放電加工是利用電能轉變成熱能產生高溫達到去除材料的目的，適用於高硬度、高強度等難切削之材料加工。由於放電過程中工件表面受到急冷急熱的作用，容易產生微裂縫、微氣孔等缺陷，使得加工面的品質變差；對機件使用壽命上造成嚴重的影響。因此如何精修放電表面，提升加工品質是本論文所要探討的主題。 本研究針對放電加工後表面粗糙度不佳的缺點，提出了三種改善表面品質的方法，分別為加工液中添加界面活性劑、放電面噴塗介電材料及結合電泳拋光技術對放電加工表面進行精修，達到精緻表面的目的。經由實驗結果分析顯示，純煤油中添加Span 20 界面活性劑，藉由界面活性分子的作用，達到分散碳渣、加工屑的效果，減少積碳、放電集中現象，對材料去除率可提升40~85%；純煤油中添加鋁粉與非離子界面活性劑Tween 80，改善了鋁粉集聚現象，使其更均勻地分散於放電加工液中，對放電後的工件表面粗糙度可改善60%；另一種方法為放電面上噴塗介電材料，以其薄膜厚度來影響放電痕高低點之電阻抗，消耗部份的放電能量，使得放電痕之隆起處逐一被去除，留下深淺均一的凹坑，因此加工後工件表面粗糙度改善率可以提升33%，表面粗度值穩定性可以增加18%；而採用電泳沉積方法可使粒徑0.3µm的Al2O3顆粒均勻吸附於旋轉電極，配合適當參數對放電後之工件進行精密拋光，可在5~10 min將初始粗糙度0.52 µmRa的放電表面改善到0.068 µmRa 似如鏡面的效果，對縮短加工時間與改善表面粗糙度效果相當明顯。 In electrical discharge machining (EDM), materials are removed by thermal energy transformed from electrical energy. This technique can be applied to remove materials with high hardness and strength, which are difficult to machine by traditional methods. During the process, the workpiece is subject to rapid heating and cooling under a violent temperature gradient. As a result, defects such as microcracks and pores are formed, to deteriorate the surface quality and shorten the life cycle. In view of such drawbacks, this study aims to enhance the high precision quality of electrical discharge machined surface. Three approaches to improve the surface roughness and to achieve a mirror-like surface quality are proposed. Three techniques including the addition of surfactant to dielectric, the spraying of dielectric material and the combination of EDM with electrophoretic deposition polishing (EDP) are further studied. For the first approach, the experimental results reveal that adding surfactant Span20 to kerosene can disperse carbon and debris through the molecular reaction of the surfactant. As a result, carbon accumulation and concentrated discharge are reduced and material removal rate can be improved by 40-85%. Similarly, the addition of Al powder and surfactant Tween 80 to kerosene helps reduce the agglomeration of Al powder, thus ensuring even distribution of Al powder in the dielectric, which in turn improves surface roughness by 60%. The second approach influences the impedance of the electrical discharge profile by spraying thin films with different thickness, which consumes part of the discharge energy. The bulges on the surface are removed by the second electrical discharge, leaving craters of even depth and thus improve the surface roughness by 33%. In addition, the surface roughness stability is together with an increase in surface roughness improved by 18%. Finally, combining EDM with EDP using 0.3µm of Al2O3 particles can improve the initial surface roughness from 0.52 µmRa to a mirror-like surface of 0.068 µmRa. In addition, the total working time required for the polishing process can also be reduced significantly to around 5 to 10 minutes.
