Patent GB-947176-A (GB)
Title: Improvements in or relating to polishing
Inventor: RAWSTRON GEORGE ORMEROD
IPC: B24B11/00
Filed: 1959-02-19
Abstract: 947,176. Polishing. RANK PRECISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON]. Feb. 15, 1960 [Feb. 19, 1959], No. 5736/59. Heading B3D. Polishing apparatus includes a workholder for carrying the workpiece having the surface to be polished and a polishing device having a polishing surface for engaging with the surface to be polished, the workholder and the polishing device being relatively movable to effect polishing, the workholder having an opening of a size for closely fitting the peripheral edge of the workpiece and surrounding such opening a surface which forms a substantially continuous extension of the surface to be polished when the workpiece is supported in such opening, and the polishing device comprising a piece of flexible sheet material bearing the polishing surface and means for applying fluid pressure to such piece of flexible sheet material to press the polishing surface into engagement with the workpiece surface and with the surrounding surface of the workholder with substantially constant force at all points of contact therewith. Figs. 1, 2 and 3 show apparatus for polishing a generally flat work surface, a convex work surface and a concave work surface respectively. In each case the polishing device comprises a piece of flexible material A preferably formed in two layers and consisting, e.g. of a piece of canvas-type material, which is impregnated with pitch and wood with an abrasive slurry, having a lining of plastic material impervious to air, e.g. thin sheet polythene. The piece A is fastened over one end of a generally circularsectioned metal body B having a central bore B<SP>1</SP> through which air under pressure is admitted to a chamber C formed between the piece A and the adjacent end face B<SP>2</SP> of the body B. In Fig. 1, the end face B<SP>2</SP> of the body B is substantially flat and provided around its periphery with a projection B<SP>4</SP> over which the piece A passes, being fastened to the outside wall of the body B by a securing ring D. In Fig. 2, the end face B<SP>2</SP> of the body B is flat or somewhat concave and is enclosed by a thin-walled tubular extension B<SP>5</SP> of the body B. The piece A passes over the annular end face of the extension B<SP>5</SP> and is fastened to its outside wall by a securing ring D. In Fig. 3, the end face B<SP>2</SP> of the body B is convex with a curvature somewhat less than that of the concave surface to be polished and the outside wall of the body has a rounded peripheral projection B<SP>6</SP> immediately adjacent to the periphery of the end face B<SP>2</SP>. The piece A passes over the projection B<SP>6</SP> and is fastened by a securing ring D. In each case the workholder has a surface portion forming a substantially continuous extension of the surface being polished around the periphery thereof. In Fig. 1, the workholder E has a recess E<SP>2</SP>, for receiving the workpiece F, surrounded by a flat annular surface E<SP>1</SP> closely surrounding and flush with the generally flat surface F<SP>1</SP> to be polished. When the surface to be polished is a surface of revolution, the workholder has an annular surrounding surface for the surface to be polished which is so shaped that in any plane through the axis of the surface to be polished the adjacent portions of such two surfaces have at least approximately the same curvature. Thus in the case of a surface of revolution to be polished which is at least approximately spherical, e.g. an aspheric surface which deviates by only small amounts from a basic spherical surface, the surface portion of the workholder at least approximates to an extension of such basic spherical surface. In Fig. 2, the workholder comprises a main annular body G having a generally hemispherical surface G<SP>1</SP> whose curvature is approximately equal to that of the surface H<SP>1</SP> to be polished. At the centre of its surface G<SP>1</SP> the body G has a cylindrical recess G<SP>2</SP> of just sufficient diameter to receive the workpiece H, e.g. an optical lens element, and of depth less than the thickness of the workpiece so that the surface H<SP>1</SP> stands out from the surface G<SP>1</SP>. Over its surface G<SP>1</SP> the body G carries a moulded annular layer J of resinous material closely surrounding the workpiece H and having a surface flush with and substantially of the same spherical curvature as the surface H<SP>1</SP>. In Fig. 3, the body G of the workholder has a generally hemispherical concave surface G<SP>3</SP> with a central recess G<SP>2</SP> for receiving the workpiece H. A moulded layer J of resinous material is provided around the recess G<SP>2</SP> on the surface G<SP>3</SP>. In each case, with air under pressure in the chamber C, the flexible piece A presses against the surface F<SP>1</SP> or H<SP>1</SP> to be polished and the adjacent portion of the surrounding surface of the workholder with substantially equal force at all points of contact therewith. The polishing device is so dimensioned in relation to the size and shape of the surface engaged thereby in use that the area of the piece A enclosed by the edge over which it passes is slightly greater than the area of contact of the piece with the surface which it engages. To ensure this in the case of a rounded surface to be polished, the area of piece A enclosed by the edge is made greater than the plane area enclosed by such edge by an amount depending on the curvature of the surface to be polished. To effect polishing, the workholder and the polishing device are relatively moved; to avoid edge effects at the periphery of the work surface F<SP>1</SP> or H<SP>1</SP>, the area over which polishing is effected by the inclusion of the portion of the surface of the workholder around the workpiece, i.e. the surface E<SP>1</SP> or the surface of the resinous layer J, whereby the work surface F<SP>1</SP> or H<SP>1</SP> constitutes only the central portion of the whole area polished, which central portion is polished adequately and substantially evenly. Instead of using a mass of resinous material on which to form the generally spherical surface portion thereof, such surface portion may be formed, e.g. on a metal or glass member surrounding the workpiece. Instead of securing the lined flexible piece to its supporting member in an airtight manner to form the air chamber, the lining may be in the form of an envelope contained between the flexible piece and the end face of the supporting member and having a valve connected to the passage through such supporting member. Furthermore when an air-tight envelope is employed it is not essential for the flexible piece to be secured to the supporting member, but such flexible piece may be carried by the envelope which in this case alone confines the air under pressure. Specifications 947,174 and 947,175 are referred to.