Study on electrode vibration in the touch-down stage of fast electrical discharge machining drilling

Fast EDM (electrical discharge machining) drilling is broadly used in the manufacturing industry. When drilling inclined holes, the machining efficiency in the touch-down stage is much lower than that of drilling normal holes for a general-purpose fast EDM drilling machine. To uncover the reasons, this paper conducted observations of inclined hole drilling processes by using a high-speed video camera. It was found that the vibration of the tubular tool electrode was the major factor that led to the unstable state, resulting in low machining efficiency. And the vibration was driven by the discharge reaction force and the hydrodynamic force. The vibration enlarged the gap width and caused open circuit pulses, which misled the servo control system to feed the electrode downward. The overfeeding eventually caused short circuits and the subsequent retraction of the electrode. The iterative feeding and retraction of the electrode resulted in low machining efficiency. Then a fluid-structure interaction (FSI) model was built to analyze the vibration behavior. The impacts of the inner flow speed and the free length of electrode were studied with the model. It was found that the vibration is inevitable on general-purpose fast EDM drilling machines. Finally, to reduce the vibration impact, a new gap servo control strategy for the touch-down stage was proposed. The strategy adjusts the reference gap voltage by fuzzy rules. Experimental results showed that with the new gap servo control strategy, the performance was improved in the touch-down stage, with the machining efficiency improved by 38.75% and the area of discharge affected zone around the hole inlet reduced by 13.44%.