Method and tool for creating a concave surface from a spectacle blank

A method for creating a concave surface on a rough blank for eyeglasses (workpiece), which already largely corresponds to the inner surface of the finished eyeglass, by means of a milling or grinding tool, during which the blocked-up workpiece and the tool are moved relative to each other in a CNC-controlled machining process with two linear axes of motion (x and y axis) and two axes of rotational movement making an angle (.alpha.) to each other, the first axis being assigned to the workpiece (b-axis) and the other axis being assigned to the tool (c-axis), in which the removal of material to shape the surface is done along a spiral path on the surface, in that the tool and the workpiece are moved relative to each other along the x, y and b axes, and a disk-shaped rotation-symmetrical tool is used as the tool, being arranged such that the lowest point of the tool in relation to the workpiece is situated in a plane defined by the b and x axes, characterized in that the removal of material along the spiral path is preceded by a plunge-cut step, during which the workpiece rotates about its axis (b) and the tool is moved at least in the direction of the y-axis, until a surface in the shape of an annular trough is achieved, adapted to the concave surface being created at least in the region of the outer rim of the workpiece, so that the surface produced on the workpiece at least in the region of the outer rim corresponds to the nominal outer contour of the optically active inner surface of the eyeglass.