US2668397A - Method and apparatus for correcting crystal blanks - Google Patents

Description

Feb. 9, 1954 R. F. HOLZ RICHTER ETAL ,668,397

METHOD AND APPARATUS FOR CORRECTING CRYSTAL BLANKS Filed June 29, 1951 2 She ets-Sheet l "hmm y if ll l Inventor's Robert E Ho richter Erriest E. verbe l b r imam/0% a Feb. 9, 1954 R. F. HOLZRICHTER ET AL ,668 3 7 METHOD AND APPARATUS FOR CORRECTING CRYSTAL BLANKS Filed June 29, 1951 2 Sheets-Sheet 2 ol w d 7 dz 1 d2 Q 2| Ti 1. V

Z2. 22 35 24 zzc ab 4 21 I a4 REFERENCE 614M340: (2Z0. [22d L lnveintorl'ast I zzb 22 23 R bert E 0 arm er ZZCZNON REFERENCE SURFACE Ernest E. Overbsy 5: /i ff Patented Feb. 9, 1954 METHOD .AND;ARPARATUS. FOR COR- RECTING' CRYSTALTBUANKS flobert -Holzricliter and- Ernest E1 ()verb'ey; Sandwich; 111., assignors tozTheaIameszKnights-f CompanwSand-wiclr, Ill.

nois

, a corporation of Illie- ADDIi'catiGnJ 1111929; 1951', Serial No; 234,242

7 Claima- Thepresen-t invention relatesto the manufactture of piezo-electric crystals and; moretparticue' larly'," to correctingthe' orientationon crystal.

blanks= during th'ecou-rse ofmanufacturee y In the production of' piezo=electric:. crystalszit: is* awell known= fact that the? temperature? frequency characteristics or the crystal may-:vary, considerably; depending upon" thee precise-e angle: of cut: For example;- in the-casec-ofianh'li cutcrystal to= be operated at room. temperature; orienting "the-crystal at an' angleiof .35Ldegrees; 13 minutes relative= to the z axis produces xaisatise factory temperature characteristicz. However; where --a* crystal of AT cut is-to. be employed in a crystal oven andoperated'rat.asubstantially constant temperature, preferred; characteristics are obtained when the: angle? is: on the order of 35 degrees;- 26- minutes:.. But:wherethe :crystal to be employed inaircraft or for military purposes where xvide temperatureswingssare tabeencountered, for exampl ,'from minus 55::degrees "C. to plus 90 degrees- C., the-performance:characteristics -requi-re-th'at' the angle Ofl'ClltilielbGtWEelk the above*li-mits;-- for example; vat". 35'? degrees; 19 mine utes.- It is apparent; thereforepthatchanging the angle of cut only a few minutes of arc hasra radical effect on theev operating-z; characteristics, and such angle v must be? maintained: within extremely close tolerance-swhere military; specificae tions are 'tohelmet When" crystal blanks are produced fIOl'IL a rough crystalgtheacrystal axes are, determined: by X -ray measurement, and? the individual blanks are -formed by azseriesi of parallel-cuts made at the appropriateangle employing a diamond saw. Unfortunately; sawing is .an inherently inaccurate loperation; ,and .itis practicallyj-impossi-ble :to control conditions closer than plus or minus severrminutes of arc. The-accuracy, desired in the:finalcrystal, by, contrast, is $011 the order of plus orcmi-nusoneor two minutes; lniordri to prevent-ta high rate. of rejection, v.tl ierefore,its is desirablezto :correctthe angle of orientation or a largeaportionofthe crystal blanks resultinglfrom the: sawing" operation; Correction has," in. the past; been timeconsuming andexpensivem Where crystalse are to be produced in quantity for a competitive market, many manufacturers have preferred to-throwrv away a large proportioniof the:blanks-rather than to bother, with. individue ally, correcting them. In a number i of instances efforts have beenimade to.correct orientation of aanumber of crystal blanks at; the (same time, but the schemes which have been devised have suf; fered from various drawbacks; for' example; the grinding away 'of "toomuchface material "from at correcting" orientation least a-porti'on of thecrystal undergoing correc tion;

