Abstract: Apparatus for and a method of scanning a workpiece are in the form of a retrofit package which provides a scanning capability for a machine tool which would otherwise have none. The basic elements of the package are a scanning probe, a computer, an interface between the probe and the computer, and a link between the computer and the RS232 (or other serial port) of the machine controller for communicating between the two. According to the method, the computer signals the machine control system to drive to a target position and stop, reads the probe outputs when the machine has stopped, calculates a new target position and signals it to the machine control system. To ensure that the machine has stopped the computer calculates time intervals based on its knowledge of the machine velocity, or motion signals based on a motion sensor or the probe outputs and only latches the probe readings when the time has expired or when the probe/motion sensor indicates that motion has ceased.