US2387135A - Method of grinding small objects - Google Patents

Description

METHODS OF GRINDING SMALL OBJECTS Oct. 1G, 1945.

Filed March 16, 1944 2 Sheets-Sheet 1 IN VEN TOR.

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Oct. 16, 1945. H. F. FRUTH METHODS OF GRINDING SMALL OBJECTS 2 sheets-sheet 2 Filed March 16, 1944 Patented Oct. 16 1945 METHOD or GRINDING SMALL OBJECTS Hal F. Fruth, Chicago, Ill., assigner to Galvin- Manufacturing Corporation, Chicago, Ill., a corporation of Illinois Application March 16, 1944, Serial No. 526,700

Claims.

The present invention relates to improved methods of grinding small fragile crystalline objects or bodies, and more particularly to improved methods and apparatus for grinding piezoelectric crystals to impart predetermined resonant frequency and activity characteristics thereto and to provide an improved crystal structure having more stable operating characteristics.

'I'his application is a continuation-in-part of copending application Serial No. 479,928, filed March 20, 1943.

In the manufacture of small crystalline articles or parts it is frequentlydesirable, if not essential, that certain face surfaces of each article or part be ground to exact dimensions and exact contour, either for the purpose of imparting desired operating characteristics to the part or to satisfy the structural requirements of Vthe particular device in which the part is to be used.l Thus in the manufacture of piezoelectric crystals, such for example, as quartz crystals, adapted for use in communication circuits and, more particularly, for

use in crystal microphones, radio transmitting i and receiving systems, and the like, the crystal blanks are iirst cut from the crystal stock and are -then finish ground to the dimensions required to provide the desired frequency and activity characteristics. .The usual crystal blank is cut in Athe form of a. wafer-like rectangular piece having ldimensions slightly larger than the desired dimensions. Each blank is then reduced to the approximate rectangular dimensions desired by lap grinding the sides and ends of the crystal on a suitable grinding wheel. All nish grinding operations are conventionally performed by hand, both the edge and face grinding operations being performed by bringing the desired surface or edge to 4bear against an abrasive surface and manually moving4 the bearing surface of the crystal across the abrading surface. For example, one very common practice is that of pressing the cryst'al face against the abrasive surface by means of a finger held against the opposite face of the crystal and moving the crystal bodily over the abrasive surface in an orbit roughly approximating a figure 8. An alternative method commonly in use, is that of supporting two or more crystals on a iiexible supporting strip, such as rubber, cork or the like, and moving the exposed faces of the supported crystals over the abrasive surface in unison. Regardless of the particular manual method employed, each crystal is, during the iinish grinding operation, periodically tested to determine the frequency characteristics thereof.

The above described methods now commonly employed to finish grind piezoelectric crystals are tediously slow, require skilled labor and hence are costly from the labor standpoint, and require the use of exceedingly fine abrading materials, such, for4 example, as optical powders and diamond dust. Moreover, the crystals obtained by using these methods are not entirely satisfactory in operation, particularly when operating under widely varying temperature conditions.

The operating difliculties that have been experienced are largely attributable to the fact that the manual and semi-mechanical methods heretofore used are essentially high contact pressure grinding methods, with the result that discontinuities, in the form of scratches, hills and valleys, filled up pores and cavities, and the like, are

produced in the crystal blank surfaces as the grinding action proceeds. Also, in the finish grinding of crystals for use in ultra-high frequency circuits, the thickness of each crystal blank is limited to a value of approximately '7 mills or less. As a consequence, high contact pressure methods of grinding, be they semi-mechanical or manual, result in excessive Waste due to crystal breakage.

It is an object of the present invention, therefore, to provide an improved method of nish grinding piezoelectric crystals which substantially obviates all of the disadvantages of conventional grinding methods previously in use.

It is another object of the present invention to provide an improved method of iinish grinding the face surfaces of small objects or bodies to predetermined dimensional and contourlstandards in batches and With a minimum expenditure of manual labor.

According to another object of the invention,

the face grinding action is obtained by tumbling'l the objects in the presence of a loose abrasive material on a variable time basis and in an improved manner such that the desired face contouring is realized.

In accordance with another object of the invention, an improved method of abrading the face surfaces of the objects is provided, such that the original face contour of the objects is retained as the abrading action proceeds, or the ture of loose abrasive material and objects to be ground that the face surfaces of the objects are only 'intermittently abraded as the agitation of the mixture proceeds.

