創新微細電極設計於微放電鑽孔加工之研究

Abstract

微細孔的加工為目前精微製造的最大宗，而更深、更快、更準的鑽孔技術一直是此領域的目標。而由於放電加工對導電材料皆可加工的特性，微細孔放電加工在此領域便為一相當廣泛應用的重要技術。本研究從電極設計的角度切入，以改善微細放電鑽孔之技術與加工品質，並嘗試製作特殊造型與應用之微細孔。為了鑽孔孔徑的正確性，考慮數種材料物理性質與加工參數的關係，文中提出一可預估放電擴孔量之策略。經由少量的實驗數據，先繪製出一預測趨勢圖，藉此可設計出適當的電極尺寸，以精確得到所需的孔徑，實驗證實此法可得到優於2μm的誤差。為了提高鑽孔效率與減少入出口差異，文中提出了雙股螺旋電極與覆層電極技術，經由實際加工測試發現，確實可減少孔徑入出口差異，與一般圓桿電極比較，減少了一半，加工時間更減少到三分之一。對於實務上有需求但微細孔加工難以製作的倒錐孔，本研究亦提出一以創新的特殊電極與製程的加工方法，成功於250μm厚不銹鋼上製作出入出口分別為100μm與218μm，錐度角約為13°的倒錐孔，此技術並可用於精修通孔或內孔倒角。最後，利用孔徑預測技術配合雙股螺旋電極與通孔精修技術，更成功於難加工材SKD11上鑽出深寬比達10的直徑100μm通孔，入出口差異在1μm內。

Micro holes have been used most frequently in the area of micro-manufacturing, and approaches to fabricate more efficiently the deeper and closer to the desired size holes by micro are essential. Up to date, the micro electrical discharge machining (micro-EDM) has been one of the key technologies to fabricate micro holes. In this thesis improvement of micro-EDM hole drilling technique so as to obtain better quality drilled hole is studied from the micro electrode design viewpoint. Special geometry electrode is also proposed to fabricate special shape micro holes for a particular application. To ensure accurate size of the drilled hole, a strategy to determine the appropriate electrode diameter in micro-EDM drilling is presented. The effects of the thermal properties of the material on hole enlargement are investigated. Regression curves showing their relationship under various settings of energy parameters in micro-EDM drilling for various materials being drilled are obtained by curve fitting of experimental data. Based on this relationship the diameter of the electrode can be estimated. The strategy is verified by experiment. It is found that size error of the drilled hole smaller than 2μm can be obtained. In order to improve machining efficiency and to reduce the size difference between entrance and exit of the drilled hole, a novel twin helices electrode is proposed. It is found that the size difference is reduced by half, and the machining time is reduced to 1/3 of those Micro-EDM drilled by using the conventional round shape electrode. More, a method by coating a layer of glaze (/enamel) on the electrode is developed. By using this kind of electrode, the size difference and machining time can be further decreased besides reducing enlargement of the drilled hole. In the thesis special geometry electrode and the associated procedures to fabricate reverse taper hole from an existing drilled hole are also proposed. Fabrication of a reverse taper hole with the entrance and exit of 100 μm and 218μm in diameter, respectively on a 250μm thickness workpiece is demonstrated by experiment. Similar procedures can be applied for finishing of the through hole as well. Finally, by combining the electrode size predicting strategy, twin helices electrode design, and hole finishing technique, a hole with both the entrance and exit of 100μm in diameter is successfully drilled on a 1mm thickness SKD11cold work tool steel.