US2387141A - Method of making piezoelectric crystal structures - Google Patents

Description

Oct. 16, 1945. H F TH 2,387,141

-METHOD OF MAKING PIEZOELECTRIC CRYSTAL STRUCTURES Filed July 3, 1943 2 Sheets-Sheet l r 0. c. sou/me IN VEN TOR.

H. F. FRUTH Oct. 16, 1945.

METHOD OF MAKING PIEZOELECTRIC CRYSTAL STRUCTURES Filed July 5, 1945 2.Sheets-Sheet 2 QNN v mu l WWW N 7 FUN HH /N /M4 n/ 4 v q INVENTOR; Haw i rad/w,

Patented Oct. 16, 1945 METHOD OF MAKING PIEZOELECTRIC CRYSTAL STRUCTURES Hal F. Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corporation, Chicago, IlL, a corporation of Illinois Application July 3,1943, Serial No. 493,419

Claims.

The present invention relates toimethods of making piezoelectric crystal structures and more particularly to improved methods of forming conductive electrode surfaces upon the faces of piezoelectric crystals.

One of the problems involved in the manufacture and use of piezoelectric crystals is that of providing a satisfactory electrode structure by means of which operating potentials may be applied across'the crystal faces. Many different types of crystal holders having an equal number of different electrode structures have been devised in an attempt to meet this problem. Also, various methods have been developed for bonding a suitable conducting material upon the crystal faces in order to produce integral conductive electrodes to which circuit connections may be made. When the electrodes are formed by the latter methods, however, difficulties are experienced in preparing the crystal surfaces for metalizing, coating the crystal faces to the depth required to obtain the desired resonant frequency characteristic, and preventing the formation of electrical and mechanical metallic connections between the metalized faces of each crystal incident to the metalizing operation. Moreover, v all such methods require a considerable amount of handling of the crystals and a considerable number of individual operations on each crystal,

with the result that the crystals are frequentlybroken and the further result that a. considerable amount of labor is involved in the production of the metalized faces on each crystal.

It is an object of the present invention, therefore, to provide improved methods for metalizing the faces of the piezoelectric crystal in order to impart a predetermined resonant frequency characteristic thereto and to provide electrically and mechanically separated metallic electrode surfaces upon the faces thereof.

frequency characteristics closely conforming to a desired predetermined frequency standard.

According to still another object of the invention, the improved method may be easily practiced without individual handling of the crystals to insurappening of all metaltic bonds between the metalized faces of each crystali In accordance with still another object of the invention, all steps of the improved method may be practiced with but a minimum number of individual operations on each crystal, with a corresponding decrease in the likelihood of crystal breakage, an increase in the speed of production, and a material saving in the amount of labor involved in metalizin the faces of each crystal.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a side sectional view of improved tumbling apparatus which may be utilized in practicing certain steps of the improved method;

Fig. 2 is a side view of improved apparatus which may be utilized to practice another step in the improved method;

Fig. 3 is an end sectional view of the apparatus shown in Fig. 2;

Fig. 4 is a fragmentary sectional view illustratin the disposition of metalized crystals within one of the grinding tubes embodied in the apparatus shown in Fig. l; and

Fig. 5 is an end sectional view of,the grindin tube shown in Fig. 4.

Before considering the apparatus illustrated'in the drawings, it may be pointed out that the resonant frequency characteristic of a piezoelectric crystal formed of quartz, for example, is primarily determined by the thickness and mass of the crystal. It may be pointed out further that to produce a crystal having a desired resonant frequency characteristic, it is the usual pracs tice to slice a crystal blank from a bar of crystal stock, following which the blank is rough ground on an abrasive wheel or the like to the approximate desired dimensions.- Thereafter the blank is edge ground to remove irregularities from the edges thereof and is face ground to reduce the thickness and mass thereof until the crystal is characterized by the desired resonant frequency characteristic. V