Accordingly; it"is anobject ofthepreseht in.

vention to provide a method and -apparatus for which enablescrystal blanks to "be-"corrected "to an extremely high de-' gree of accuracy, and which may lie-employedwith equal efiectiveness =reg-ard'less of whether the amount of correction' which is irequiredis large or small. It is anothenobrjectzto provide'a; method and apparatus for crystalianglecorrece tion which enables crystals of anytyp e 20f ion to: be corrected quickly in'z a high: quantity pro'dncetioniline-and at" an: extremely: small costzper. crystal. Itisa related obj ect i: to: provide aaproe cedure and: apparatus for correctingt. crystal blanks which may be. efil'ciently' and: safelm eme ployed byrelativelysunskilledzhelp.

It? is: another object to: provide method-rand: apparatus for crystal: angle correction which? is capable of treating azrelativelyclarge nnmberiof, crystals atithe same: timeyetwhichiremovess only a'minimunramountiof quartztfromveach-ofs:them; thereby having especials application to crystals which are: extremely thinLand-LfragiIe: It 1818; related object to provide a method'and'apparatus for: crystal.- correctiorr whichnleayes: 1 theiaces of thescrystal;blank-smooth andeactive, andzwhich avoidsstheznecessitygforiextensivesubseqnentdam ping for. the: purpose: of restoring activity: and

' avoidingwtwinningi It is a: furtherrobject .to provideta-nxapparatus which; although 'of.:high;,accuracy, may beteasily andi inexpensivelyaconstructed:and which may-be employed in; various types of; conventional. lapping :machines: t

Otherobjectsand advantages: of: the invention will become: apparent as the-ldiscussionproceeds irrcon-nectiorrswith'zthe-v followingxdrawings:

Figure '1 showsea generalw view; of 'a: setup for; correctingqcrystal blanks. inraccordances' with(the; present-invention and in connection with arstand arol :type: of flapping-machine, theacorrection ringemployedz herein: being, illustrated: in exploded.

relation to the machines,

Fig; 2. is: a cross-sectional: view showing the correction ring, and the? crystal blanks. attached thereto during-the correctingoperation. v I

Fig-.1 3; shows. the 1' underside of the correction ring with .crystals closely; spacedxaround the: pe-' riphery thereof;

Fig: .4i shows'iar crystal blank. before :and after) correction with. the? removed portions: em.-.- phasized by: shading:

Fig. 5 is a fragmentary view of the correction ring and attached crystal showing the manner of correcting the first crystal face.

Fig. 6 is a similar view showing the correction of the remaining crystal face, and

Fig. '7 shows an alternative construction of correction ring used in practicing the present invention.

While the present method and apparatus are subject to various modifications and alternative constructions, it will be understood that only the preferred embodiment is disclosed herein and that the invention is not limited thereto, but includes the various modifications and alternative constructions falling within the spirit and scope of the appended claims.

Referring now to Figure 1, a lapping machine base is disclosed, which will be recognized as being of Hunt-Hoffman type extensively employed in this art. It includes a lap plate 18, having an outside rack or gear I l, and an inside gear l2. The lap plate and outside rack are stationary While the inside gear is rotatable by any suitable power source. Meshing with the inside and outside gears is a carrier 13, having a series of gear teeth Hi, to impart orbital rotation movement to the carrieras the inside gear i2 is rotated.

Eccentrically arranged in the carrier 13 is an opening 15. In accordance with the present invention, a novel correction ring it is provided which fits freely within the opening 15 and which is held in place by a weight ii. The parts are assembled as set forth in the sectional view in Figure 2. As shown in detail in the latter figure, the correction ring 16 has a frusto-conical face 20 machined on one side thereof, and a fiat face 2| machined on the remaining side. The conical angle or slope is indicated at A in Fig. 2 and is generally very small, on the order of one to forty-five minutes, depending upon the exact amount of correction which is desired.

