Process Monitoring for Efficient Micro Electrical Discharge Machining

Micro electrical discharge machining (micro EDM) is a proven technology for machining small holes and cavities in any kind of electrically conductive material, irrespective of its hardness. Nevertheless, due to the extensive tool wear and complexity involved in the electrical discharging process, efficient and accurate control of the machining process is still a difficult task to accomplish. This research aims to gain new insights into electrical discharge phenomena through process monitoring in order to develop efficient strategies for controlling the micro-EDM process. Part of this research focuses on the drilling of through micro holes. An approach based on processing the gap voltage signal is used to analyse the phenomena occurring once the tool breaks through the workpiece. Based on the result of the experimental observations, a novel technique for detecting the completion of the drilling process is developed and validated by drilling inclined holes. In this technique, the threshold limit for detecting the hole completion is adjusted during the drilling process through monitoring the discharge pulses without asking for any set of training data or input from the machine operator. The application of process monitoring in micro-EDM milling of inclined surfaces is also addressed in this research. The behaviour of the discharging process when milling inclined surfaces is investigated through process monitoring. An adaptive control strategy is accordingly developed. In this strategy, simultaneous monitoring of the discharge pulses and tool displacement is used for adapting the milling process to the expected tool wear behaviour. The control strategy is tested in an industrially-relevant application, showing that the desired machining accuracy can be achieved without the need for intermediate tool measurements.