In accordance with the present invention; a batch of crystals which are to have the same resonant frequency characteristics and are to be provided with metalized electrode faces are, following the initial cutting and rough grinding operations, subjected to an additional controlled moved from the crystals, and the thickness and random grinding operation by means of which I scratches and other surface irregularities are re- .duency may require no further treatment.

a value well above the desired resonant frequency. After the random grinding step is com pleted all surfaces of each'crystal are built up by metallzing the same, whereby the mass and thickness ofeach crystal structure is sufliciently increased to lower themeasured resonant frequency ofeach crystal to a value which either equals or is 'lower than'the desired value. Following the surface metalizing operation, the

' raise the resonant frequency ofeach crystal to g the rear and front walls lid and l lb, respectively,

of the carriage ll. Suitable collars I81! and I81),

, plate is is rigidly mounted upon the right end of crystals as a batch are subjected to a controlled random edge grinding step by means of which the metallic bonds between the metalized face surfaces of each crystal are completely severed, thereby electrically and mechanically to isolate the metalizedelectrode surfaces oi each crystal one from the other. Also during the random edge grinding operation, the edges of each crystal structure arebeveled or chamfered in order to remove therefrom any irregularities or discontinuities which might tend to produce spurious oscillation of the crystal when subjected to use under certaintemperature conditions. After the random edge grinding operation is completed, the crystals are measured as to the resonant frequenciesthereof and are grouped or classified in accordance with' their 'resonant frequencies.

Those crystals having resonant frequencies which closely approximate the desired resonant fre- On the other hand, the crystals having resonant freq encies lower than the desired resonant frequency are, as groups,- subjected to random finish grinding operations on a controlled time basis such that the resonant frequencies of the crystals desired value.

The novel method, as briefly described above and referred to with greater particularity in the following explanation, may conveniently be practiced by utilizing apparatus of the character il lustrat'ed in the, drawings. More specifically, the tumbling apparatus, as shown in Fig. 1, is well adapted for use in initially grinding the crystal blanks to raise the resonant frequencies thereof above the particular desired resonant frequency. It is also well adapted for use in finish grinding the'metalized faces of those crystals which, following the edge grinding operation, arefound' to possess resonant frequencies below the desired inclusive, of the drawings, onthe other hand. is well suited for use in edge grinding the metalized crystals, for the purpose of opening the bonds between the .metalizedlface surfaces of each crystal after-the metaliz'in'g pleied.

Briefly considered, the random grinding apparatus, as shown in Fig. 1 of the drawings, comprises a base Ill having a. carriage or housing ll tiltably mounted thereon by means of hinges l2,

. which are secured to the base l0 and to the lower edge of the rear wall Ila of the carriage H. The carriage ll houses'drivlng means I: which includes a motor-supporting member ll, a rotatable shaft l8, and a-speed reducing gear train. constep is comin each class or group are accurately raised to the the shaft l5 and carries four outwardly projecting, angularly spaced apart resilient arms 20, which,.in combination, define a .nest for receiv-' ing the cup iii. The four arms-20 may be bent over from an annular base 2i, so that a oneplece constructionis provided, and the annular base 2i may be suitably secured to the outer surface of, the plate l9 by spot welding the inner edge thereof to the adjacent plate surface. With the tumbler cup It nested within the resilient arms 20, the bent over and portions 20a of the arms 20 engage the base of the neck Ilia to hold the cup It in place. If desired, a rubber band Zia may-be used to increase the pressure of the arms 20 against the side walls of the cup ii.

For the purpose of imparting rotary movement to the shaft l5, thereby to rotate the tumbler cup it, a motor 24 supported upon the member I4 is provided. This motor is arranged to drive the shaft l5 through a speed reducing gear train of the motor shaft, and that the driven gear 28 is rigidly mounted upon the tumbler-drive shaft l5. Cap screws or bolts 30 extending through holes drilled in the side walls Ila and lib of the carriage I l and threaded into the side walls of the member It may be utilized for supporting the motor 24 and the speed reducing gear train within the carriage ll.

