In order to compensate for errors in the positioning of a tool by a servo motor (4) resulting from mechanical inaccuracies in the machine tool slides (2) and drive (3), an analogue error compensation signal (12) is generated by a compensation unit (1) in which is stored calibration data relating to the components of the positioning error. The error compensation signal (12) is summed by a summation unit (14) with the uncompensated analogue position error signal (7) generated at the output of a CNC controller (5), and the resulting signal (15), as modified by the usual velocity feedback signal 10′ from the servo motor (4), is used to drive the servo motor (4). The resolution of the error compensation signal (12) is arranged to be greater than that of the position error signal (7), which is generally limited by the number of bits of the digital to analogue converter in the output stage of the controller (5). As the demand position is closely approached the servo (4) is driven substantially by the error compensation signal (12).