In-situ process monitoring and adaptive control for precision micro-EDM cavity milling

The linear tool wear compensation method (LCM) is commonly applied in micro-EDM 3-D milling to compensate the tool length wear in order to achieve high machining accuracy. Traditional LCMs mainly rely on empirical models and off-line wear measurements, whereas the process dynamics are not taken into account. When machining complex 3D cavities, an increasing number of tool wear compensation cycles have usually to be performed in order to maintain the targeted machining accuracy. This negatively affects the duration of the overall machining cycle. To realize efficient precision micro-EDM cavity milling, without the necessity to predefine Z-axis tool feed in the NC trajectory before machining, an in-situ process control system is developed to adaptively control the tool wear compensation factor based on the discharge pulse behavior. Experiments have shown that the change of the compensation factor can be detected and also a continuous increase of the factor (over compensation) leads to the saturation of the mean effective pulse frequency. Pulse monitoring therefore provides valuable information for understanding the process dynamics and for selecting the machining parameters towards better machining efficiency. Furthermore, the information gathered in-situ can be utilized to predict the tool wear and perform in-situ tool wear prediction. To implement this on machine-level, a combined off-line and in-line adaptive control of the tool wear compensation factor is proposed and experimentally validated by milling different 3D cavities. The off-line adaptive control is only necessary when the predicted machining depth error exceeds a certain limit. In this way, more than 80% of the off-line adaptive control cycles can be eliminated, whereby a total save of cycle time up to 18% has been reached, while still maintaining the desired dimensional and form accuracy.status: publishe