US2687603A - Method of lapping quartz crystals - Google Patents

Description

Aug. 31, 1954 F. WHITE METHOD OF LAPPING QUARTZ CRYSTALS 2 Sheets-Sheet 2 Filed June 26, 1951 Xcp xxx

INVENTOR.

Patented Aug. 31, 1954 UNlTED METHOD OF LAPPING QUARTZ CRYSTALS Fred L. White, Chicago, Ill., assignor to Crane Packing Company, Chicago, Ill., a corporation of Illinois Application June 26, 1951, Serial No. 233,535

3 Claims.

This invention relates to a method of finishing thin articles such as quartz crystals and particu larly to an improved method of lapping the sur face of such crystals.

In the electronics industry, a large number of quartz crystals are used for regulating frequencies, etc., these crystals being on the order of five-thousandths of an inch thick and having circular, rectangular or other regular geometric shape, The suitability of the crystals for the work that they are to do depends to a large extent upon the constancy of the thickness of the crystals and upon the fiatness of their surfaces. It is desirable that the fiatness of the surfaces be as perfect as possible and that the parallelism of the surfaces of the crystal be maintained within a tenth of a millionth of an inch. These standards are extremely high and are correspondingly diificult to obtain by any heretofore known means. When such accurately formed crystals are required in large quantities, however, the problem of producing them on a production basis becomes ahnost unsolvable.

Several lapping methods and machines have been proposed for lapping thin quartz crystals. In general, these devices involve the use of a fixture having a recess for each crystal, the recess being shallower than the desired thickness of the crystal.

In one form it has been proposed to exert pressure upon a crystal by the recessed fixture and by forming the surface of the recess as a lap surface. Thus both sides of the crystal theoretically are lapped at the same time, the lapping compound presumably finding its way around the crystal to the top surface thereof and the crystal rotating relative to the recess to secure an abrading action. Where the crystal is initially Wedgeshaped, the Compound will accumulate between the crystal and recess at the thin parts of the crystal and will remain there throughout the lapping operation. In order to secure parallelism, however, it is necessary to operate on the high parts of the crystal first so as to bring them down to the thickness of the thinnest part. If the crystal is uniformly lapped over its entire surface, the wedge-shape will persist throughout the lapping operation with the result that the finished crystal is no more parallel than it was initially.

It has been found that the accumulated compound acts as a filler to cause the crystal to be lapped over its entire surface and thus prevents the selective lapping necessary for parallelism.

The principal object of this invention is to provide a method of lapping thin articles such as quartz crystals, to a high degree of parallelism.

A more specific object of this invention is to provide a method of lapping thin articles such as quartz crystals which will selectively lap the thickest portions of the crystals first and will then extend the lapping operation gradually over the remainder of the surface until parallelism has been achieved.

A more specific object of this invention is to provide a method of lapping thin articles such as quartz crystals which will eliminate uncontrolled accumulations of Compound upon the surface of the crystal removed from the lap.

A still more specific object of this invention is to provide a method of lapping quartz crystals to a high degree of parallelism by lapping one side of the crystals at a time, the side opposite the lap being subjected to air pressure such that there is a constant movement of air over the thin parts of the crystal thereby fioating the crystal on a thin film of air and preventing an accumulation of lapping Compound on those thin 'portions.

These and other objects and features of this invention will become apparent from the' following detailed description when taken together with the accompanying drawings, in which Fig. 1 is a plan View of an annular lap, a. fixed cross-bar frame member for holding lapping fixtures on the lap, and a grit-distributing means;

Fig. 2 is an enlarged side elevation of the fixture used to hold the articles to be lapped;

Fig. 3 is an enlarged fragmentary section of the fixture taken through one of the crystalholding means, the section being taken along line 3-3 of Fig. 2

Fig. 4 is a bottom view of the crystal-holding means of Fig. 3 taken along line 4-4 of Fig. 3; and

Fig. 5 is a bottom view of one of the fixtures such as is shown in Fig. 2 and also in Fig. 1.

For purposes of illustration, this invention will be described With reference to the lapping of thin circular quartz crystals, but it is understood that it may be applied as well to crystals having other shapes.

Referring now to the drawings for a detailed description of the invention, there is shown a fiat annular lap n which is rotatable by suitable means (not shown) and which is provided with radially disposed serrations l I (shown more clearly in Fig. 2) the purpose of the serrations being to remove excess grit and cuttings from the surface of the lap '0. Rigidly mounted over lap IO is a framework comprising a bar '2 and a cross-bar '3, each of which has an opening therein to receive a fixed pin '4. Each pin in turn serves to hold either an article holding fixture '5 or a conditioning ring IS. The fixtures and ring are rotatable about the axis of the pin.

Conditioning ring '6 rests on the lap of its own weight and is of the type disclosed in a copending application of Earl J. Bullard, Serial No. 14,619, filed March 12, 1948 now Patent No. 2,565,590, dated August 28, 1951. l The ring is free to rotate about pin '4 under the influence of frictional forces developed between the ring and lap, and is of substantial weight so that as it rotates on the surface of lap '0 it will continually abrade the surface and thereby continually dress the lap. The details of construction and operation of ring '6 are not a part of this invention and hence will not be described here.

