US2854794A - Treating sapphire objects of irregular shapes - Google Patents

Description

R. T. LUEDEMAN ING SAPPHIRE OBJECTS OF IRREGULAR SHAPES Get. 7 i95 TREAT Filed Sept. 22, 1955 ROBERT 7'. LUEDEMA/V INVENTOR.

United States Patent TREATING SAPPHIRE OBJECTS OF IRREGULAR SHAPES Robert T. Luedeman, Leonia, N. J., assignor, by mesne assignments, to Daystrom, Incorporated, Murray Hill, N. J., a corporation of New Jersey Application September 22, 1955, Serial No. 535,917 3 Claims. (Cl. 49-77) This invention relates to the manufacture of sapphire objects of irregular shapes and, more particularly, to the manufacture of jewel bearings.

Forces due to surface tension, as present in non-metallic materials when heated to a temperature near their melting points, have been used to smooth out surface irregularities in connection with glass fabrication. However, glass, in contrast to crystalline materials such as sapphire, has an extremely wide plastic range and no true melting point. Therefore, control during the heating cycle and the method of heating are not at all critical. The high melting point of sapphire, which is about 3742 F., and the necessity of heating to temperatures below but still very close to that melting point, but not closer than about 100 F. and not further removed than about 600 F., in order that the surface tension forces may operate effectively, restricts the available means of heating and makes close control mandatory.

Flame polishing has been used, at relatively uncontrolled temperatures of 3100 F. to 3650 F., for simple objects. However, on irregular objects, such as instrument jewel bearings, the action of the flame produces ripples and other surface distortion, thus usually rendering the part unsuitable for its intended use. The normal flame-fired, boule-annealing furnaces are also unsatisfactory because of poor temperature control and the possibility of contamination by particles of refractories used to line them.

In order to remedy these defects, I propose to firepolish by heating the sapphire objects in a vacuum, either by induction methods using a metallic radiating partscontainer, or by resistance elements. Vacuum is important in that it acts as an inert thermally non-conducting atmosphere. This permits the furnace to be constructed with no refractories in the hot zone, using readily controllable electric power, and with the parts not subject to high velocity gas streams.

An object of this invention is to improve on the fire polishing of crystallate materials, such as sapphire, in order to avoid surface distortion such as rippling.

An object of this invention is to manufacture sapphire V jewels, in which the final step is fire polishing in a vacuum at a temperature below but very near the melting point of the sapphire.

An object of this invention is the production of bearing jewels particularly adapted for use in electrical instruments, in which the concave or interior conical surface which receives the rotating bearing is fire polished, after the usual finishing with abrasives, to an extraordinary degree of smoothness, better than that produced by the finest abrasives, without rippling or other distortion.

These and other objects and advantages will become apparent from the following detailed description when taken with the accompanying drawings. It will be understood that the drawings are for purposes of illustration and do not define the scope or limits of the invention,

reference being had for the pended claims.

In the drawings, wherein like reference characters denote like parts in the several views:

Figure 1 is an enlarged plan view of a fire polished sapphire bearing embodying my invention;

Figure 2 is a sectional view on the line II-II of Figure l, in the direction of the arrows;

Figure 3 is a view corresponding to Figure 2, but showing the jewel bearing mounted for use in an electrical instrument; and

Figure 4 is a vertical sectional view of apparatus including a vacuum induction furnace, which may be used for treating irregular sapphire objects in accordance with my invention.

Referring to the drawings, there is shown in Figures 1 and 2, considerably enlarged, a sapphire bearing 11 which is generally cylindrical on its outer lateral surface, fiat on its lower surface, and has a concave, generally-conical surface formed as in indentation 12 in its upper surface. The vertex of the conical surface 12 is not exactly a point, but is spherical on a very small radius, such as 2 or 3 mils. The bearing spindle, or pivot, which is to be received in this cavity 12, has its end correspondingly formed, except that the tip is to a somewhat smaller radius, such as /2 mil less, and the angle of its cone is somewhat more acute. It is necessary that the bearing surface provided in the jewel be very smooth and very accurately formed, so that not only is friction minimized, but shifting of the axis of pivoting prevented.

If made of artificial sapphire, the part 11 may be cut from the raw stock or boule and shaped by known methods to take the form illustrated in Figures 1 and 2. After this, it is polished by abrasives, a very fine abrasive being last used, so as to produce a bearing surface as smooth as possible by such method. Finally, a quantity of such instrument cup jewels are placed in a refractory metal cup or crucible 13. Such a crucible may consist of such material as molybdenum, tungsten, or

latter purpose to the ap- 'other high-melting point metal.

