Abstract: A system is disclosed for detecting the state of a rotating machining tool, such as a drill bit, in a machining head. The system employs an electromagnetic detector having a coil connected in series with an oscillating circuit. As the tool rotates, its lands and flutes cause detectable variations in reluctance which are used to modulate the output of the oscillator circuit. The modulated oscillator output is monitored as an indication of the state of the tool.