GB1179818A - Method and apparatus for grinding and polishing aspheric surfaces of revolution - Google Patents

Abstract

1,179,818. Grinding. D. VOLK. 12 March, 1968 [16 Oct., 1967], No. 12075/68. Heading B3D. A method of generating, grinding or polishing aspheric surfaces of revolution by abrasive engagement between a workpiece surface and a non-matching tool surface, one of the surfaces being a positive aspherical surface of revolution of generally toric shape having a continuously increasing transmeridional radius of curvature from an equator or a nearby circle of latitude outwardly, the other of the surfaces being a generally negative aspherical surface of revolution of generally conicoid shape, supporting the positive surface of revolution for rotation about its axis of revolution, placing said surfaces in contact, maintaining said surfaces in contact with each other, rotating each of the surfaces about its respective axis while causing relative oscillation between the surfaces in a generally meridional direction of the positive surface to cause the contacting areas of the non-matching surface of the tool and of the workpiece to abradingly engage in a continuously changing pattern while maintaining the axes of revolution of the two surfaces at all times intersecting. The method may be used in generating, grinding or polishing optical surfaces on glass, metal or plastic to form concave surfaces of revolution having an apical umbilical point, prolate ellipsoids, paraboloids, hyperboloids, spheres, conicoid and bell-shaped surfaces, and the surface may serve as a mould for the casting of lens or as the curved surface of a contact lens. The curved surface may be formed on a tool of cast iron or other material to which loose abrasive is supplied or the surface may be charged with diamond particles. The tool has an axis of revolution X<SP>1</SP>, X and an elliptical arc A<SP>1</SP>, B, A, Fig. 4 having minor axis B, O and major axis O, A. The transmeridional radii of curvature 1, 1<SP>1</SP>; 2, 2<SP>1</SP>; 3, 3<SP>1</SP>; 4, 4<SP>1</SP> from points normal to the arc to the axis of revolution X<SP>1</SP>, X increase O, B to O, A. When a concave surface is oscillated between the positions shown solid and dotted in Fig. 5, as the tool is rotated the area of contact varies from a single area near the centre of the surface to two areas near the periphery. In other embodiments, the arc of the tool is a cycloid or involute curve. In apparatus using the method, the tool 37, Fig. 25 is mounted on a spindle 26 which is driven by an electric motor 27 and carried on an arm 25 adjustable about a horizontal pivot 24 on a base 10. The surface to be ground is mounted in a holder 39 having a conical recess engaged by a pointed rod 40 whereby the surface is free to engage the tool and is rotated by frictional engagement with the tool. The rod is pressed towards the tool by a weight 43 and is slidable in a block 42 carried by an arm 46 secured to a horizontal shaft 48, in turn secured to a second arm 50 from which extends a rod 52 engaged by an adjustable pin 53 on a crank 57 driven by a motor, rotation of the crank effecting oscillation of the surface over the rotary tool. Adjustment of the arm 25 about pivot 24 permits various surfaces to be generated using the same tool. The surface formed may have a central spherical or aspherical area adjoining a pheripheral zone which is spherical or aspherical. In other embodiments, the oscillation of the shaft 40 is effected by a cam and the rotation of the workpiece is by a drive from a motor. The positions of the tool and workpiece may be reversed.