The grit or lapping compound is supplied to the surface of the lap from a tank l" having therein a motor-driven agitator (not shown) and from which the grit is fed through a wire '8 to the side of ring '6. The continuous rotation of ring '6 causes the lapping compound to be distributed evenly across the annular surface of lap '0.

Referring now to Fig. 2, each fixture '5 is comprised of a circular plate '9 having a recess 20 formed on the bottom thereof. A somewhat thinner plate 2' is secured to plate '9 by any suitable means (not shown) to form an air-tight joint between the two plates. In this manner a chamber 22 is formed into which air under pressure is admitted through a central passage 23. Plate '9 is secured to a short shaft 24 which is mounted in a sealed antifriction bearing 25 of any commercially obtainable type, and said bearing 25 is in turn mounted in the enlarged end 26 of pin '4. Said enlarged end has a shouldered recess 21 in which is formed a groove 28 for the reception of a ring of packing material 44 such as the well-known rubber O" ring. Said "O" ring 44 serves to provide a fiud-tight seal between the outer race of the bearing 25 and the enlarged end 26 of pin '4 and also holds the outer race of bearing 25 against rotation. Since shaft 24 has a passage 23 therein, communication is established from chamber 22 to a chamber 29 formed in the enlarged end 26 of pin I-4. Said chamber 29 in turn communicates with a central passage 30 in pin '4 and thence through a fitting 3' to a fiexible hose 32, a shutoff valve 43, pressure regulating valve 33, and an air pump 34. The pressure of the air is preferably maintained at from two and one-half to five pounds per square inch. suitable filter means for preventing the entry of foreign substances into chamber 22 through the air passages may also be provided if the air supply from pump 34 is found to contain particles of dust, dirt, etc.

Referring now to Fig. 3, it will be observed that plate 2' has a plurality of threaded openings 35 therein, each of which receives a finger-like fixture 36 having a central passageway 3" therein communicating with chamber 22. The bottom of each fixture 36 is formed as shown in Fig. 4 and comprises radially disposed slots 38 connecting the central passageway 3" with an annular groove 39, said annular groove being disposed in proximity to the outer periphery of fixture 36 but being of lesser diameter than the diameter of the crystal to be lapped. The provision of the slots 38 and annular groove 39 creates in effect a series of intervening sector shaped land portions having co-planar surfaces which are, during actual lapping Operations, maintained spaced from and parallel to the plane of the rotating lap '0. In order to hold the crystal within the boundaries of finger-like fixture 36, a ring 40 is secured to the outside of fxture 36, the ring extending beyond the end of fixture 36 to form a crystal-receiving recess 4'. Said recess, however, is of less depth than the ultimate thickness of the crystal to be lapped and is of slightly greater diameter than the crystal to prevent binding and to facilitate loading and unloading the fixture.

Thus, as shown in Fig. 3, the crystal 42 is confined within the boundaries of ring 40 but extends beyond the end of ring 40 so as to contact the surface of lap '0. Also as shown in Fig. 3, crystal `42 is subjected to air pressure on its upper surface, i. e., the surface removed from lap '0, from the annular groove 39 and the radially disposed slots 38. This air pressure performs the important function of preventing the entry of lapping compound between the upper surface of crystal 42 and the end of fixture 36.

The bottom of the fixture body 36 and the confining ring 40, in combination, provide in effect a shallow cup shaped recess in which a crystal 42 is adapted to be nested. The application of air to this recess behind the crystal serves to *'float" the crystal, so to speak, on a thin film of air so that the crystal is maintained slightly spaced from the plane of the sector shaped land portions formed on the bottom of the fixture body 36. It has been found in actual practice that this film of air will tend to distribute itself uniformly behind the crystal and equalize any tendency for the rear face of the crystal to become inclined relative to the plane of the bottom surface of the fxture body 36. The inner periphery of the ring 40 will, of course, confine the crystal within the recess but will not affect the Orientation of the crystal with respect to the bottom face of the fixture body. The pressure of air maintained within the passageway 3" is carefully regulated to the desired degree. If insufiicient air pressure is maintained the crystal will not move away from the underneath face of the fixture body uniformly and chattering of the crystal may occur. If too great a pressure is maintained, the crystal will be unevenly floated in the cup-shaped recess provided for it. However, if an unduly high pressure is attained the crystal may be literally 'blown" out of the recess.

Pins [4 may be raised or lowered relative to the bar '2 so that when it is desired to remove a fixture, the end 26 of pin '4 may be pulled away from antifriction bearing 25, the latter merely sliding out past the "O" ring until it is free of the enlarged end 26. The freed fixtures |5 may be placed on a bench or other suitable support in an inverted position to facilitate removal of the finished thin crystals from the fixture and the insertion of other crystals to be lapped.

