線切割放電加工機鼓形量之探討與抑制

Abstract

線切割放電加工程序由於使用銅線為電極，因此或多或少都會在工件側腹上會留下鼓形量的幾何誤差，造成厚度方向不垂直，影響尺寸精度，實務上係以多道次精修的方式解決此問題，但時間及成本上並不經濟。過往研究認為鼓形量成因有二，其一為線極振動造成工件中央易發生放電，其二為工件中央排渣困難，加工絕緣液比電阻值下降，造成二次放電。本研究之在探討影響鼓形量之主要以何者為重，並藉此找出抑制策略。

本研究首先針對厚度60mm之SKD11，從線張力和伺服電壓的改變觀察鼓形量之變化，並和放電延遲時間進行關連以找出合適控制指標進行控制策略。實驗結果顯示，隨著張力的上升，線振因素所佔的重要性會下降，抑制線振後排渣的模式將是影響鼓形成因的主因。本研究並發現鼓形量的形態另有二種模式(近直線、不規則)，針對其它兩種鼓形是由於渣流動和間隙大小不同，造成放電點集中程度不同，放電集中程度不同所對應正常放電比也不同。適當的正常放電比會應對有良好的鼓形量。綜合上述發現，本研究提出了一套簡單的控制策略，在伺服進給(G95)的工作條件下，加工時之正常放電比作調整放電休止時間的依據。經實驗證實，正常放電比參考值0.40是一理想參考在本研究的條件之下，針對厚度60mm能將一般伺服進給鼓形量6μm改善到鼓形現象消失，厚度80mm從11μm下降至2μm。

WEDM is a machining process using thin wire electrodes. Because of the flexibility of the wire electrodes, there would be vertical alignment geometric error left on the machined surface. In most workshop practice, the error is corrected through multiple surface-finishing process, which is not efficient in time and cost.The previous studies over this issue proposed that the vertical alignment geometric error were out of two mechanisms – either vibration of the wire electrodes, or the ineffective of debris removal – both making the uneven distribution of the electoral discharge alone the wire electrodes.

In this research, with SKD11 alloy of 60mm in thickness as the workpiece, the influences of the wire electrode tension, the servo voltage, and ignition delay time (Td) over the vertical alignment geometric error were observed, and also in conjunction with the ignition delay time of electrical discharged (Td) in order to figure out the suitable control index for feedback automatic control. It showed that, as the wire electrode tension increased, the influence of the wire vibration diminished, which suppressed the removal of the debris making the uneven distribution of electrical discharges. This research also noted that the vertical alignment geometric error could be categorized into two major types: nearly linear type, and irregular type. Different types of the error were attributed to two factors – the flow of debris removal, and the width of discharged gap – which reflected to different normal discharge ratios. Under the adequate normal discharge ratio, the vertical alignment geometric error would be much reduced. Based on the finding stated above, a clean-cut feedback control strategy was proposed in this research. Under the machining setting of servo feeding (G95), the normal discharge ratio was used as the control index to change the discharge off-time for the feedback control. Through on-site experiments, it was proved that, under the machining condition setting of this research, the vertical alignment geometric error could be much decreased with the normal discharge ratio of 40%. For workpiece of 60mm in thickness, the geometric error decreased from 6μm to nearly zero. For workpiece of 80mm in thickness, the geometric error decreased from 11μm to 2μm.