Patent GB-940732-A (GB)
Title: Improvements in or relating to polishing
Inventor: RASTRON GEORGE ORMEROD
IPC: B24B13/005
Filed: 1959-02-18
Abstract: 940,732. Polishing. RANK PRECISION INDUSTRIES Ltd., [trading as TAYLOR, TAYLOR & HOBSON], Feb. 15, 1960 [Feb. 18, 1959; Feb. 10, 1960], Nos. 5686/59 and 4645/60. Heading B3D. A work-holder for a workpiece having a surface of revolution to be polished comprises an outer work-holder part having a spherical or an approximately spherical end surface substantially conforming to the surface of the workpiece to be polished and encircling an opening within which the workpiece closely fits, the axis of such opening being collinear with a radius of the end surface of the outer work-holder part, an inner work-holder part for supporting the workpiece within the opening, the two workholder parts being relatively movable for adjustment purposes in the direction of the axis of the opening, and removable stop means for engaging the end surface of the outer workholder part and overlying the opening therein, so as to determine the position of adjustment in which the edge portion of the workpiece surface lies accurately adjacent to the edge portion of the end surface of the outer work-holder part encircling such opening, whereby such end surface forms a substantially continuous extension of the workpiece surface, the stop means being removed when the workpiece is located in such operative position. A work-holder, Fig. 1, comprises an annular member L, e.g. of metal, having a convex hemispherical surround surface L<SP>1</SP> whose curvature is substantially equal to that of a convex spherical surface A<SP>1</SP> to be polished on a workpiece A, the inner diameter of the member L being slightly greater than that of the workpiece A. An internal screw-thread L<SP>2</SP> co-operates with an external screw-thread M<SP>1</SP> on a tube M, e.g. of metal, fitting within the member L whereby the members L, M can be relatively adjusted along the axis of the member L by relative rotation of the members L, M. A cylinder N axially aligned with the member L is movable axially towards the member L so that its annular end face N<SP>1</SP> engages the surround surface L<SP>1</SP>. The end face N<SP>1</SP> is accurately shaped to match that of the surround surface L<SP>1</SP>. The outer and inner diameters of the cylinder are greater and less than the inner diameter of the member L. The workpiece A to be polished is waxed in position to the end face M<SP>2</SP> of the tube while the latter is retracted into the part L so that the work-surface A<SP>1</SP> is below the surround surface L<SP>1</SP>. The part M is then adjusted relatively to the part L to move the workpiece A against the end face N<SP>1</SP> so that the work-surface A<SP>1</SP> is precisely flush with the surround surface L<SP>1</SP>. The cylinder N is then withdrawn. When the work-surface is concave the part L is formed with a concave surround surface, and the end face N<SP>1</SP> is convex. When the worksurface is aspheric the surround surface has a curvature substantially equal to that of the basic spherical surface. In a modification, Fig. 2, a work-holder comprises an annular member P, e.g. of metal or glass, having a surround surface P<SP>1</SP> suited to the surface A<SP>1</SP> to be polished on the workpiece A. A tube Q, e.g. of metal or glass, is axially movable within the member P and urged towards the surface P<SP>1</SP> by a spring R. In the mounting of the workpiece an oven S is used having a boss S<SP>1</SP> in screw-threaded engagement with the member P. The roof of the oven carries, in axial alignment with the boss S<SP>1</SP>, a fitting T for receiving a stop cylinder V of a size suited to the diameter of the workpiece and having an annular end face V<SP>1</SP> shaped to match that of the surround surface P<SP>1</SP>. The fitting T constitutes a guide for a peripheral flange V<SP>2</SP> on the cylinder V which is urged downwardly to engage the surround surface P<SP>1</SP> by a spring W bearing against a plug T<SP>1</SP> in screw-threaded engagement with the upper end of the fitting T; the guide allows sufficient freedom of lateral movement for the cylinder V to ensure that it seats correctly on the surround surface P<SP>1</SP>. An inward projection T<SP>2</SP> on the fitting T prevents the cylinder V passing right through the fitting T into the oven when no work-holder is in position. A light spring T<SP>3</SP> opposes the spring W. The workpiece A is waxed within the member P so that the work-surface A<SP>1</SP> lies below the surround surface P<SP>1</SP>. The spring R presses the tube Q against the underside of the workpiece so that when the oven S is operated to melt the wax the workpiece A is urged upwardly until the work-surface A<SP>1</SP> engages the positioning surface V<SP>1</SP> on the cylinder V. The spring W exerts a stronger downward pressure than the upward pressure of the spring R so that further upward movement of the workpiece is prevented when the work-surface A<SP>1</SP> is flush with the surround surface P<SP>1</SP>. The oven is then allowed to cool so that the wax solidifies to secure the workpiece to the member P, whereafter the plug T<SP>1</SP> is unscrewed to release the downward pressure of the spring W on the stop-cylinder V which is raised out of engagement with the work-holder by the spring T<SP>3</SP>. In a further modification, Fig. 3, a workholder comprises a metal base member W<SP>1</SP> to which is rigidly secured an annular glass member W<SP>2</SP> having a surround surface W<SP>3</SP>. A tube W<SP>4</SP> is acted on by a spring W<SP>5</SP>. The base member W<SP>1</SP> is secured to a base-plate X carrying supporting posts X<SP>1</SP>, X<SP>2</SP> for a vertically adjustable cross-piece X<SP>3</SP> having, in alignment with the axis of the work-holder, a cylindrical fitting X<SP>4</SP> for a weighted plunger Y, the lower end of which co-operates with a recess Z<SP>2</SP> in the end wall Z<SP>1</SP> of a stop cylinder Z. With the plunger Y withdrawn so that the stop cylinder Z can be placed in approximately correct position the workpiece A is waxed within the members W<SP>1</SP>, W<SP>2</SP> with the work-surface A<SP>1</SP> below the level of the surround surface W<SP>3</SP>. The plunger is then lowered through the fitting X<SP>4</SP> into engagement with the stop cylinder Z to position the latter exactly. The assembly is then placed in an oven for the purpose of axially displacing the workpiece A to its correct position as previously described, the plunger Y being so weighted that the downward pressure on the cylinder Z considerably exceeds the upward pressure on the workpiece due to the spring W<SP>5</SP>. Instead of using an oven heat may be applied to a metal part of the workpiece to melt the wax. The stop may comprise two or more cylindrical segments having suitably shaped end faces.