In order to adiustably tilt the carriage H with reference to a horizontal surface upon which the base i0 is supported, thereby to change the angle of inclination of the tumbler cup IS with reference to the horizontal, adjusting means comprising-a link 3| and a'wing nut assembly 32 value. The apparatus illustrated in Figs. 2 to 5.

necting the drive shaft of the motor and the shaft IS. The shaft l6 functions to support and rotate a tumbler cup l6 which is located exteriorly ofthe carriage II. The cup I8 is of quare cross section along the body thereof and has a round neck lia suitably threaded at the end to receive an internally threaded cover 22.

More specifically, the shaft is is iournaled in is provided. More specifically, the lower end of the l nk Si is pivotally connected to an edge' ened'in order hat the carriage l l and the parts supported thereby may be tilted to any desired angular position, and then tightened to retain the carriage II ,and the tumbler cup It in the selected position.

Although the motor 24 may be of any desiredadlustable speed type, it. is preferably an adjustable speed direct current motor, and may be connected for energization from a direct. current source connected to the terminals 35. An ad- ,iustable rheostat 36 is provided in one side of the circuit for energizing the motor 34 in order that the speed of operation of the motor may be varied as desired.

Inutilizing the above described apparatus to practice the present improved methods, piezoelectric quartz crystal blanks which have been cup with the cover 22 fitted over the open end thereof is inserted between the arms 20. Operation of the motor 24 to rotate the tumbler cup at the speed established by the setting of the rheostat 35a is now initiated. During rotation of the tumbler cup, random relative movement occurs between the crystals and the abrasive material so that all surfaces of the crystals are ground or abraded. The extent of the grinding action per unit time interval is of course determined by the speed of the tumbling action as established by the speed of rotation of the cup l6, and by the character and fineness of the abrasive material used.

More specifically considered, the preparation of a given batch of quartz crystals for metalizing may be accomplished by grinding the selected crystals of the batch at a relatively high grinding speed until the resonant frequencies thereof are well above the particular resonant frequency desired for each crystal. In thisregard it will be understood that the crystal blanks asinitially cut from the crystal stock are of such dimensions that the resonant frequency of each crystal is well below the particular resonant frequency which is desired. During the preparatory grinding step, therefore, a considerable amount ofgrinding may be required in order to bring the resonant frequencies of the respective crystals to the desired high values. For this preliminary grinding step, a grinding media may be employed which comprises grains of gravel or fused alundum having granules of boron carbide or silicon carbide coated upon the surfaces thereof by a suitable vitrification process or by coating the gravel or alundum grains with an air setting sodium silicate mixture containing the particles of boron carbide or silicon carbide.

After the crystal blanks have been mixed with tent of random movement of the many crystals is approximately the same. Accordingly, the

- crystals are ground at substantially the same rate. In fact, using a grinding material of the character described above, it has been found that the resonant frequencies of the crystals may be changed at a substantially uniform rate of approximately 700 cycles per hour when the tumbling cup I! of the apparatus in use is rotated about a horizontal axis at a speed of 26 revolutions per minute. If, therefore, the approximate resonant frequencies of the unfinished crystals are known, and the particular frequency,

to which each crystal is to be ground is also known, the time interval during which the mixture is tumbled durin the preparatory grinding step may be so chosen that the resonant frequencies of all crystals in the .batch are raised to values wellabove the desired frequency.

As outlined above, after the preparatory grindinl operation is completed, the crystals are segredated from the mixture, are cleaned, and are subiected to a metallzing step whereby all surfaces of each crystal are coated with metal to a desired depth. More specifically, the metalizing of the crystal surfaces may be effected by a sputterin process utilizing gold, silver or other precious metals, which is carried out in such a manner that particles of the particular metal in use are Sii bonded to and form a coating of suitable depth over all surfaces of each crystal. Alternatively, a combination spraying and plating process may be employed. In practicing the latter method, gold, for example, is first sprayed upon the crystal surfaces and particularly the faces thereof, in any entirely indiscriminate manner, following which the gold film is plated with gold to build up the metallic coating to the desired thickness. In this regard it will be understood that no care need be exercised in preventing the metal from being deposited upon the side surfaces of the crystals. Moreover, the amount of metal which is deposited upon the surfaces of each crystal re-' quires no accuratecontrol, other than that which is necessary to insure that the coating will be of sufficient depth to decrease the frequency of the composite structure below the desired resonant frequency.

