US2438916A - Work holding nest - Google Patents

Description

April 6, 1948. H. HAvsTAD I 12,438,916

WORK HOLDING' NEST l Filed sept. 24, 1945' AT TORNE Y Patented Apr. 6, 1948 WORK HOLDING .NEST Harald HavstaiL Northport, N. Y., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application September 24, 1945Seral No. 618,343

This invention relates to a lapping mechanism and, more particularly, to a Work holdingnest for use in such a mechanism.

An object of the invention is to facilitate the production of very thin elements such as piezoelectric crystal plates.

A more specific object ofthe invention is to facilitate the simultaneous ygrinding of both major surfaces of a plurality of very thin elements such as piezoelectric crystal blanks.

It is well known that in the production of certain elements, an example of which isy quartz piezoelectric crystal plates, it is usual at one or more stages in the production to makeluse of a grinding procedure known as lapping; this `.procedure commonly involves simultaneously lapping the upper and lower surfaces of a plurality of elements and involves rotating the elements between two superposed abrasive discs or laps. The elements are usually moved around between the two stationary laps by means of a work holder in the form of a dat plate, usually referred to as the nest. In order that the elements or Work may project both above and below the surface of the nest for engagement with the abrading surfaces of the upper lap and lower lap respectively. it is obvious that the thickness of the nest' must be substantially less than that of the work. At the same time the nest must have su'fcient strength and rigidity to withstand the stresses set up when it is being rotated in its fully loaded condition. f

In the instances of the production of a certain type of quartz piezoelectric plate the problem is encountered of lapping the crystal blanks down tc a thickness of 0.1 millimeter (.004 inch). 'The invention the thin metal plate of a lapping nest is maintained in a stretched condition thereby resulting in the requiredcornbination of thinness with rigidity. l

In accordance with a specific embodi-rrlent` of the invention, a novel lapping nest comprises a thin stretched metal plate provided'with properly shaped and dimensioned apertures for re-rV ceiving the elements to be ground. The plate is i secured at its periphery to a metal ring, being maintained in a stretched condition by said ring.

A centrally locatedhub is provided on the nest for coupling to the driving arm. Preferably the 3 claims. (c1. 51-1e1l' plate is formed of e metal having e substantially' higher coefficient of thermal expansion than that of the metal from which thering is formed. This permits formation of the stretched nest by` a method which compris-es the general steps of `first heating both plate and ring to a predetermined, relatively high temperature, then attaching the plate to the ring and allowing the combination to vcool down to room temperature. The plate is` expanded by the heating considerably more ,than is the ring due tothe difference in the respective expansion coefficients so that as the combination' is cooled down to room temperature the plate'isl stretched on the ring like a drumhead withV all bulges,.bends and indentations removed. U By following the novel procedure contemplated by. the invention, nests as thin as 0.05 millimeter (0.002 inch) have been produced and used successfully/.under conditions of mass production.

A complete understanding of the arrangement contemplated by the` present invention and ap-` preciation of the various advantageous features thereof may be gained from .consideration of the` following detailed description and the attached drawings in which:`

Fig. 1 is a sectional view showing a nest of the` type contemplated by the present invention, posi tioned between `the upper and lowerlapsofj a typical lapping mechanism;

Fig. 2 is a plan .View of the nest taken on line 2-2 0f Fig. 1;

Fig. 3 is a sectionalview of the nest taken von line 3--3 of Fig. 2;`and

Fig. 4 is a view of an arrangement which may" be utilized in the assembly of the nest.

Referring now to the drawings, nest I0, which includes nest plate Il, is shown in Fig. 1 in operative position between upper lap l2 and lower lap I3; A number of elements I4, l5, I6 and Il, which we will `assume for example are thin quartz piezo-` electric crystal wafers, are shown positioned'in4 the nest.y In accordance with the usual lapping practice, nest IU is rotated while laps l2 and I3 are stationary. Force for rotating the nest is applied through crank 2| from a suitable source not illustrated. As iswell understood, the shape of the crank may be varied and the nature of the driving force may likewise be modified in various ways in order to bring about various desirable motions of the nest.

As nest In is rotated the elements I4, I5, etc.. with which the nest is loaded, are moved about, their upper'surfaces being in contactwith and `ground by the abrading face of lap I2 and their lower surfaces being similarly in contact with and ground by the opposed iabrading face of lap I3. It will be readily apparent from Fig. 1 that in order to permit this simultaneous grinding of both upper and lower surfaces of the elements I4, I5, etc., the thickness of nest plate II must be denitely less than that of the elements in order to permit said elements to extend both above and below the nest plate. It will be apparent further that the nest must have suiilcient strength to withstand the stresses set up when it is rotated in loaded condition; the resistance down.

offered to this rotation increases as the grinding, f

progresses, that is, as the surfaces of the elements are brought into closer and closer conformity to the opposed abrading faces of the laps. It will be apparent further from Fig. 1 that the nest must have sufficient rigidity to assure maintenance of its position exactly midway between the upper and lower laps, that is, the nest plate must not sag at any point and it must likewise be entirely free o-f bulges. This is a particularly exacting requirement where extremely thin elements are being ground.