According to a further object of the invention, 'a controlled method of finish grinding piezoelectric crystals is provided in which the flnishground crystals are produced with a minimum expenditure of manual labor and on a variable time basis to conform to a pre-established frequency standard.

It is also an object of the present invention to provide an improved method of nish grinding piezoelectric crystals in batches at high speeds and with a minimum expenditure of manual labor.

In accordance with still another object of the invention, the speed of agitation of any given mixture of crystalsand abrasive material is so controlled that-first the faces and then the edges oi the crystals are abraded,"or vice versa, all without removal of the crystals from the abrasive material.

It is a further object of the invention to utilize coarse, cheap, abrasive materials in finish grinding small Waferg-like crystalline bodies, while obviating the difficulties accompanying the use of such materials in practicing conventional grinding methods, and yet producing ground surfaces which are more highly polished and contain fewer scratches than the surfaces obtained by using optical abrasives in the practice of con-. ventional high Contact pressure manual grinding methods.

It is a still further` object of the invention to provide an improved method of finish grinding piezoelectric crystals wherein low contact pressures are at all times maintained between the crystal surfaces and the abrasive material.

The invention, both as to its organization and the methods 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 front view of improved apparatus for practicing the present improved methods;

Fig. 2 is an end view. partially in section, of

- the apparatus shown in Fig. 1;

Figs. 3. 4. 5 and 6 are end sectional views illustrating changed positions of one of the tumoi the slot formed therein and a clamping screw IIc which extends through one of the lugs andv is threaded into the other lug. At their upper ends, the legs of the yoke I3 are threaded to re.

lar bracket I2, the bearing member I8 is likebling jars embodied in the apparatus when filled Y.

with a charge of abrasive material and crystal blanks: and

Fig. 'l is a family of face surface contour curves illustrating the change in face configuration of typical crystals which is obtained by practicing the present improved methods.

Referring now to the drawings and more particularly to Figs. 1 and 2 thereof, the crvstal grinding apparatus there illustrated comprises a horizontal base I haw'ne.r two upstanding tubularbrackets II and I2 rigidly supported thereon in spaced apart relationship. At the uriner ends thereof, these two brackets pivctally support two bearing members I1 and I8 within which a drive shaft I6 is journalled. More specifically, the tu5 bular bracket I`I issplit downwardly from its upper end along one 'side thereof. and is adapted `to receive the shaft of a bearing yoke I3, which is provided with legs for supporting the bearing member I1 therebetween. The shaft portion of the yoke I3 is clamped within the split end of the bracket I I by means of two lues extending wise mounted for universal movement with respeci'. to this bracket and the base Ill. This bearing mounting arrangement permits' self alignment oi the shaft I6 within the bearing surfaces of the two bearing members I1 and I8 without exacting adjustment of thepositions of these two members. lThe shaft IS functions to support two tumbling jars 25a and 25h," each of which is of square cross sectional configuration alongA the body thereof and has a smaller neck portion suitably threaded at the end to receive an internally threaded. cover 24. To support these jars, 'face plate members I9a and I9b are ilxedly mounted upon opposite ends of the'shaft I6 for rotation therewith.' These members, in the engagement of the hub portions thereof with the ends of the bearing members II and I8 also serve to prevent axial movement ofthe shaft I6. YThe face plate portion of each member I9 carries four' one-piece construction is provided, and the anguiar base 2I may be suitably secured to the outer face surface of the associated member I9 by spot welding the inner edge thereof to the vadjacent platesurface. If the tumbling jar 25a, for ex- `ample, is within the resilient arms 20a, thebentover end portions 22a of the arms engage the base of the jar neck to hold the jar in place, If desired, a rubber band 23a may be used to increase the pressure of the arms 20a against the side walls of the jar 25a.

For the purpose of imparting rotary movement to the shaft I5, thereby to rotate the two jars 25a and 25h about the long axes thereof, a-motor 26 is provided which is rigidly mounted upon the base l0. This motor is arranged to drive the shaft I6 through a speed reducing gear train which comprises a worm gear 29 carried by the shaft I6 and engaging a worm 28 mounted for rotating with the motor rotor shaft 21. Although the motor 2G may be of any desired adjustable speed type, it is preferably an adjustable speed direct current motor, and may b e con nected for energization from a direct current source indicated by the bracketed terminals 32.

outwardly from this bracket upon opposite sides An adjustable rheostat comprising the resistor element 3U andan adjustable wiper arm 3l, .is

provided in one side of the circuit for energizing the motor 26 in order that the speed ofoperation of this motor may, within limits, be varied as desired.

