Abstract: In a machine tool, a component such as a grinding wheel acts on a workpiece to form the latter into a circularly asymmetric shape, for example, a crankpin. Movement of the grinding wheel is controlled by a control signal which is derived from theoretical relative positions and positional velocities of the grinding wheel relative to the workpiece. In the course of at least one revolution of the workpiece, the respective positions of the grinding wheel relative to the workpiece are measured for each of a succession of angular positions of the workpiece. The measured positions are compared with corresponding theoretical positions and data indicative of any differences are stored, and then used to modify the control signal during a subsequent revolution of the workpiece so as to compensate for any of said differences and thereby reduce the magnitude of any error which would have resulted from the measured positional differences.