After the surface metalizing step has been completed by utilizing one of the methods described above, the metalized crystals are subjected to the edge grinding operation for the purpose. of completely severing any metallic bond which may have been formed between the metalized faces of and 3. Side members ll and ll! which are flanged at the ends thereof to overlie the end panels Illa and lllb of the carriage III are provided for rotatably supporting the carriage upon the two bracket pieces 2 and Ill. More specifically, the flanged portions of the two side members H4 and III are riveted to the carriage panels ll la and ll lb adjacent the tops thereof and carry pivot pins II! and H1 at aligned central points thereof, which are suitably journaled in bearing members respectively carried by the two bracket pieces 2 and Ill. The carriage III is arranged detachably to support a grinding tube assembly 9 which comprises six grinding tubes Ill of the same length and internal diameter. These tubes are arranged with their long axes in parallel and offset from the center of rotation of the tubes. They may be formed of-glass, metal or any other material having the required hardness to resist abrasion. Rollers ll 9 and I20 are utilized to rotatably support the tubes ll! upon the carriage Ill. To this end, six equiangularly spaced-apart and aligned openings are out through the two rollers Ill and I" for receiving the six tubes.

For the purpose of rotatably supporting the roll-- ers ill and I2. upon the carriage lll, two driving rollers l2la and I2": rigidly mounted upon a shaft I22 at opposite ends of the carriage Ill, and two idler rollers I23 mounted upon a shaft I24 at opposite ends of the carriage iii, are employed. The two shafts I22 and I2! are suitably journaled in the top corner portions of the carriage end panels Illa and lllb.

A driving motor I25, which is mounted upon the floor I II c the carriage Ill, is provided for imparting rotary movement to the rollers II! and I20, thereby to rotate the grinding tubes lit 1 about the longitudinally aligned centers oi the two rollers. This motor is equipped with a motor pulley I26 which is connected by a driving belt 12'! to drive a second pulley l2! rigidly mounted upon the roller drive shaft Hi. In order to continuously tilt the carriage iii between two extreme positions, wherein the grinding tubes lib are oppositely inclined with respect to the horiiontal, a second motor m is provided, which is crank connected to the end panel till: 01 the carriage. More specifically, the rotor shaft of this motor carries a wheel I30 having an eccentric pin iti mounted thereon which is iournaled within one end of a crank arm M2. The opposite I lit and I20. Tothis end, the rollers may be bined movements of each crystal through its enprovided with tube receiving openings having diameters slightly greater than the tube diameters, so that rubber grommets {188 may be inserted within the roller openings. The tubes lit may then be inserted through the aligned grom- :met openings of the two rollers sov that a semirigid assembly is provided which is completely removable fromthe supporting rollers Hi and i123.

' It will be noted that the upper inner edge portions of the two carriage end panels liter and llib serve as friction bearing suriaces against which the side surfaces of the rollers M9 and lid may bear to prevent movementoi the grinding tube assembly ,9 in a direction longitudinal of the grinding tubes. In other words, with the tube assembly positioned upon the rollers iii and are or the carriage, the engagement of the rollers lit and "I with. the upper inner edge portions of the and panels i i la and llib serves to prevent the tube assembly from sliding oil of thecarriage.