In utilizing the above described apparatus to practice the present improved grinding methods, piezoelectric quartz crystals which have been cut from the crystal stock and .then machine ground to rectangular dimensions slightly larger than those desired, are mixed with a charge of loose abrasive material and a liquid cleaning agent in oneor both of the two jars 25aand 25h, following which the jars with the covers 24 screwed over the opened ends thereof are inserted between the arms 20a and 20h. Operation of the motor 26 to rotate the tumbling jars 25a and 25h at the speed established by the setting of the rheostat wiper arm 3| may now be initiated. At this point it may be noted that when a tumbling jar having given cross sectional dimensions is used, the desired face abrasion of the crystals is only obtained when the jar is rotated at a speed falling within a predetermined speed range. Thus when a square jar having side dimensions of approximately '7l/2 inches is rotated at speeds of approximately revolutions per minute and less practically no relative movement occurs between the crystal blanks of the mass and the loose abrasive material with which these blanks vare intermixed. As a result, `the grinding action is excessively low. This lack of relative movement between the crystal blanks and the abrasive material and the resulting slowness of the abrasive action, may be attributed to the fact that as rotation of the jar proceeds at such slow speeds, the entire mass of crystals,- abrasive material and liquid cleaning agent within the jar simply slides over the inner surface of the jar with only a shifting of the cross sectional configuration of the mass. As thespeed of rotation of the tumbling jar is increased, the mass of crystals, grinding material and liquid cleaning agent is frictionally carried up the leading side surface of the jar so that the upper surface of the mass becomes tilted with respect to the horizontal. When a predetermined tumbling jar speed of rotation is exceeded, the angle of inclination of the mass with respect to the horizontal exceeds the inherent angie of repose of the mass. When this critical angle of inclination is exceeded. the upper surface part of the mass breaks away from the remaining part of the mass and rides down the lower part of the mass to the bottom of the tumbling jar. More specifically, the tumbling ac.. tion which occurs when the jar is rotating at a speed above the described critical value is well illustrated in Figs. 3, 4, 5l and 6 of the drawings, wherein the disposition 'of the mass within the tumbling jar at successive angular positions of the jar is shown. From avcomparison of the mass distribution as shown by these figures, it will be observed that a mass breakdown occurs between the Fig. 5 and Fig. 6 positions of the jar, the moving part of the mass being generally indicated as overlying the division line 9 shown in Fig. 5, and that after each breakdown of the mass the upper surface thereof is leveled and then tilted in the direction of rotation of the jar.

During the sliding movement of only the upper surface part of the mass, those crystals which are included in this mass part are moved relative to the abrasive material and are subjected to a low contact pressure grinding operation at the face surfaces thereof. At this point it may be noted that after rotation of the tumbling jar at a definite speed is well established, the crystals breakdown of the mass in the manner described,

the bottom part of the massi. e. that below the line 9, remains substantially static in that very little relative movement occurs between the constituents thereof. Accordingly those crystals which are disposed at the juncture 9 between the lower static part of the mass and the upper moving part of the mass are moved relative to the abrasive material at both face surfaces thereof with the resultthat both said surfaces are ground. It has also been observed that those crystals vwhich appear at the surface of the sliding portion of the mass move toward the bottom of the mass with greater rapidity than the abrasive material included in the moving mass part. During such relative movement between the sliding abrasive and the contacting face surfaces of the crystals riding over this abrasive, abrasion of the indicated crystal faces obviously occurs.

With the tumbling jar rotating at a constant speed above the described critical value, the above described breakdown of the mass within the jar occurs intermittently at periodic intervals. iThus with a four sided jar operating at a. speed of from twenty-five to forty revolutions per minute, it has been found that the mass within the jar breaks down at a rate of` approximately three times for each revolution of the jar. During the periods separating the breakdown intervals the entire mass remains substantially static in that relatively little relative movement occurs between the abrasive material and the crystals making up the mass. During such periods, therefore, practically no abrasion of the crystal surfaces is produced. At the end of each mass breakdown operation. a new portion of the mass obviously occupies the leading position insofar as movement of the mass is concerned. It will be understood,

