Elucidation of the mechanism of the deteriorating interelectrode environment in micro EDM drilling

In drilling with micro electrical discharge machining (EDM), gradual deterioration of the interelectrode environment with increasing drilling depth results in bad process control, low machining efficiency and poor shape accuracy. The reasons are attributed to three key factors: tool wear, bubble and debris. Tool wear changes the initial tool shape and results in poor machining accuracy. Bubble behavior affects dielectric exchange and debris evacuation by virtue of the bubble flushing effect. Debris plays the decisive role on the dielectric contamination. This paper reveals the characteristics of three factors at the micro level and elucidates how they affect the drilling process. First, the influence of tool wear on the material removal process is investigated by presenting the evolution of micro hole shape based on the in situ observation of micro hole profile. Second, the bubble flushing effect is quantitatively evaluated depending on the standard statistical analysis of bubble behavior. Third, the complex debris agglomeration is observed and explained by the colloidal chemistry theory, then the dielectric contamination is quantitatively evaluated based on weighing the debris accumulation amount in the gap. Results show that tool wear leads to an enlarged discharge area, especially the lateral discharge area, and results in uneven material removal. The nature of bubble flushing effect is a self-vibration process, which is weakened seriously with increasing feed depth due to severely decrease of vibration frequency. The existence of debris agglomeration indicates that debris particles have complex behavior but not just freely distributed in the dielectric. Comprehensive analysis indicates that deterioration of the interelectrode environment is inevitable in micro EDM drilling, attributed to the weakened bubble flushing effect and increasing difficulty of debris evacuation and control of discharge area. Visible and quantitative investigations provide specific evidences of how three factors affect the discharge process compared to the vague description from empirical consensus in previous researches. The insights elucidate complex interelectrode phenomena, enhance the understanding of mechanism of micro EDM and provide the theoretical guide for the manufacturers. [Display omitted] •The uneven material removal process in micro EDM drilling is visibly exhibited by the evolution of micro hole shape at different feed depths.•Distribution of bubble escaping from