Arranged side-by-side around the bevelled or undersurface of ring [8 are a plurality of crystal blanks 22 (see also Fig. 3). These blanks are fastened to the surface 20 by means of paraffin wax or the like.

In order to understand the manner in which correction is achieved, it will be helpful to refer to Fig. 4 which shows a typical rough crystal blank greatly enlarged and with the amount of r correction exaggerated. The crystal blank has two faces which have been designated 22a, 22?). Face 220. will be assumed to be the reference surface. It is desirable to designate a selected one of the surfaces as a reference surface since normally the faces of the blank will not be exactly parallel to one another upon completion of the sawing operation. It will be further assumed that the proper plane of orientation of the crystal blank is denoted by the line P-P which is located at the desired angle relative to the Z axis. It will be seen that the desired orientation can be achieved by removing the wedgeshaped portions which have been cross-hatched in the figure and indicated at 22c, 22d. The resulting blank, indicated by the unshaded portion in Fig.4, therefore, has an angle of orientation which is related to the reference surface by a small angle A. It is this angle A which is referred to as the angle of correction. As the discussion proceeds it will be apparent that a single reference surface is sumcient and that it is unnecessary to know the relative angle of orientation of the opposite or non-reference surface.

The angle of correction A between the reference surface and the plane of orientation of the crystal is established by employing X-ray techniques known to one skilled in this art. Employing such techniques, the crystal is mounted in the X-ray analyzer with the reference surface away from the operator. A reading is made of the number of minutes of arc between the reference surface and the desired plane of orientation of the crystal, in other words, the amount by which the reference surface is in error. To preserve the direction that the cut is to be taken, and in carrying out the present invention, a pencil mark is applied to one of the faces of the crystal, preferably while the crystal is still in the X-ray analyzer. For a purpose which will become clear as the discussion proceeds, such pencil mark is applied, not to the reference surface, but to the non-reference surface 221), the surface which faces the operator. This pencil mark, which is indicated at 23 of Fig. 4, is preferably in the form of a line which is placed on the nonreference surface 22b along the edge which is diagonally opposite from the portion of the reference surface which requires minimum correction. Since the error in parallelism is usually rather small, the mark 23, as a result, denotes that portion of the reference surfacewhich requires minimum correction. The pencil employed for the purpose is preferably hard so as to resist ordinary cleaning and handling and to be removable only as a result of the later lapping of the crystal face.

In a quantity production run of crystal blanks there will, of course, be a considerable variation in the degree of correction required. In the case of some of the blanks it will be necessary to remove material to correct the orientation of the reference surface by, say, two minutes of arc. Other blanks will require the reference surface to be corrected by three minutes of are, still others four minutes, and so on. In carrying out the present invention, all of the blanks requiring the same amount of correction are segregated to form a three-minute lot and four-minute lot, and

so on.

After all of the crystals have been analyzed and segregated, they are given an additional pencil marking on the opposite face which is, as stated, the reference surface. Preferably such marking is applied with a red pencil at a region of the reference surface which lies diagonally opposite from the first mark 23. The new mark has been designated at 24 in Fig. 4. Preferably the mark is one which is non-symmetrical and therefore not subject to reading from the reverse side of the crystal. In practice, the numeral 4 is used.

In accordance with the present invention, a group of crystal blanks from the same lot and requiring the same reference correction are mounted side-by-side in ring formation upon the conical face of a correction ring having a corresponding angle. The crystal blanks are mounted on such correction ring with their reference surfaces in contact with the ring and with the reference marks 23 on all of the blanks facing inwardly, as shown in Fig. 3. With the crystals thus mounted, it will be apparent that the desired orientation is defined by a plane which passes symmetrically through all of the crystal blanks. The portions bearing the reference marks 23 will be the last to be removed during the lapping process.

