Insert-mounted thin film sensors for real-time monitoring of tool conditions

A new type of sensor has been developed; it is mounted directly on a tungsten carbide insert and is capable of real-time sensing of the tool condition. The sensor uses thin films of ZnO deposited onto the insert by r.f. diode sputtering. The sensor has an extremely wide bandwidth, being able to detect resonances in the machine structure between 100 Hz and 10 kHz as well as acoustic emission in the 100-500 kHz range generated by metal cutting. During forced tool fracture, distinctive signals were observed from the sensor that could be used for cutting operation shutdown. The measured stress sensitivity averaged 0.05 V m N^-1 (standard deviation of 0.01 V m N^-1 from eight depositions) which is about 80% of the value for single-crystal ZnO. The sensors are quite rugged, with most of them still being operational at the end of the useful cutting lifetime of the inserts on which they were mounted. Temperature measurements indicate that the response of the sensors decreases by about a factor of 3 between room temperature and 100°C.