| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 99 === Quartz is the critical material used in MEMS due to its beneficial properties, such as piezo-electric effect and stable chemical properties. However, it is difficult to machine between the efficiency and accuracy using conventional methods. Electrochemical discharge machining (ECDM) is an emerging non-traditional machining process that involves high-temperature melting assisted by accelerated chemical etching. However, the electrochemical reaction affects the coalesce status of gas film in ECDM. The structure of gas film is in turn affected by efficiency and accuracy. Therefore, this research uses different methods to improve the stability of gas film and machining efficiency in ECDM.
First, discharge energy varies with tool material of electrode. Different tool materials have different transition voltages, which determine the gas film formation, and hence the hole diameter and average current achieved. Otherwise, Surface roughness of tool electrode is key determinant of gas film formation. Poor surface roughness increases contact angle of gas bubbles adhered on electrode surface, causing them to coalesce and form a thicker gas film, resulting in largest hole diameter machined. During machining process, there is no significant tool wear observed after repeated gravity-feed machining of 50 micro-holes by using tungsten carbide tool electrode.
In ECDM process, the stability and formation of gas film is in turn affected by the machining efficiency and quality. In order to improve the stability of gas film structure, this study attempt to use the magnetic effect keeps bubbles move quickly form the tool electrode. According to the experimental results, the stability of standard deviation in hole diameter was increased by 80.7% while hole diameter was also decreased by 24.6%. Besides, both machining efficiency and accuracy were found to worse with increasing machining depth. In particular, the machining gap between the electrode and micro-hole is completely filled up by the gas film when using the cylindrical tool electrode. To solve these problem, this study proposed using a tool electrode with a spherical end whose diameter (150 μm) is larger than that of its cylindrical body (100 μm). In other words, during machining by the spherical end, the thickness of gas film formed on the surface of electrode body would be smaller than that of the micro-hole machined. Comparison between machining depth of 500 μm achieved by conventional cylindrical tool electrode and the proposed spherical tool electrode shows that machining time was reduced by 83% while hole diameter was also decreased by 65%.
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