A light source generates a beam of light which is incident on a detector which generates a trigger signal when the beam incident upon it is occluded by a tool. To avoid trigger signals being produced when the beam is temporarily occluded by coolant drips passing through the beam, the detector produces a detection signal each time the beam is occluded, but only produces a trigger signal if a further detection signal is produced in a specified time interval from the generation of an earlier detection signal. One method of generating the time interval is by rotating the tool so that the beam is occluded at time intervals (t1) each time a cutting edge of the tool enters the beam. If no second signal is generated within an interval (t2) at the end of interval (t1) no trigger signal is produced.