1. A numerical control unit responsive to an instructed feedrate for controlling a machine tool to move along linear axes in a spatial coordinate system and rotate about one or more of said linear axes, said numerical control unit comprising: means for machining a workpiece at an instructed feedrate along any of said linear axes to maintain a desired relative speed between said tool and said workpiece; and means for machining a workpiece by rotating said machine tool about a selected one of said linear axes, said selected one of said linear axes thereby becoming an axis of rotation, at a rotational feedrate by correcting, in accordance with a linear interpolation command provided for said axis of rotation, said instructed feedrate based on a radial distance between said axis of rotation and said machine tool so that said rotational feedrate about said axis of rotation equals said desired relative speed between the tool and workpiece.