therefore, that successively different parts of the mass are slipped to the bottom of the pile during successive breakdown intervals. Due to this continuous shift in the relative positions of different parts of the mass, different groups of crystals are obviously included in the sliding mass parts during successive breakdown operations. It will thus be apparent that as the tumbling of the mass continues, each crystal describes an orbit which is entirely repetitious insofar as the downhill sliding part of the orbit is concerned and is somewhat repetitious insofar as movement of the crystal within the mass is concerned. It will also be apparent that each crystal is face abraded intermittently and at a rate which is considerably tend so to position themselves within the mass l slower than the periodic breakdown rate of the mass. As each mass part sliding action ends, those crystals involved therein are tumbled end over end so that diiierent face surfaces of each crystal are abraded as the tumbling proceeds. The desired change in position of each crystal required to insure the same average grinding of both faces thereof during the succeeding breakdown operations in which the crystal is involved, is further obtained by the changing position of the crystal within the mass during those intervals `when it is buried in thestatic part of the mass and hence is not involved in the breakdown operations. It has been found that for a given tumbling period, the faces of all crystals of a given mixture are substantially uniformly abraded, indicating that on. an average basis the rate and paths of circulation of the crystals through the upper and lower parts of the mass are about the same for all crystals.

More specifically considered, vthe finish grind-j ing of a given batch of quartz crystals is, in accordance with the present invention carried y-out in two or more' steps. During the first or primary step, the crystals are ground to approach the particular resonant frequency characteristic desired for each crystal. In this regard it will be understood that `the crystal blanks as initially cut are lap ground to such dimensions that the resonant frequency of each crystal is well below the particular resonant frequency which is desired, Durr.ing the primary grinding step, therefore, the extent of grinding is limited not to exceed an amount whichv will bring any of the crystals to a 'resonant frequency higher than that desired.A

It has been found that the primary grinding step may be satisfactorily 'carried out by mixing the crystal blanks with a charge ofabrasive material having the following specifications. The tumbling jar 25a, for example, is filled 5/8 full of body material granules consisting of 3-5 or 5-8 mesh mined time interval at a pre-established .tumbling rate. As the tumbling operation proceeds,.the coarse body material enhances the contact pressure obtained between the crystal faces and the abrasive material, and hence acts to speed up the grinding operation. The oil wetting agent i. e. the aerosol solution. acts to break down and remove any oil deposits or films which may be present upon the crystal faces as the tumbling action proceeds.

abrasive powder, particularly the former, in scrubbing the crystal surfaces. The end result is that the surfaces of the crystals vare thoroughly cleaned as the face grinding proceeds.

After the primary grinding for the selected time interval is completed, the crystals are segregated from the mixture, are rinsed or Washed in water. dried, measured to determine the resonant frequencies thereof,` and' are grouped or classified according to their measured resonant frequencies. In the usual case, certain of the crystals in the batch willbe found to have been ground to resonant frequencies which are so close to the desired predetermined frequency as to require no further grinding. The remaining crystals i. e. those having resonant frequencies well below the desired value, are subjected to further grinding. To this end, the non-acceptable crystals are remixed with the original charge of body material This cleaning action is en-j hanced by the action of thev body material and` that those crystals having acceptable resonant frequencies are removed from the batch of crys- -tals at the end of each grinding operation. I

`It has been found that the above described method of grinding produces an exact and predictable frequency increase for crystals of a given size on a time basis with a frequency variation of less than ten per cent. Thus when using the same body granules and operatingv at the same tumbling speed of 35 revolutions per minute, a

frequency change of from 5.5 kilocycles per hour to which a new charge of silicon carbide and and testing is repeated until the predominant f portion of the crystals have been ground to the desired resonant frequency, it*` being understood to 5.8 kilocycles per hour has been -obtained forty times in succession in the grinding of 4.3 megacycle crystals. Using this same tumbling speed, other frequency changes which have been obtained with the same degree of reliability on crys tals of the same and different sizes are outlined in the following table:

Body Freq. in- Crystal frequency granule Abrasive crease per size hour 4-6 220 silicon carbide 5035 cycles. 3-5 220 silicon carbide 5500 cycles. H 650 cycles. 5-8 125 cycles. 5-8 220 silicon carbide. 3800 cycles. 4-6 220 silicon carbide. 6500 cycles. 5-8 220 silicon carbide-. 8500 cycles.