To load fixture '5 with crystals when it is removed from the machine and in an inverted position, each crystal is preferably daubed with a slight amount of oil which will cause the crystal to adhere to the end of the finger-like fixture 36. The crystals are placed in the recesses 4' and the fxture is then righted and inserted into the enlarged end 26 of pin '4. The entire fixture, including the pin, is then lowered until the crystals are in contact with the lap surface. Antifriction bearing 25 is sealed as aforesaid and is also sealed in recess 2" by "O" ring 44 so that although each fixture '5 is free to rotate about its pin '4, it is unnecessary to provide a swing joint between the pin '4 and fitting 3l.

When the machine is loaded, that is, the three fixtures shown in Fig. 1 have been mounted on their respective pins I 4, the air pressure is turned on by means of valve 43, the lap is rotated and the grit is fed to the lap from the tank I" and feed wire '8. As the lap rotates, it will induce rotation in each fixture '5, thereby promoting a uniform lapping of the crystals.

Not all of the crystals will be initially of the same thickness. If the thick crystals are all located on one side of a diameter of the fixture '5, the fixture will tip slightly and there may be a tendency for the crystals to be lapped slightly nonparallel, which of course is undesirable. It is preferred, therefore, to measure the thickness of the crystals to be lapped before they are inserted into the fixture and then to space the thickest crystals about fixture '5 as uniformly as possible. Thus it may be desirable to alternate a thick crystal with a thin one so that the forces acting on the fixture '5 Will be evenly distributed about the axis of rotation of the fixture and will therefore induce an even rotation in the fixture from the beginning of the lapping cycle. This will insure a uniform removal of material from the crystals and will promote parallelism in the finished product.

As the lapping cycle progresses, the crystals may be observed from time to time by stopping the rotation of the lap 'U and checking the fiatness of the crystals with an optical fiat. When it appears that the crystals are completely lapped, the lap is stopped, the air pressure is cut off, the pin '4 is raised and fixture '5 is removed from the enlarged end 2 6. The crystals are removed from the finger-like fixtures and other crystals to be lapped are placed in the fixture to repeat the cycle.

By the foregoing method and apparatus, circular crystals of .002" thickness have been successfully lapped to a parallelism of plus or minus one millionth of an inch in one inch of the length, the time cycle of course depending upon the condition of the crystals at the beginning of the lapping operation.

It is understood that the foregoing description is illustrative of a preferred embodiment of this invention and that the scope of this invention is not to be limited thereto, but is to be determined by the appended claims.

What is claimed is:

1. The method of lapping an article having oppositely facing surfaces which are slightly out of parallelism With each other to bring said faces into parallelism which comprises applying the article to a plane lapping surface of a rotating lap With one of said surfaces in contact with the lap and exerting pressure'on the article by establishing a planar reference surface parallel to and spaced from the plane lapping surface, and maintaining a thin film of air between said reference plane and the other surface of the article to fioat" the article from the reference surface and orient the first surface of the article so that its angular inclination or deviation from the plane of the reference surface is equal to the original angular inclination or deviation from said other surface.

2. The method of lapping an article having oppositely facing surfaces which are slightly out of parallelism with each other to bring said faces into parallelism which comprises applying the article to a plane lapping surface of a rotating lap with one of said surfaces in contact with the lap and exerting pressure on the article by establishing a planar reference surface parallel to and spaced from the plane lapping surface, and introducing air under pressure between said reference plane and the other surface of the article to "float" the article from the reference surface and orient the first surface of the article so that its angular inclination or deviation from the plane of the reference surface is equal to the original angular inclination or deviation from said other surface.

3. The method of lapping an article having oppositely facing surfaces which are slightly out of parallelism with each other to bring said faces into parallelism which comprises applying the article to a plane lapping surface of a rotating lap with one of said surfaces in contact with the lap and exerting pressure on the article by establishing a planar reference surface parallel to and spaced from the plane lapping surface, and continuously introducing air under pressure between said reference plane and the other surface of the article while allowing such air to escape between said surfaces to "float" the article from the reference surface and orient the first surface of the article so that its angular inclination or deviation from the plane of the reference surface is equal to the original angular inclination or deviation from said other surface.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 653,531 Richmond et al July 10, 1900 1,075,714 Hornig Oct. 14, 1913 1,198,403 Bagnall Sept. 19, 1916 1,214,009 Coleman Jan. 30, 1917 1,230,595 Osterholm June 19, 1917 1,851,028 Worall Mar. 29, 1932 2,080,880 Schenk May 18, 1937 2,136,036 Avery Nov. 8, 1938 2,184,955 Gerlach Dec. 26, 1939 2,187,748 Macellaro et al Jan. 23, 1940 2,274,332 Jarrett Feb. 24, 1942 2,296,883 Trask Sept. 29, 1942 2,345,884 Powers et al. Apr. 4, 1944 2,405,417 Fruth Aug. 6, 1946 2,509,211 Clement May 30, 1950 2,539,561 Wolfskill Jan. 30, 1951 2,565,590 Bullard Aug. 28, 1951

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