. In utilizing the above-described apparatus for the purpose oi grinding the edges 0! the metalized piezoelectric crystals, the crystals. are disposed within the tubes H8. For the purpose of retaining the crystals within the tubes, eachtube is provided at each end thereof with arubber stopper I31 which may .be readily removed for the purpose of inserting or withdrawing crystals from the tube chamber. In order to produce random changes in the position or each crystal relative to the tube within which it is confined, .a bailie or obstruction I3! is provided at one or more points each along each tube. Each bailie may comprise a small narrow strip of adhesive tape extending longitudinally or the tube in the manner shown in Figs. '4 and 5 of the drawings. Depending upon the type or random movement of thecrystals which is desired, the baille strips may be positioned at different points intermediate the-ends ofv each tube or may be provided at the ends or each tube. V

, in the operation or the apparatus shown in Figs. 2, 3, 4 and 5 of the drawings, each 01' the tubes III is loaded with. a charge oi metalized crystals and abrasive material in the manner aea'mei more fully explained below, aiterwhich the stoppers iii are inserted in the open ends oi the tubes for the purpose oi confining the abrasive material and crystals therewithin. -'I'his operation may be performed by removing the grinding tube assembly 9 from the carriage and holding "the same in any angular position which is convenient for loading the individual tubes with a charge of the metal crystals and abrasive material. The grinding tube assembly is then positioned upon the carriage with the rollers llland ltd in engagement with the driving rollers III and the idler rollers I23. The motors I25 and 629 are now energized for the purpose of simultaneously rotating the tubes H8 and tilting these tubes back and iorthso that the angle of inclination or the tubes with respect to the'horizontal is continuously changed. As the tube assembly 9-13 tilted, each crystal within each tube is slid longitudinally from one end of the enclosing tube to the other. The rotary movement of thetubes causes each crystal to slide over the inner surface oi the enclosing tube in a substantially circular path. As a result 0! the comclosing tube, the path followed by any selected point on any selected crystal during movement of the crystal from one end of the enclosing tube to the other is in the form or a helix. The manner in which the positions of the crystals with respect to their respective enclosing tubes "are changed at random and the edge grinding onu, 400 screen mesh grinding material maybe .uti-

stations are carried out, is fully explained below.

As will be apparent from the foregoing ex planation, each metalized crystal structure isioi substantially rectangular pattern having a length which is somewhat greater than its width. These dimensions are taken into account in determining the inner diameters of the tubes HE which are utilized in the edge grinding apparatus. Preferably, the dimensions of the-metalized crystals as related to the tube diameter are about as indicated in Fig.5 of the drawings. As there shown, the diameter 01 theillustrated tube II isso chosen that when a metalizedcrystal: is supported at its edges with its axis extending longitudinally of the tube, a second crystal may be supported thereabove with the long axis thereof extending transversely of the tube and with the adjacent suriaces of the two superimposed crystals in spaced-apart relation. With this arrangement, the upper crystal is prevented from increasing the contact pressure between the inner surface of the tube and the engagingedges oi the lower crystal, and the presence or the lower crystal beneath the upper crystal is prevented from reducing the contact pressure between the inner surface of the tube and the engaging edges of the upper crystal.

As indicated above, to condition the disclosed edge grinding-apparatus for grinding the metalized edges or the crystals, the tubes 8 areea'ch loaded with a portionof the crystal batch and a charge of abrasive material. The abrasive ma terial may comprise a small amount of diamond powder or boron carbide, for example, theparticular grinding speed being determined toa large extent by the size of the grinding particles employed Thus, it rapid grinding is desired, diamond powder of from No.--11o toNo. screen mesh may be used. On the other hand, ifv less rapid grinding and a higher polish of the chamiered surfaces are to be obtained, No. 320 to No;

lized. The amount ofgrinding material usedin each tube will of course depend upon the dimensions of the tube. For example, when tubes having a length of about 60 inches and an internal diameter of 1.0625 inches are employed, satisfactory grinding is obtained if approximately two carats of from No. 100 to No. 150 screen mesh diamond powder is used in each tube. On the other hand, if the final grinding powder of from No. 320 to No. 400 screen mesh is employed, approximately two to three carats of the grinding material should be used in each tube.