With this data available as to the degree of frequency change per unit interval of grinding time, for crystals of a givensize, an alternative method of grinding procedure may be employed. This alternative process is carried out by classifying the crystal blanks according to their resonant frequencies before the grinding is started. More specifically, the crystal blanks are lap ground to have resonant frequencies ranging from l5 to 65 kilocycles less than the particular desired value, and are then classified in groups according to the resonant frequencies thereof so that the resonant frequencies of the crystals in i( each group are not more than 10 kilocycles apart and the frequency bands of the different groups are non-overlapping. After the classification is completed, those crystals that need the most grinding to raise the resonant frequencies thereof to the .desired value are first ground for a predetermined time interval. At the end of this interval, the crystals in the next adjacent 10 kilocycle band are added to the mix without removal of the crystals of the first group, and the grinding is continued for an additional predetermined time interval. The remaining crystals are successively added tothe mixture on a group basis in the order of increasing group frequency at the respective ends of additional predetermined grinding intervals.l After the crystals in the group requiring the least grinding have been added to the mixture, the grinding operation is continued until all crystals of the batch` require additional grinding for an additional interval of from one-half to one hour in order to raise the resonant frequencies thereof to the particular desired value. At this time the predominant portion of the crystals are from three to ten kilocycles lower than the desired frequency. The crystals of the batch are now reclassified according to the frequencies thereof, following which the grinding procedure of successively adding the crystals of different groups to the grinding mix is repeated. At the end of the first grinding op-l eration, approximately ve per cent of the crystals will be found to have the desired resonant frequencies and may be removed from the batch. At the end of the second grinding operation apare performed, the" crystals involved therein ap- A proach with increasing nearness the particular desired frequency. In order to prevent over grinding, it is desirable to reduce the grinding rate during the final grinding operations. This may conveniently be done by progressively decreasing the size of the body material used in the grinding mix. As will be evident from the above table, by decreasing the size f the body material granules, the frequency change produced per unit of grinding time is correspondingly reduced. Accordingly by decreasing the body material granules used in the successive grinding operations, the final grinding operations may be much more accurately controlled on a time basis, and hence over grinding may be avoided. If exceedingly slow grinding is desired, the abrasive powder may be omitted entirely from the mixture.

'I'he size of the body material granules employed should also be determined, in part at least, by the size of the crystal blanks to be ground. Thus the crystal blank thickness varies inversely with increasing crystal frequency. Hence, high frequency crystals are much more fragile and likely to break during the grinding operations than are low frequency crystals. This tendency toward breakage of the crystals is, moreover, increased with an increase in the size of the body granules employed. Also, and as will be evident from the above table, for a given grinding mix the frequency change produced per unit of grinding time increases rapidly with increasing resonant frequencies of the crystals. It is preferable, therefore, to use smaller granules of body material in the grinding of ultra high frequency crystals than are employed in the grinding of the lower frequency crystals. On an average basis, from three to five mesh granules of body material have been found to be entirely satisfactory in the grinding of crystals having desired resonant frequencies lower than megacycles. It has also been found that body material consisting of 4-8 mesh granules is entirely satisfactory for use in the grinding of crystals having desired resonant frequencies above 5 megacycles.

As previously indicated, the change in contour of the crystal faces occurring during the grinding operation is definitely a function of the tumbling speed. It has been found that the character of the grinding aggregate or body material, ine. the mass of each granule as determined by the size and density of the granule, is a relatively'v small factor insofar as the production of a de` sired crystal face configuration is concerned. Primarily, this configuration is determined by the tumbling speed employed in effecting the abrading action. Thus at very'low tumbling speeds below a value of approximately revolutions per minute, practically no relative movement occurs between the blanks and the abrasive material and hence an almost negligible grinding action is obtained. At increased tumbling speeds, i. e. those in excess of the critical speed at which periodic mass breakdown occurs, the amount of face grinding per unit of grinding time becomes appreciable and the grinding occurs substantially uniformly over the crystal faces, with the result Cil that the face surfaces tend to retain their orisi-nal contours as the grinding proceeds. At still highertumbling speedstheabrading action is increasingly effective toward the center of each crystal face. Accordingly, a concave configuration is imparted to each `crystal face as the grinding action proceeds. The degree of concavity increases with increasing tumbling speed. The results obtained at different tumbling speeds are graphically illustrated` by the crystal face contour curves shown on a magnified scale in Fig. 7 of the drawings. As there illustrated, the curve A represents the average faceV contour of a num- Aber of crystals which are to be tumbled at differute, the -predominant portion of the grinding occurs at the centers of the crystal faces such that the initially convex face of the particular crystal in question is gradually flattened, and as the grinding operation continues a slightly concave contour is imparted thereto. The three contour curves C-I, C-2 and C--3 illustrate the-face configurations which are obtained by subjecting a typical crystal to the grinding operation for ini vtervals of l2, 24 and A36 hours at a tumbling speed of 40 revolutions per minute. These three curves clearly indicate, when compared with the corresponding curves A-l, A-Z and A-3, that the increase in tumbling speed results in an increased concentration of the grinding action at the centers of the crystal faces such that these faces more rapidly tend to assume concave configurations as the grinding action proceeds. From a study of the illustrated curves, it will be clearly apparent that at some tumbling speed below the tumbling rate 35 revolutions per minute a substantially uniform face abrading action will be obtained, such that the original face contours of the crystals will be retained as the grinding 0peration proceeds. When'a four sided tumbling jar having side dimensions of 'l1/2 inches is used, the tumbling speed at which uniform face grinding occurs has been determined to be approximately 30 revolutions per minute. As the last statement indicates, the change in face contour of the crystals is not only a function of the tumbling speed but is also a function of the. jar coniiguration and the size of the Jar. More generally defined, the changel in face contour is a function of the number of mass breakdowns which are produced in a given time interval.