As illustrated in the fragmentary view of Fig;

theicorrection ring: is then- :placed inzthe carrier l3 with the weight I1 thereon. The lap plate is -coated with an abrasiveof fine-lapping grade suspended-in oil; water or similarvehicle. The weigh-tds such as to produce efficient cutting actionfor the particular grade of lapping material employed, the selection being a matter-Well within the: skill of :"the art:

The. correction ring. and-itsload of crystals are rota-ted With-a:.-compund orbital movement until thezwedge-shapedportion indicated at nc -in Figsa 4:and-5. is removed; The orbital movement is:compound since the carrier is moved orbitally With-respect to the lapping-plate and 'the ring ismoved 'eccentrically with-respect to-the carrier; Complete removal of the-unwanted portion 220 of the--.crystals :is indicated by-the lapping ad of all ofthepencil marks on the entire series, and the progresscan'b'e checked from time to time during the lapping process to determine when this-point has been reached. Upon completion ofthelapping step'it is found that the'lapped faces --lie in. a plane which 1 is precisely parallel to the plane ofdesired orientation'of eachof the crystals. This is the desired result; Its achievement, however, israther-surprising considering-the fact that the correction ring is only loosely guided by'the carrier and the fact that the correction ring is positioned in-the lapping machine solely by the non-reference surfaces of the crystal blanks, the orientation of which is not only undetermined-but varies somewhat from b1ank=-to--blank.

After the material 220 has been lappedaway,- the-crystal'blanks are removed from the tapered surface 20,=forexample, by heating the paraflin which hasbeen used-as 'a'cement; As a next step -in--the correction; the surfaces which have been-lapped arecementedto the-opposite or flat surface 2| of'the correction ring'as Fig. 6; and the-reference surfaces are exposed for-lapping Proper orientation is assured by placing the red pencil mark towardthe inside of thering and a final check-is made to see that the crystal is right side up by noting whether all of the marks are ,readable. Theweight I! is againapplied ;and;=the lapping machine turned on. The work is inspected from :time1to time untilit-is noted that the marks 24 have'all been removed." The time requiredfor the lapping of the first "face serves as arough guide for lapping the second.

Upon completing the lapping of the second face of the crystal blank; the faces Will'be substantially parallel to one another and the crystal will be ready for additional lapping, as required to bring it--to the desired frequency. Such additional lapping is convenientlycarried beyond the desiredfrequency point-and the faces are, as a final operation, loaded by plating them with silver or the like in order to adjust the frequency of the crystal to a high degree of accuracy.

It'will'be apparent that the above procedure enables alarge number of crystals, for example fourteen or more, to be treated simultaneously. The amount of quartz removed from each of the crystals during a the correction thereof is "en! stantiallythe same.- It'will be apparent that the amount removed is the very minimum amount necessary -in order to establish the desired degree of.:correction. It 'is to be noted that the crystals during correction acquire a lapped-finish,=.as contrastedwith-a diamond finish. As istwelkknown to one skilledin this artga lapped finisl1 ,-isv highly: desirable since .it insures a high indicated in degreeof activity; insures frequency-"stability- -over a longperiod oftim'e; and prevents=the phenomenon" known as twinning? Fiirther more, the production 0f 5 a lapped finish-greatly simplifies the subsequent finishing operations and makesfit completely unnecessary toperform; extensive lapping to repair-the damage-doneby the i heawie'r abrasives commonlyemployed in crystal correction. Ithas been necessary in the past to lap. 'as much as .010. inch from the 'faceof a crystal in order to correct the damage-doneby rough grinding operationsiand to insure a high degree of-accuracyand' stability? It will be apparent, therefore that the present -'procedure particularly applicable to high frequency --crys= tals -which areextremelythin and i which may not have sufficient quartz available to 'p'ermit the use of conventional: correction techniques:

It has been-found that'a widerange of correction is possible employing the present 'proce dures; Aslittle as one minute 'of correction may he -effected, or; if "desired; the correction may be as much as one degreeor more; Regardlessiof the degree of correction, th'e-'-fabrication"of' the' correction rings therefor is an extremely simple: precedure and one-which maybe accomplishedwith precision .vhile using only an' ordinary de' gree of care. Acorrectiorr ring may bechucked in a ccnventional lathe and an-cangle cut-taken to form the tapered surface-20. The angle n'eed not be measured directly butmay be ascertain'ed indirectly with a high-degree of accuracy simply by calipering the inner and outerrfedges of the ringand measuring the distances d1- anddziFi'g: 5). The difference between these two distances divided by the Width wot the ringis the tangent of theiangle. In'the event-thatthe angl'e produced-by a given cut isv not sufiiciently accurate; a very slight tool adjustment-maybe=made and a second cut taken.- This process may. be .repeated until' the desired angle is :achieved. If desired, thetapered surface may be-made'nsing a grinding wheel instead of a cutting-tooI- inthe tool: holder;

