Observation of hole formation process in plasma arc drilling

Drilling is widely applied in materials processing. Novel drilling methods, including electrical discharge machining, laser drilling, and plasma arc drilling, have emerged in recent years to overcome such problems as tool wear and low efficiency in the drilling of thick plates or difficult-to-machine materials. Plasma arc drilling is an effective and high-speed drilling method for thick plates. However, the mechanisms of plasma arc drilling, such as the hole formation and dross formation processes, have not yet been fully elucidated. In this study, a number of experiments under different drilling conditions were conducted to investigate the hole formation process in mild steel plate of 12 mm in thickness, using a high-speed video camera. Hole formation in plasma arc drilling is a highly complex process since both melting and vaporization are involved. The mechanisms of molten metal ejection as a material removal process were verified. The formation and removal of dross at the entrance and exit sides were elucidated by our observations of molten metal ejection. Further experiments were carried out to investigate the effect of the anode motion peculiar to plasma arc drilling. The results clarified the hole formation process and some specific related phenomena. The insights gained into the hole formation process will be useful in improving hole quality and reducing dross around the hole.