Thus it has been found that over the entire tumuration and tumbling speed required to produce a desired face contour change may readily be determined with only `a rsmall amount of experimentation.

From the above explanation it will be apparent that by selecting a group of crystals having substantially matching face contours,y mixingthe crystals with a predetermined abrasive material of the character described, and tumbling the overall mixture for a predetermined time interval, a uniform and predictable change in the face contours of the crystals will be obtained.

On an experimental basis, it has been found that in the grinding of piezoelectric crystals to specific desired resonant frequencies in the manner explained above, tumbling speeds of from 25 to 40 revolutions per minute may be satisfactorily.

. is desirable to select a tumbling speed at which a tendency to produce concave face contours is provided. With a jar of the size indicated, it has been found that a tumbling speed of approximately 35 revolutions per minute will provide the required concaving tendency necessary to flatten ing interval required to raise the resonant frequency of the crystal to the desired value.

At tumbling speeds in excess of 40 to 45 revolutions per minute the above described mass breakdown action is obliterated with the result that the crystal blanks are knocked about within the tumbling jar to `bring the edges thereof into contact with .the body granules and the abrasive material; As a result, the major portion of the grinding action occurs at the crystal edges and very little face abrasion of the. crystals is produced. The end result is that at tumbling speeds in excessof approximately 45 `revolutions per minute, the edges of the crystal blanks arel beveled or chamfered without appreciable face grinding. This beveling action is extremely desirable since it serves to remove edge irregularities from the crystal blanks, with the result that .the activity of each-crystal is increased ona time controlled basis and the further result that dips or valleys are removed from the temperature-activity characteristic curve of each crystal. This ob- 'served edge grinding at high tumbling speeds may conveniently be utilized to impart the desired activity characteristics to a batch of crystals after the crystals have been face ground to the desired resonant frequency. Thus after the crystals ofla batch have been face ground to approximately the desiredresonant frequency, they 'may be tested and catalogued as tofrequency on a group basis, following which the crystals of a particular group may be returned to the tumbling jar, tumbled at a. low tumbling speed of approximately 35 revolutions per minute for vthe interval required to bring the crystals to the exact desired frequency. Thereafter, and without removing the crystals from the jar, the speed of tumbling may be lncreased to a value in excess of 50 revolutions per characteristics to a certain portion of the crystals being ground. The described process may also be reversed. Thus, the high speed tumbling for the purpose of grinding the edges oi the crystals may the usual crystal blank during the overall grindbe continued for a selected predetermined time interval calculated to impart theydesired activity very low during abrasion of these surfaces.

precede the final low speed tumbling, iil desired.

Although the methods have been descrlbedas utilizing silicon carbide as the grinding media, it

