Abstract: In order to realize high-quality and high-efficiency machining of CFRP micro-holes with large depth-to-diameter ratios, discharges debris and gap phenomena during electrical discharge machining (EDM) are firstly observed and analysed using a scanning electron microscopy and high-speed camera, in order to reveal the mechanisms that lead to the difficult problems of CFRP machining by EDM. The analysis results show that single carbon fiber adjacent point discharge fracture removal and jet force bending fracture removal lead to large size carbon fiber discharge debris. The length dimension of the carbon fiber discharge debris is much larger than that of the common metal discharge debris particles, which makes it difficult to be expelled from the small discharge gap, resulting in unstable machining conditions. This not only affects the machining efficiency and quality, but also causes the manufacturing of high depth-to-diameter ratio micro-holes using EDM to be very difficult to achieve. To solve the above problems, an EDM method using spiral electrodes is proposed. Spiral electrodes not only optimize the gap flow field to promote discharge debris removal, but also increase the inclusion space of the discharge debris, thus effectively solving the problem of large-size carbon fiber discharge debris that is difficult to be expelled efficiently. Results demonstrated that EDM using spiral electrodes can stably achieve the machining of CFRP micro-holes with a diameter of 300 μm and a depth-to-diameter ratio of more than 13, and the machining quality and efficiency are significantly better than those of rod electrodes.

Key words: electrical discharge machining, spiral electrode, CFRP, micro-hole, high depth-to-diameter ratio