Contents lists available at ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: High aspect ratio micro-hole drilling aided with ultrasonic vibration and planetary movement of electrode by micro-EDM Z.Y. Yu (2)*, Y. Zhang, J. Li, J. Luan, F. Zhao, D. Guo 報告學生：鄭雅方 報告日期：

Introduction Mechanism analysis of the new method Structure of experimental equipment Experimental results and discussion Summary Outline

There are many micro machining technologies available to generate micro-holes, However, each process has its limitations in cost, machining efficiency, properties of the workpiece and aspect ratio of a micro-hole. Micro-EDM, as one of the micro machining processes, has demonstrated its ability to drill micro-holes in metallic alloys. Although the machining speed is low, micro-EDM can generate micro-features in any electrically conductive material regardless of hardness of the workpiece material. Introduction

The effect on the improvement of material removal rate (MRR) in EDM was due to the variation of the dielectric hydrostatic pressure caused by ultrasonic vibration. To further improve machining performance, ultrasonic vibration was introduced in deep micro-hole drilling by micro-EDM.It is reported that the machining efficiency of micro-hole drilling has been increased more than 60times

The planetary movement provides an uneven gap between the wall of the hole and the side of electrode. The viscous resistance is reduced in an uneven gap. The gaseous bubbles escape from the uneven gap more easily than that from an even one. The ultrasonic vibration forces the dielectric in the narrow discharge gap to move up and down, changing the contact angles of dielectric on the surfaces of the hole wall and the side of electrode, Consequently, the viscous resistance in the discharge gap is reduced and the bubbles are broken and removed easily. Machining efficiency is improved. Mechanism analysis of the new method

Based on the analysis, it is expectable that the aspect ratio of micro-hole by micro-EDM can be increased under the combination of ultrasonic vibration and planetary movement of an electrode. To verify this new method, experimental equipment is designed and experiments are carried out.

Structure of experimental equipment Fig. 1. Structure of experimental equipment. Table 1 Machining conditions.

N represents as normal machining conditions P as aided with planetary movement U as aided with ultrasonic vibration UP as aided with planetary movement and ultrasonic vibrationwith a total electrode feed of 6.97 mm UPS as aided with planetary movement and ultrasonic vibrationbut with a total electrode feed of 3.4 mm Experimental results and discussion Fig. 2. Electrode feed vs. time.

The average diameters at the entrance and at the exit, as shown in Fig. 3, are 139 µm and 102 µm, respectively. Fig. 3. Micro-hole. (a) Hole entrance and (b) hole exit.

a short electrode with a length of 3.6 mm was used to drill a micro-hole aided with planetary movement and ultrasonic vibration. The total electrode feed length was 3.4 mm. It can be seen that the electrode feed rate is almost same as the one that the electrode feed length was 6.97 mm. Fig. 4. Electrode after machining.

As shown in Fig. 5(a), the material removal rate was improved with ultrasonic vibration

Fig. 5(b) shows the tool (electrode) wear ratios (TWR) under different machining conditions.

It seems that the planetary movement of an electrode has no influence on the discharge gap as shown in Fig. 5(c).

The achieved aspect ratios of micro-hole by EDM under different machining conditions are shown in Fig. 5(d).

The viscous resistance in the narrow discharge gap increases with an increase of hole depth. The removal of the bubbles and debris in working area is difficult, resulting in frequent occurrence of abnormal discharge, causing excessive tool wear. This paper proposed the reduction of viscous resistance in the discharge gap in micro-hole drilling by EDM by using planetary movement and ultrasonic vibration as aids. The micro-hole with an aspect ratio of 29 has been drilled successfully by micro-EDM aided with planetary movement and ultrasonic vibration. Summary

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