US2371786A - Method of producing abrasives - Google Patents

Description

March 20, 1945.

A. F; TURNER METHOD OF PRODUCING ABRASIVE Filed May 31, 1943 ARTHUR F.TURNER INVENTOR.

AT TOR N EYS Patented Mar. 20,1945

METI-IOD OF PRODUUIN G ABRASIVES Arthur F. Turner, Brighton, N. Y., assignor to Bausch & Lomb Optical Company, Rochester, N. Y., a corporation of New York Application May 31, 1943, Serial no. 489,162

4 Claims. (01. 51-293) This invention relates to abrasive and polishing materials and the apparatus and 'method of forming the same.

In' certain industries, such as the optical industry, wl: ere it is highly essential that the abrasive or polishing materials used during the final grinding and polishing operations be extremely fine, considerable difilculty has been heretofore experienced in grinding the materials to the fineness desired. Not only has considerable difilculty'been experienced in grinding the materials to the fineness necessary, but it has been also diflicult to make certain that no particles larger 7 than the maximum to be tolerated will be present in the finished material. Although a number of screening and separating processes .are usually performed before the material is considered ready for use, there is always the danger that excessively large grains may be present in the material. Itwill be appreciated that several or even one excessively large grain can destroy a highly polished surface.

The present invention avoids the diiiiculties of the prior art in that the abrading material is not broken up into extremely fine particles but is actually formed at the outset of particles having a controlled desirable thickness. This is accomplished by forming the final product by a I high vacuum thermal evaporation process wherein the vapors of the abrasive-forming material are allowed to condense ona suitable support.

As the thickness of the film formed by the distilled material on the support can be controlled, t e thickness of the abrasive or polishing flakes formed by removing the film from the support can also be controlled. Thus the process of the present invention is the reverse ofthe ordinary process of forming fine abrasives wherein the difierent size particles must be separated:

out by, sedimentation or an equivalent process.

In the embodiment of the invention now preferred, the support is formed with a substantially spherical surface and is held in an .in

verted position over the-material tobe evaporated. .1'I'he support is preferably formed 01 a:

transparentmaterial such as glass so that the 'nlm being deposited can be examined. during 7 the evaporation process. v r The material to be evaporated is heldby a crucible heated from above by a heating element formed of a flat tungsten spiral and to secure an some transparent material such as glass. In thev A feature of the present invention, therefore, is the ability to'prepare a flne'polishingor abrading material of a definite and uniform thickness. Thus it is possible to form such material free of any large particles which might cause scratching or stripping.

If desired, the support on which the vapors are condensed can be used as a storage chamber after the evaporation process has been completed by sealing the film-carrying surface or two supports can be brought together to form a singlestorage container and the line of jointure sealed to prevent foreign matter from becoming mingled with the abrasive held by the storage container. If it is desired to store the material of the pres-= ent invention ready for use the material can be removed from the interior of the support and stored in a medium such as water until used;

Other features and advantages ofthe present invention will more clearly appear from the following description taken in conjunction with the accompanying drawing in which:

Fig. 1 is a sectional view partly in elevation of the apparatus of the present invention.

Fig. 2 is a sectional view partly in elevation of a modified form of the apparatus of the present invention.

I'he apparatus of the present invention, referring now to the drawing, comprises a suitable base plate Ill which supports an evacuable container shown here as a bell jar ll of glass or other material. A vacuum tight seal is formed between the lower edge of the bell Jar ll andthe base plate ID by any suitable sealing means Ill. The bell jar II is connected by some conventional means such as the pipe l3 to a high .vacuum pump shown here diagrammatically at M.

Mounted on the base plate 10 are a plurality of stanchions l5 supporting a carrier IS on which is placeda support l'l, preferably formed of illustrated embodiment of the invention, the support has been shown as. substantially hemispherical in shape although it is to be understood hat t e support can-take other forms having spherical or substantially spherical undersur faes. The carrier I8 is formed with a large central opening ll having a diameter equal to or greater than the diameter of the support ll; 'I'he material to wit in a crucible .l9 mounted above the base mouth of the be evaporated is supported plate II by any suitable support v2|. The material to be evaporated is heated from above bya heating element 2| comprising a flat spiral of tungsten wire supported above the crucible l9 by small rods 22 of current conducting material carried by suitable bushings 23 of insulating material mounted in apertures in the base plate I 0. The heating element 2| is connected through the posts 22 and lead-in conductors 24 to a source of current, not shown.

In carrying out the process of the present invention after the apparatus has been set up as disclosed-in Fig. 1, the bell jar is sealed and evacuated by means of the pump it. After the bell v jar has been evacuated and the pressure reduced therein to the minimum desired, the heating element 2| is energized to heat the material disposed in the crucible to the evaporating temperature thereof, whereafter the vapors of the material will travel toward and condense on the under surface of the hemispherical support l1. This process would be carried on until the material held by the crucible I 9 was completely evaporated or until the film formed by the condepsed vapors on the undersurface of the support I! had reached the desired thickness.