A graded set of rings maybe made up in-th'is manner for-the various amounts of correction, for-example, with intervals of one minute :or'two minutes of arc; Each of suchrings should pref; erably' have i the same inside and outside di'ame-e ter i to enable use of: a single carrier and single auxiliary weight- Only one-such set of -rings is required regardless of whether the correction is plus or minus, assuming the pencil marking pro cedure is uniformly followed as described above.

In operation, use of the rings is so simpleand: mechanical that-a relatively unskilled operator may beemployed, even wherethe'degree of cor rection is extremely slight and maximum accu racy-essential; The operator need-merelyzbe told to use a ring having a designated number forall of. the crystal blanks in thecontainerfhaving the corresponding number; Several lapping machines equipped with thepresent. correction rings can easily be tended at the same time;

It will be apparent to one =-skilled in the art that the invention is not limited to theme of "a ring and separable weight. If desired,- the ring may be made of heavier cross sectionso that the desired weight is incorporated therein. The present arrangement enables a singleweight-to be used with a complete set of rings.

It will also be-aparentithat -'the invention is not limited'to a correction ring .havingan' in ternall'y conical-surface but" is equally applicable to rings in whichithe': conical surface "20 'fa'ces outwardly. The crystal blanks in such case would be similarly oriented with respect to the lapping surface so that the marked portion of each blank is low and therefore the last to be removed. This is shown in Fig. '7, corresponding parts being indicated by corresponding reference numerals with subscript a. While the arrangement of Fig. 2 is preferred, either construction enables the same amount of material to be lapped from a group of diametrically arranged blanks.

- In the following claims the term ring shall be understood to mean a crystal lapping mount in which there is provision for arranging crystals in ring formation and is not limited to carriers which have a hole or opening at the center.

We claim as our invention:

1. Apparatus for correcting the angle of orientation of a plurality'of piezoelectric crystal blanks comprising in combination a correction ring having a frusto-conical surface on one side thereof which is coaxial with respect to the ring, said frusto-conical surface being of such width as to accommodate a plurality of crystal blanks arranged side by side in ring formation thereon and with the slope of said surface equal to the angular correction to be imparted to the first face of each of said crystal blanks, said correction ring having a plane face on the opposite side thereof for the mounting of said crystal blanks in ring formation for corrective lapping of the remaining faces of said crystal blanks when the ring is in inverted position, a lap plate having an extensive flat surface, a carrier having a fiat apertured face through which extends the said correction ring, said fiat face lying on the lap plate, and means operatively engaging the carrier for translating the same with orbital movement on the lap plate.

2. Apparatus for correcting the angle of orientation of a plurality of piezoelectric crystal blanks comprising in combination a correction ring having a frusto-conical surface on one side thereof which is coaxial with respect to the ring, said frusto-conical surface having a plurality of crystal blanks arranged symmetrically side by side in ring formation on such conical surface and cemented thereto, the slope of said surface being equal to the angular correction to be imparted to the presented faces of each of said crystal blanks, said correction ring having a plane face on the opposite side thereof, a lap plate having an extensive fiat surface, a carrier having a flat apertured face through which extends the said correction ring, said flat face lying on the lap plate, and means operatively engaged in the carrier for translating the same with orbital movement on the lap plate.

