放電加工製程在實際使用上常需加工各種大小的尺寸。加工尺寸的不同使放電加工後的材料表面特性改變，造成產品品質的不穩定。故本研究以不同尺寸的電極控制放電加工的面積，來釐清放電加工尺寸效應對放電加工表面形貌、表面粗糙度及表面裂紋等放電加工表面特性的影響。
研究結果顯示，影響放電加工後的表面粗糙度主要為放電電流與放電持續時間，表面粗糙度隨兩者上升而增加，放電加工尺寸與加工材料種類則對表面粗糙度影響較小。在表面形貌方面則以放電電流與放電持續時間為影響表面放電痕大小的主因，放電加工尺寸雖不影響放電痕大小但卻使表面積碳區域範圍產生改變。
在表面裂紋方面以較長的放電持續時間配合較低的放電電流的放電條件表面最容易出現裂紋，隨著放電加工尺寸的減少則能有效減少裂紋的產生。粗略歸納下裂紋可分為7類，當持續時間與放電加工尺寸的增加，出現的裂紋種類也增加。對於嚴重開裂現象可藉由增長放電休止時間的方式避免，增進加工後的表面品質。
綜合以上實驗結果，放電加工的表面品質可以藉由增高放電電流、降低放電持續時間、增長放電休止時間、減少單道次的加工尺寸等方式，達到明顯的改善。

Electrical discharge machining(EDM) is often used in the practical process for various sizes of components. Different sizes of electrodes change the EDMed surface and cause the quality varied. This study is mainly to investigate the effect of electrode size on the roughness, appearance and crack of EDMed surface.

The results show that peak current and pulse on time (two EDM parameters) attribute to the major factors to increase surface roughness. Besides, increase the size of the electrode rise the surface roughness only as it exceed a critical condition. Peak current and pulse on time both also play important roles on the surface appearance for changing the size of craters. Although the electrode size don’t change the size of crater, it change areas of carbon deposition.

Surface cracks are easily occurred in the combination of long pulse on time and low peak current and eliminated as the decrease of electrode area. In this study, increasing pulse on time and electrode size increase the sorts of cracks that can be generally divided into seven kinds.Serious cracks can be avoided by increasing pulse off time and improve surface quality.

As a conclusion, the increasing of peak current and pulse off time as well as the decreasing of pulse on time and electrode size improve the EDMed surface significantly.

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