It has been found that a uniform deposit may be obtained from the relatively large flat source formed by the exposed surface of the material held by the crucible 19- if the latter is so mounted that the surface of the material is substantially tangent to the sphere of which the spherical undersurface of the support forms a part. This relationship can be brought about by mounting the crucible relative to the support'so that the surface of material is spaced downwardly from the vertex of the surface to be coated a distance equal to substantially twice the radius of curvature of the surface of the support. That the thickness of the film deposited is uniform will be evident by the uniformity of the interference colors over the entire surface on which the film is being deposited.

It has been found that the deposited film generally begins to loosen and crack away from the surface of the support when the film reaches substantially one micron in thickness. The evaporation is continued, however, beyond this point and preferably until the film reaches a thickness of approximately two to three microns.

' The thickness can be checked by counting interference bands on a small section of the sup port which is shadowed from the direct molecular beam from the material being evaporated by a small shield 25 supported adjacent a portion of the undersurface of the support I! by a bracket 26 carried by one of the stanchionslS. It has been found that the deposit formed in the shadow of the shield can be built upto a much greater thickness without the film cracking and pulling loose from the supporting surface. The interference bands are counted adjacent the edge of the shadow where the thickness of the film deposited is nearly that of the film formed on the remainder of the undersurface of the support.

After the film has reached the desired thick- :ness the bell jar, can beremoved from the base sealed and the abrasive material stored in the storage container so formed until ready for use.

The abrasive or polishing flakes formed when the film is removed from. the surface of the support will vary in area but will never have a thickness greater than the controllable thickness of the deposited film. The lateral dimensions of the flakes will change as they are broken up during the polishing operation but as the thickness of the fiake can be controlled by the film forming operations, polishing material can be formed which is absolutely free of any particles'having a thickness greater than a predetermined one and which might scratch or otherwise mar the surface being polished.

Any material which can be evaporated and which has abrasive or polishing qualities can be used to form the materialof the present invention. I have found magnesium fluoride and sodium aluminum fluoride to be readily evaporable and both form excellent polishing materials when the finished material is formed by the process of l the present invention. Abrasive materials of the two fluorides mentioned have been very successfully used for polishing calcite. 'Stannic oxide, often used for polishing steel and like metals, has been found to evaporate readily and extremely fine polished metallic surfaces have been attained by using distilled material formed by evaporatingthis oxide.

In polishing material heretofore used, it has usually been found desirable to use a polishing support could be used as long as the thickness of the distilled material was not allowed .to exceed the maximum thickness which could be tolerated. In Fig. 2, there is shown a modified form of the support ll. In this form, the supmay be placed together with the line of juncture 15 port comprises a hollow spherical member 3|] having an opening 3| in the wall thereof, just large enough to permit a crucible 32 and a heating element 33 with its support to be placed therein. Inthis form of the present invention,

the member' 30 can be either placed in an evacuable container such as the bell jar I I or the openin'g 3| sealed and the member 30 itself evacuated;

It will be noted that the crucible 32 is again spaced downwardly from the vertex of the inner surface of the member 30 a distance substantially equalto the diameter of the same and that the inner surface of the member if extended would be substantially tangent to the surface of the material held within the crucible 32.

The opening 3! can be sealed after the completion of the evaporation process so that the distilled material may be stored therein untl it is to be used. To permit. the opening 3| to be quite easily sealed, it may be formed with a neck portion 34 and a suitable stopple placed in thebore of the neck portion. I

It is to be understood that while-the preferred embodiments of the present invention have been described and illustrated herein, the invention is not to be limited thereby but is susceptible of changes in form and detail within the scope of the appended claims.

comprises the steps of evaporating the abrasive I claim:

1. The method of forming an abrasive and polishing compound comprising the steps of evaporating an abrasive-forming material in an evacuated space in close proximity to a surface of a transparent body so that vapors from said material may condense thereon; discontinuing the uated container; collecting the vapors of said material by mounting a support adjacent to said material having a concave surface on which saidvapors may condense; removing said support from said container; and placing a seal over said surface to'prevent foreign matter from mingling with the condensed material until said material is): be removed from the surface and used.

'3. A method of preparlna an abrasive material having a predetermined flake thickness which 25 Q material in an evacuated container, condensing the vapor on a support, and determining the thickness of the layer of the material deposited on the support by observing interference bands caused by light rays which strike the condensed layer of material on the support whereby the abrasive flakes may be held'to a predetermined thickness, and thereafter removing the material from the support.

4. A method of preparing an abrasive material having a predetermined maximum flake thickness which comprises the steps .of evaporating the abrasive material in' an evacuated chamber, condensingthe vapors on a surface of a transparent support, and determining the thickness of the layer of material deposited on said support by observing through the support the interference bands caused by light rays which strike the layer of material whereby theabrasive flakes may be held to a predetermined thickness, and thereafter removing the abrasive material from the support. I

ARTHUR F. TURNER.-

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