After each tube is loaded with a batch of metalized crystals and a charge of abrasive material and the tubes H8 are suitably positioned upon the carriage ill, operation of the driving motors I25 and I30 may be initiated for the purpose of starting the grinding operation. Due to the continuous and reverse tilting movement of the tubes H8, each metalized crystal within each tube is slid longitudinally of the tube over the inner abrasively coated surface thereof. Concurrently therewith, and due to therotation of the tube assembly 8 by the motor I25, the tubes are rotated about the axial centers of the rollers H9 and I20, so that the inner surface of each tube is slid beneath the edge surfaces of each crystal which it houses. After rotation and reciprocation of the tube assembly is started, the crystals tend to align themselves along the lengths of their respective enclosing tubes with only the straight edge sections thereof bearing against the inner surfaces of their enclosing tubes. The abrasive material is of course soon -spread over the inner surface of each tube to produce an abrasive surface against which the metalized edge portions of the crystals bear. Also, the contact pressure at the points of engagement of the edges of a particular. crystal with the abrasively coated inner surface of its enclosing tube is primarily determined by the weight of the crystal. This is due to the fact that the longitudinal movement of the crystals through the tubes tends to prevent the crystals from becoming stacked one above the other. Due to the abrasive action of the abrasive material, the metal at the edges of each crystal is soon cut away as the crystals are slid over the surfaces of their enclosing tubes. As the abrading action continues, the edges of each crystal are also cut away to widen the gaps between the metalized side and face surfaces of the crystal and to remove any high points which may be present in the crystal edges. In this connection it is pointed out that the movement of the crystal edges longitudinally of the tubes prevents transverse scratches from being produced in the chamfered crystal surfaces which are soon formed at the edges of each crystal between the metalized side and face surfaces thereof. Similarly, the

sliding oi the crystal edges circumferentially of the abrasive surfaces against which they bear' prevents the chamfered surfaces from being scratched longitudinally, incident to the sliding of the crystals back and forth within their enclosing tubes. I

Preferably, the speed at which the tilting motor I29 is operated is so related to the lengths of the tubes I I8 and the angle of inclination of the tube assembly with respect to the horizontal that a metalized crystal disposed at the elevated end of a tube will just slide to the other end of the tube before the angle of inclination of the tube is reversed; It has been found that if tubes fiftyeight inches long are employed and the maximum angle of inclination with respect to the honinner surfaces thereof beneath the edges of the the maximum abrading action is produced byrotation of the tubes to slide the abrasively coated crystals which are supported upon these surfaces. However, the longitudinal movement of the crystals prevents the transverse scratches from being produced in the chamfered edge surfaces during the rapid rotation ofthe abrading surfaces.

The longitudinal movement of the metalized crystals along the tubes is also utilized to change the positions of the crystals within the tubes so that different metallically coated-edge sections thereof are brought to bear against the abrasively coated inner surfaces of the tubes. To consider one aspect of the crystal tumbling movement, it may be assumed that the uppermost crystal as shown in Fig. 5 of the drawings is slid into a position where it is transversely aligned with the baffle strip I38. As the illustrated tube H8 in which this crystal is disposed is rotated, one edge of. the strip I38 engages one side of the crystal so that during continued rotation of the tube the crystal is turned over to reverse the edges thereof which bear against the abrasively coated inner surface of the tube. The reversal in the position of the crystal within the tube may occur several times during the period when the crystal is disposed in transverse alignment with the strip I38. During this period the crystal is not only turned over so that alternate faces thereof face downwardly, but in addition is turned and for end So that all eight of the metallically coated edges thereof are at one time or another brought to bear against the abrasively coated inner surface of the tube. Incident to the turning of the crystal end for end, the metallically coated corner edges thereof are also subjected to the action oi the abrasive material so that they too become smoothly rounded. The particular set of oppositely disposed edges which may remain in contact with the abrasively coated surface of the tube during the longitudinal sliding movement of the crystal through the tube depends upon the particular position at which the crystal is brought to rest after it is moved out of transverse alignback and forth movement of the crystals with respect to the abrasive surfaces does not follow any predetermined or ascertainable pattern but differs for each crystal. On an average basis, however, it has been found that all eight edges and eight corners of each crystal are, in a given time interval, chamfered or ground to almost precisely the same extent. This time interval is 'of' sufilcient length to insure that a substantial gap will be produced between the metallic side and face surfaces at all points around the edges of the crystal. Since the abrading action is accomplished in a wholly random manner and at exceedingly low contact pressures between the engaged edge surfaces of the crystals and the abrading surfaces, the surfaces of. the chamferl produced on the edges of the crystals are almost entirely free of scratches or other irregularities and are highly polished.

