Abstract: A tool wear detector is formed by applying a resistor, the resistance of which is permanently altered in accordance with a time/temperature relationship (and by change in size e.g. as a result of wear), to a tool in a manner so that the resistor and the portion of the tool to be monitored are subjected to corresponding temperature cycles. Measuring the resistance of the resistor at any time provides an indication of the cutting conditions to which the monitored portion of the tool has been subjected and thereby an indication of the remaining life of the portion of the cutting edge of the tool being monitored.