will be understood that other abrasive materials,

such, for example, as boron carbide, alundum, tungsten carbide, molybdenum carbide and tantalum carbide may be used. The only apparent limitation upon the type of abrasive material which may be used, is that the material from which thepiezoelectric crystals are formed be less hard than the abrasive material. It will also be understood that by utilizing the present improved methods of finish grinding piezoelectric crystals, the cost of the finish grinding operation ls reduced to an. exceedingly low figure. On a relative basis, it may be noted that the cost of hand grinding a crystal ranges from seventeen to twenty-live times the cost 'of finish grinding a crystal when the above described improved methods are employed. The methods disclosed herein are also characterized by the additional important feature that the nish -grinding operation may be carried out with very little waste, in that only a small percentage of the finished crystals arerejected because of being broken or overground. When` hand grinding methods are used, on the other hand, great skill is required in order to prevent crystal breakage and to prevent the crystals from being overground or, in other words, ground to a resonant frequency higher than the desired resonant frequency. Another advantage of the .present improved methods relates to the fact that in practicing these methods it is practically impossible to produce scratches or other blemishes in the crystal faces ev'en though much eoarser .grinding abrasives are used in practicing the methods than are employed in practicing conventional manual methods. This is due to the fact that the contact pressures between the abrasive material and the crystal surfaces are relatively Thus the total weight acting on the entire crystal area of each crystal to produce contact pressure between the abrasive material and the crystal face will not exceed 'I or-8 grams. In manual grinding, on the other hand, the total Weight acting on each crystal face area is usually in excess of two pounds during each excursion of the crystal across the abrasive surface. As a result, if a single coarse granule of the abrasive material or quartz becomes lodged between the crystal face and the abrasive surface, a deep scratch may be produced in the crystal face during one excursion of the crystal across the abrasive surface. 0n a comparative basis, the amount of frequency change produced in a crystal by one small manual sweep of the crystal across an abrasive nishing plate by a finisher is about equal to that produced by utilizing the present improved method with a slow frequency moving abrasive to grind the crystal yfor one hour. This indicates the diierence between the contact pressure employed in the present improved method and that normally utilized in manual nishing methods. Another advantage of the present improved methods resides in the fact that crystals may be produced thereby having rounded or beveled edges, all surfaces of which are substantially free from hills and valleys or other surface discontinuities. As a result, the crystals are less susceptible to sudden and transient frequency changes, occasioned by subjecting the crystals yto different operating temperatures.

Although the invention has been described 'with l specific reference to a 'particular embodiment thereof, it will be understood that various modil flcations may be made therein, which are within the true spirit and scope of the invention as defined in the appended claims. v

1. The method of grinding the surfaces of small fragile objects and of imparting a predetermined surface contour to the objects as the grinding proceeds, which comprises selecting a group of said objects having substantially matching surface contours, mixing the selected objects with a loose'abrasive material, disposing the mixture within a square container having side dimensions of from seven to eight inches, and rotating said container about its long axis for a predetermined time interval at a speed within the range-of from twenty-five to forty-five revolutions per minute, the particular speed within said range being selectable to determine the character of said predetermined contour.

2. 'I'he method of grinding the faces of small fragile objects and of controlling the face contour imparted to the objects as the grinding proceeds, which comprises mixing the objects with a charge of loose abrasive material, intermittently tilting the entire mass until the angle of repose of the mass is exceeded at a selected tilting rate of from 75 to 120 tilts per minute and for a predetermined time interval, and sliding the upper part of the mass over the lower part of the mass to produce face abrasion of the objects overlying said lower part of the mass each time theangle of repose of Asaid'mass is exceeded.

3. The method of face grinding. piezoelectric crystals to establish a predetermined frequency characteristic for each crystal, which comprises mixing successively smaller portions of a. batch of crystals with loose abrasive material, agitating each mixture to produce random grinding of the crystal faces by the loose abrasive material, frequency testing the crystals at the end of each grinding period, and separating .those crystals having acceptable frequency characteristics from the batch as the frequency testing proceeds.

4. The method of face grinding piezoelectric crystals to establish a predetermined resonant frequency characteristic for each crystal, which comprises separating the crystals into groups according to the resonant frequencies thereof, mixing the crystals of the lowest frequency group with a charge of loose abrasive material, tumbling the mixture, and successively adding the crystals to the mixture group by group and in the order of increasing frequency after the mixture is tumbled for different and increasingly greater time intervals.

5. The method of imparting a predetermined resonant frequency to each crystal in a batch of piezoelectric crystals, which comprises repeatedly tumbling the crystals for predetermined time intervals in the presence of body material granules and a loose abrasive material, and progressively decreasing the size of the body material granules during succeeding tumbling steps.

6. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies and a relamixture thereby to fiow the mixture so as to cause the crystals to migrate about in and slide relative to the abrasive materia1 with low contact pressure engagement therebetween.