As previously indicated, after the edge grinding operation is completed, the crystals are segregated from the grinding material, are classified according to the resonant frequencies thereof,

and those crystals which have frequencies below relative sliding movement between said abrasive surface and the metalized edges of said crystals,

thereby to open the metallic connection between the metalized faces of each crystal at the edges thereof which bear against said abrasive surface,

and utilizing the relative movement between said the desired value are subjected to a finish 'grinding step in order to raise the frequencies thereofto'the desired value. Apparatus of the character shown in Fig. 1 of the drawings may be employed for the final finish grinding operation of those metalizedcrystals having frequencies which are too low.- Preferably, the finish grinding step should not proceed at a too rapid rate and should be-carried out on a controlled time basis. To this end, the crystals of the respective frequency surface and the crystals to change at random the metalized edge portions of the crystals which bear against said surface, thereby to insure complete severance of the metallic connection between the metalized face surfaces of each crystal.

3. The method of' forming electrode surfaces upon the faces of piezoelectric crystals, which comprises metalizing substantially all surfaces of each crystal, supporting the crystals upon an abrasive surface with only the metalized edge portions thereof bearing against .said surface,

groups are separately mixed with difierent charges abrasive material are made in the different tumbler cups of several tumbling mills of the character shown in Fig. l'of the drawings, and each mix is separately tumbled or agitated at a predetermined tumbling rate for a particular time interval which is dependent upon the difference between the average resonantfrequency of the particular class-or group of metalized crystals and the desired resonant frequency. Assuming, for example, that there are three different classes or groups of metalized crystals which respectively contain crystal structures of different resonant frequencies, and also that the same tumbling speed is used for the different mixes, three different intervals of tumbling time are required for the three different mixes. As the final grindproducing relative sliding movement between said abrasive surface and the metalized edges of said crystals, thereby to open the metallicconnection between the metalized faces of each'crystal at the edges thereof which bear against said abrasive surface, utilizing the relative movement being' steps are completed for the crystals of the tween said surface and the crystals to change at random the metalized edge portions of the crystals which bear against said surface. thereby to insure complete severance of the metallic connection between the metalized face surfaces of' each crystal, and agitating the metalized crystals I speed that relative movement between the abra sive material and the crystal is accomplished to grind the face portions'of the metalized crystal to remove metal therefrom, and utilizing the relative movement between the abrasive material and the crystal to change at random the metalized outside portions being ground.

5. The method of grinding a piezoelectric crystal having faces and having a metalizedcoating on substantially all surfaces, which comprises grinding the metal on the edge surfaces to mechanically. and electrically disconnect the'metalized face surfaces of the crystal, mixing the crysand producing relative movement between said crystal and said surface in order to open the metallic connection between the metalized face surfaces of said crystal.

2. The method of forming electrode surfaces .upon the faces of piezoelectric crystals, which comthereof bearing against said surface, producing tal with loose abrasive material within a container to provide a mixture of crystal and abrasive material therein, agitating the mixture at such a speed that relative movement between the abrasive and the crystal is accomplished to grind metal from the metalized face surfaces'and produce a change in the frequency characteristic of the crystal thereby, and utilizing the relative movement of the. loose abrasive material and the crystal to upset the crystal in amanner such that both faces are ground during the entire operation,

HAL F. FRUTH.

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