3. The method of correcting the angle of orientation of piezoelectric crystal blanks having a reference surface having a predetermined angle to the desired plane of orientation which comprises the steps of arranging a plurality of blanks requiring the same amount of correction side by side in ring formation with the reference surface on each of the crystal blanks symmetrically inclined in the locus of a cone having a slope equal to the magnitude of the desired correction, lapping said arranged crystal blanks simul taneously until the presented faces of the blanks form a plane surface, arranging said crystal blanks in similar ring formation but with the lapped surfaces coplanar and with the romaine ing faces presented for lapping, and then lapping said remaining faces of the crystal blanks simul- 8 taneously until such remaining faces form a plane surface.

4. The method of correcting the angle of orientation of piezoelectric crystal blanks which comprise the segregating a plurality of blanks each having a reference surface which is in error relative to the desired plane of orientation by the same angle of correction, mounting said blanks side by side in ring formation on a frusto-conical surface having a slope equal to the angle of correction, said crystal blanks being so symmetrically arranged that the desired orientation of said crystal blanks is defined by a plane passing symmetrically through all of the blanks, subjecting the presented faces of said crystal blanks to relative lapping movement on a flat lapping surface, continuing such lapping movement until all of the lapped faces of the crystal blanks lie in a common plane which is parallel to the plane of desired orientation, and then lapping the remaining faces of the crystals into parallelism with the faces which hav been previously lapped.

5. The method of correcting the angle of orientation of piezoelectric crystal blanks which comprises the steps of selecting a plurality of crystal blanks each having a reference surface which is in error relative to the desired plane of orientation by the same angle of correction, marking the non-reference of each crystal blank adjacent the edge thereof which does not require lapping away of material, mounting said blanks side by side in ring formation with their reference surfaces in contact with a frusto-conical surface having a slope equal to the magnitude of the desi'ed correction, said crystal blanks being so arranged that the marks are all exposed on the innermost edges of the crystals respectively, subjecting said crystal blanks to simultaneous lapping movement on a flat lapping surface, continuing such lapping movement until all of the marks on said crystals are obliterated indicating that the lapped faces of the crystal blanks lie in a common plane of desired orientation, and then lapping the remaining faces of the crystals into parallelism with the faces which have been previously lapped.

6. The method of correcting the angle of orientation of piezoelectric crystal blanks which comprises the steps of segregating a plurality of blanks each having a reference surface which is in error relative to the desired plane of orientation by the same angle of correction, mounting said blanks side by side on a mounting block in a two-dimensional pattern with each crystal blank being angled with respect to the mounting block by an angle equal to the desired angle of correction and with said crystal blanks being so symmetrically arranged that the desired orientation of said crystal blanks is defined by a plane passing symmetrically through all of the blanks, subjecting the presented faces of said crystal lan s to relative lapping movement on a flat lapping surface, continuing such lapping movement until all of th lapped faces of the crystal blanks lie in a common plane which is par-. allel to the plane of desired orientation, and then lapping the remaining faces of the crystals into parallelism with the faces which have been pre: viously lapped,

7. The method of grinding piezoelectric crys: tal blanks on a fiat abrasive coated lap plate which comprises selecting a plurality of crystal blanks requiring the same amount of angular correction, cementing the blanks side by side in ring formation on a symmetrically weighted mounting member having a irusto-conical surface, said frusto-conical surface having. a slope equal to the desired angle of correction and said crystal blanks being symmetrically arranged on the frusto-conical surface for corrective removal of a wedge-shaped face portion on each of the blanks, and then transporting the mounting member across the lap plate with the presented faces of the crystal blanks in contact with the lap plate and with the mounting member free to press downwardly on said crystal blanks under the force of gravity.

ROBERT F. HOLZRICHTER. ERNEST E. OVERBEY.

References Cited in the file of this patent Number UNITED STATES PATENTS Name Date Underwood Dec. 31, 1901 Hoke Mar. 13, 1928 Hunt Jan. 19, 1943 Indge Feb. 6, 1945 Penberthy June 12, 1645 Watkins Oct. 1, 1946' Ramsay Jan. 24, 1950 Heinrich Jan. 16, 1951 Wolfskill Jan. 30, 1951 OTHER REFERENCES American Machinist, June 26, 1924, pag 973.

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