According to the features of the present invention the required thinness with extreme rigidity of plate II is achieved through the novel expedient of maintaining said plate II in a stretched condition. 'I'he plate is maintained in the tensioned"condition by ring 22 to which it is attached (a preferred method of assembling the plate and ring will `be described subsequently).

Two hubs 23 and 24, riveted to plate I I at the center, provide means for attachment of driving crank 2 I.

Suitable holes are provi-ded dn plate II for reception of the quartz wafers or other elements to be-ground. The shape of these holes will depend upon the shape of the elements being ground. In the present case round holes have been illustrated for reception of round elements; rotatable eyelets may be fitted in the holes if desired to provide free relative movement between the elements and the plate. If desirable pentagonal openings may be provided when square or rectangular elements are to be carried in the nest.

A preferred method of preparing the nest assembly whereby the carrier plate is maintained in a tensioned or stretched condition will now be described with particular reference to Fig. 4.

An electrical heater 4I of the hot plate type is illustrated schematically in Fig. 4 together with current source 42 and switch 43 for controlling the heater circuit.-

A metal sheet 4.4 having a thickness of the order of one-half the thickness of the elements to be ground is first selected. Applicant has found a-phosphor bronze sheet suitable for his purpose and it will be assumedV for purposes of illustration that sheet 44 is of phosphor bronze although other suitable metals may be used. Sheet 44 should be slightly larger than the contemplated size of the completed nest, for example, in producing a nest seven inches in diameter a sheet seven and one-half inches square may` well be used. The sheet selected must be of uniform thickness and free of bows, warp or buckle.v The sheet should be rolled to obtain proper tensile strength.

A ring of solder flux about three-quarters of an inch in widthis provided on one surface of sheet or plate 44 extending to about one-quarter inch from the edges of the sheet.

.Nest ring 45, which in the present instance Y Switch 43 is now closed to energize heater 4I and the ring and plate are heated to approximately 238 C. until the solder melts and sweats to plate 44. Solder is now flowed around the outside of ring 45 as indicated in Fig. 4.

It should be noted that at this point plate 44 i has expanded considerably more as a result of the heating than has ring 45 this being due to the fact that the coeiiicient of thermal expansion of phosphor bronze is substantially higher than that of steel.

Asbestos plate 5I is now rested on the upper face of nest ring 45, and block 52; Weighing in the neighborhood of twenty pounds, is in turn placed on plate 5I. Switch 43 is then moved to open position to interrupt the energizing circuit of heater 4I. Ring 45 and plate 44 are now cooled down suiieiently to cause the solder to be thoroughly set, block 52 being left in position and other steps being taken to assure firm contact between the ring and plate 44 during this initial cooling. y

After thesolder has thoroughly solidified, the weight 52 is removed and the assembly of ring and plate is allowed to cool down to room temperature after which asbestos plate f5I is removed. Theassembled ring and plate are then removed from heater 4I and the excess portion of plate 44, i. e., the portion extending beyond the outer edge of ring 45, is trimmed away.

It was pointed out above that at the end of the heating period plate 44 had expanded considerably more as a result of the heating than had ring 45, this being due to the difference in the respective coefficients of thermal expansion. It will be apparent, therefore, as the plate is attached to the ring While in this relatively more expanded condition, that as the assembly is cooled, ring 45 will reach its normal, stable con-V dition while plate 44 is still in a partly expanded condition. Plate Y44 is held in this stretched condition by ring 45, a position in which practically all bulges, bends and indentations are removed and in which the plate is very rigid and capableof withstanding considerable handling without damage.

Plate 44 in its tensioned condition may be likened to a drumhead.

After the excess portion of plate 44 has been trimmed away as outlined above, the holes for reception of the elements to be lapped may be punched or drilled in the plate and the hub portions, for reception of the driving crank, attached thereto byriveting or other suitable method.

Nest plates as thin as 0.002 inch have been produced by the above method which have been found to be capable of standing up under the conditions encountered in the production of quartz crystal plates on a mass production basis. The nests, as produced, have the required rigidity tostand up under the stresses set up when the nest isY rotated in the lapping mechanism in a fully loaded condition. Further, the nests present a truly at surface without bulges or bends and maintain exactly the required central position between the upper and lower laps.

While certain specific embodiments of the invention have been selected for detailed description, the invention is not limited in its application to such embodiments. For example, diierent metals from those described may be utilized for the ring and the plate if they have suitable characteristics and a dilerent method of stretching the metal plate may be followed. In short, the embodiments disclosed should be taken as illustrative of the invention and not as restrictive thereof.

What is claimed is:

1. In a lapping mechanism, a worlc holding nest comprising a circular metallic plate and a metallic ning attached to the periphery of said REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,486,337 Hoke Mar. 11, 1924 1,486,341 Hoke Mar, 11, 1924 1,509,823 Blood Sept, 30, 1924 1,541,003 Smith June 9, 1925 1,541,865 Smith June 16, 1925 2,410,752 Sells et al. Nov. `5, 1946

Images