeelectr ic crystal blank: guided thereby-oven said el'bfifl/Cfid lap; the surface 0? the: said Weight means. pressing the saidpiezoelectric crystal blankbeing of convex configuratiom so thatsaid crystal blank is lapped to concave configuration.

6:- Apparatus for lappingpiezoelectrie crystal blankstopaper thin dimensions and close toler ances; comprising the combinatien of-a lap, at

rigid guide memberfreely resting on saidlap; said I guide member having:.- a thickness equal: to several times the thickness of the crystal blank; said'rigid" guide member having a-hole formed therethi'ough for receiving a piezoelectric crystal blank; weight means at least partial-ly positioned insaid hele for uniformly pressing? said piezoelectric crystal blank-against said-lap; means for- 30 Number rotating saidlap and means i forengaging: the

n lapiiing-i iezeelec trie cri stal circumference of said rigid g'iiid" nieniber aficl fer race-ting said r'igi'cl guide member and th'e piezelectric crystal blank guidedthe'rebyf over References Cited in the" file" of this= patent- UNIT 1 ED" STATES 1 mm Ts

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