7. The process of producing piezoelectric crystals having a desired resonant frequency from unfinishedcrystals of lower frequency; which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies which are of a size of the order of 3 to 8 mesh and'a cutting material ,which is of a size of the order of to 250 mesh, and agitating said mixture thereby to flow the mixture so as to cause the crystals to migrate about in and slide relative to the abrasive material with low contact pressure engagement therebetween. l

8. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals having different lower resonant frequencies; which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies and a relatively fine cutting material, agitating said mixture for a predetermined time interval thereby to flow the mixture so as to cause the crystals to migrate about in and slide relative to the abrasive material with low contact pressure engagement therebetween, testing the crystals to determine the change in frequency thereof, removing from the batch those crystals having substantially said desired resonant frequency, and repeating the defined steps on successively smaller portions of the remaining crystals untilthe predominant portion of the crystals of said batch have substantially said desired resonant frequency.

9. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals having different lower resonant frequencies; which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies anda relatively fine cutting material, agitating said mixture for av predetermined time interval, thereby to-iiow the mixture so as to cause the crystals to migrate about in and slide relative to the abrasive material with low contact pressure engagement therebetween, testing the crystals to determine the resonant frequencies thereof, removing from the batch those crystals having substantially said desired resonant frequency, repeating the defined steps on successively smaller portions of the remaining crystals until the predominant portion of the crystals 0f said batch have substantially said desired resonant frequency, and decreasing the rate of grinding of said crystals during the repeated grinding steps.

l0. The method of face grinding piezo-electric crystals to change the functional characteristics thereof which comprises mixing said crystals with a charge of loose abrasive material, continuously moving the entire mass until the angle of repose thereof is exceeded which causes the cascading of the upper part of the mass over the lower part of the mass to produce face abrasion of the crystals disposed above the juncture between the two mass parts each time the angle of repose is exceeded, and with said crystals migrating in the mass during the cascading action, and during portions of the cascading action changing the positions of the crystals at random in such a way that over an entire grinding operation different faces are abraded as one part of the mass cascades relative to another part of the mass.

1I. The 'method of grinding the edgesand the faces of a batch of piezo-electric crystals to de` sired functional characteristics which comprises mixing the crystals with acharge of loose abrasive material, tumbling the entire mass 'at a rela-- tively low rotational speed to grind the faces o! said crystals to a'. desired frequency characteristic, and then tumbling the entire mass at a higher rotational speed to grind the edges of said crystais to attain the desired functional character- V istie.

,12. The process of imparting a desired func- A 'tional characteristic to piezo-electric crystals, the

characteristics of which originally differ from said desired characteristic, which comprises depositing a. batch of crystals having various functional characteristics in a container, supplying abrasive to the container to thereby provide av mixture of characteristics of which originally diier from said desired characteristic but may be changed to said desired characteristic by removing certain external portions thereof; which comprises depositing a batch of crystals having various functional characteristics diferent than desired and a charge of loose granular abrasive material in a container to provide a mixture in said container, agitating the mixture atsuch a speed that relative movement between the abrasive material and the crystals is accomplished to grind portions of the crystals and produce a change in the characteristics of the crystals, testing the crystals to determine those crystals which have attained the desired characteristic, removing from the batch those crystals having said desired characteristic, I ,and repeating the defined lsteps in the order named on successively smaller portions of said batch until said desired characteristic` is imparted to the predominant portion of the crystals'of said batch.

14. 'The process of impartingt adesired functional characteristic to piezoelectric crystals, the characteristicsof which originally differ from said desired characteristic, which comprises mixing crystals and abrasive material in a container to provide a mixture of abrasive and crystals in contact with one another in the container. agitating the mixture at such a speed that relative movement between the abrasive and the crystals is accomplished to grind outer surface portions of the crystalsv and produce a change in the characteristics 'of the crystals, meanwhile upsetting said crystals in thecontainer in a manner such that theposition of crystals for grinding is varied during the entire grinding operation, and testing the crystals to determine those crystals which have attained the desired characteristic.

15. The method of mass abrading of piezoelec- -tric crystals which comprises mixing said crystals in a containerwith a charge of loose wettish abrasive material with the latter in such quantity as to substantially surround the crystals, rotating the container at a speed within a range from approximately 10 R. P. M. to approximately 45 R. P. M. to continuously move the mixture, and bringing the mixture to a position such that the angle' of inclination of the mixture exceeds the angle of repose to cause the upper part of the mixture to cascade over the lower par-t thereof to produce abrasion of the crystals substantially at the juncture between the two .mixturegparts during the cascading action, with the crystals migrating in the mass during the cascading action, and during portions of the cascading action changing the positions of the crystals at random in such a way that over an entire abrading operation different portions of the crystals are abraded as one part of the mixture cascades relative to another part of the same.

HAL F. FRUTH.

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