The crucible is supported inside of a desirably transparent refractory enclosure or bell jar 14 which may consist hard glass or Vycor, which is 96% silica, desirably in the form of a cylinder with an open bottom end and closed at its top by a spherical portion 15. The enclosure 14 rests on a metal plate 16 or other support and is vacuum-sealed thereto as by means of wax or other suitable material 17. The crucible 13 is supported above the plate 16, as on a suitable hollow refractory material support 18, and the air inside the bell jar 14 may be evacuated by means of a suitable pump through the tube 19. Surrounding the enclosure 14 is a coil of wire 21 connected to a suitable source 22 of high-frequency power through a switch 23.

The apparatus described is operated by placing the sapphire parts in the crucible or cup 13, supported as illustrated in Figure 4, and a vacuum produced inside of the enclosure or hell jar 14. After production of a high vacuum, power is introduced to the coil 21, as by closing switch 23, thereby inducing circulating currents in the crucible l3 and raising its temperature desirably to between 3600 and 3620 F. Such a temperature is maintained for about 20 minutes, whereupon the power is cut off and the apparatus allowed to cool. After this, the vacuum may be relieved, making it possible to remove the bell jar 14 and take out the jewels which have been treated. Such a treatment is designed to produce the maximum smoothing of minute imperfections with minimum gross dimensional change. The temperatures and times may be increased with increasing size of the irregularities to be reduced, the particular treatment used what lower, such as 3690" F., because above a certain temperature,.dependent on the heating cycle ofthe part, the surface becomes rough and channeled. This is'probably related to the temperaturejgradients through the pieceythe relative fluidity and volatility of the surface layer, and the crystalline nature off the material. In general, I have found that best results are obtained with a relatively low temperature and a relatively long treatment time. However, the lower limit because of practical considerations appears to be within the range of 3200 F. to 34.00" F. After the treatment has been coneluded, the rateof cooling is not critical but should be low enough to avoid thermal shock.

Figure 3 shows one of the jewels 11, produced in accordance with my. invention, mounted in metal part 24. The'peripheral portion thereof is held in place in a cavity 25 by bending over the upper edge of the part 24, as indicated at 28. The part of the holding device 24 below the cavity may be threaded, as indicated at 29, for mounting it in the instrument mechanism in accordance with conventional practice.

Although a preferred embodiment only has been disclosed, it will be understood that modifications may be made, not only in the form of sapphire pieces being treated, but in the form of the apparatus used in the treatment. For example, the heating may be effected by a coil mounted within the bell jar 14, instead of without, thereby producing a better coupling. Resistance heating, rather than inductiveheating, may be employed if desired.

Having now described my invention in detail in accordance with the patent statutes, those skilled in this art will have no difliculty in making changes and modifications in the individual parts or their relative assembly in order to meet specific requirements or conditions.

Such changes and modifications may be made without departing from the scope and spirit of my invention, as set forth in the following claims.

Iclaim:

1. The method of forming a smooth wall defining a relatively deep cavity in an instrument bearing made of a crystalline material, said method comprising abrasively polishing the c'avity'to' predetermined dimensions and then heating the bearing in a vacuum to a temperature of l00-600 F. lower than the melting point of the material, the time during which the bearing remains so heated being insufiicient to produce a. dimensional change in the said cavity.

2. The invention as recited in claim 1 wherein the bearing material is sapphire and the cavity is conical.

3. The invention as recited in claim 2, including the step of cooling the bearing at a rate slow enough to avoid thermal shock.

References ('Iited in the file of this'patent UNITED STATES PATENTS 1,422,216 McDougal et al. July 11, 1922 1,537,036 Miller May 5, 1925 1,621,446 Watson Mar. 15, 1927 2,048,556 McArthur Iuly 21, 1936 2 ,398,382 Lyon Apr. 16, 1946 2,405,892 Lederer et al. Aug. 13, 1946 2,448,511 Barnes et-al. Sept. 7, 1948 2,485,553 Barnes et a1 Oct. 25, 1949 2,511,216 Miller June 13, 1950 2,546,002 Jelinek Mar. 20, 1951 2,597,469 Gatzka May 20, 1952 2,608,031 Barnesetial Aug. 26, 1952 2,641,954 Scharf et al.. June 16, 1953 2,659,181 Yenni et a1 Nov. 17, 1953 2,735,421 Cook Feb. 21, 1956 FOREIGN PATENTS 687,159 Great Britain Feb. 11, 1953

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