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.

在微放电加工（EDM）钻孔中，随着钻孔深度的增加，电极间环境逐渐恶化，导致过程控制不良，加工效率低，形状精度差。其原因归结于三个关键因素：刀具磨损、气泡和碎屑。刀具磨损会改变初始刀具形状并导致加工精度变差。由于气泡冲洗效应，气泡行为会影响介电交换和碎片排出。碎片对介电污染起着决定性作用。本文从微观层面揭示了三个因素的特征，并阐明了它们如何影响钻井过程。第一的，基于微孔轮廓的原位观察，通过呈现微孔形状的演变，研究刀具磨损对材料去除过程的影响。其次，根据气泡行为的标准统计分析对气泡冲洗效果进行定量评估。第三，通过胶体化学理论观察和解释复杂的碎片团聚，然后通过称量间隙中的碎片积累量来定量评估介电污染。结果表明，刀具磨损导致排料面积扩大，尤其是横向排料面积增大，导致材料去除不均匀。气泡冲洗效应的本质是一种自振过程，由于振动频率急剧下降，随着进给深度的增加，这种作用会严重减弱。碎屑团聚的存在表明碎屑颗粒具有复杂的行为，而不仅仅是在电介质中自由分布。综合分析表明，微细电火花加工中极间环境的恶化是不可避免的，原因是气泡冲洗作用减弱，碎片排出和放电区域控制难度增加。与以往研究中经验共识的模糊描述相比，可见的定量调查提供了三个因素如何影响排放过程的具体证据。这些见解阐明了复杂的电极间现象，增强了对微电火花加工机理的理解，并为制造商提供了理论指导。