CNC線切割放電加工之等能量密度熔蝕技術

Abstract

本研究聚焦於「CNC線切割放電加工之等能量密度熔蝕技術」研究，並以「割一修一」的兩道次加工，求得模具良好的殘料移除及維持加工面的表面粗糙度為目標。CNC線切割放電加工適用於精密且複雜的模具製造，特別是以SKD-11冷模工具鋼為材料的模具。為使模具獲致高尺寸精度與良好的表面粗糙度，微細銅線線極對加工面需進行多道次補償加工，這項技術傳統均仰賴經驗豐富的技術人員作逐道次微調加工。為減少人為操作誤差，並快速獲致高尺寸精度加工，本研究提出「等能量密度熔蝕技術」，並以「割一修一」的兩道次加工為目標。實驗規劃先藉由第一道次粗割加工創造出一系列精密微小段差，再以第二道次精修加工，求得各段差之精修預留量所對應的放電能量，以便移除第一道次粗割加工後的粗割殘留量。實驗以高頻訊號擷取裝置及多通道示波器進行放電波列偵測與平均加工電流量測，分析項目包括加工時間、工作座標、正常放電頻率、電弧放電頻率、短路放電頻率、工作電壓與線極進給率等。為減少工件校正誤差及製程時間，實驗設計一「線上量測裝置」，工件於第一道次粗割加工及第二道次精修加工後都不拆卸，直接以數位式電子量錶對試片進行線上量測。研究發現，工作電壓對平均加工電流、放電頻率、精修移除量及加工面的表面粗糙度等，不具明顯關係；而線極進給率對平均加工電流、放電頻率、精修移除量等具正相關。實驗資料經統計迴歸分析得知，在不改變放電作用時間與放電休止時間條件下，「等能量密度熔蝕技術」能精確移除第一道次粗割加工後的各段差粗割殘留量。不論於直線或圓弧的第二道次精修加工，皆可提升目前精度的50%(< 3 μm)，突破傳統需改變放電能量的加工方法，因此加工面的表面粗糙度能維持在良好的程度，能實現「割一修一」的高精度與高效率的加工目標。此項研究結果與人工智慧演算法所得精度結果相當，深具商用價值。This study focuses on the"Equi-Energy Density Erosion Technology" of CNC w-EDM, and aims to achieve good residue removal from mould and maintain the surface roughness of the machined surface by two-pass of the "one-cut & one-skim" process. CNC w-EDMing is highly suitable for precision and complex mould manufacturing, especially for the moulds made of cold-working tool steel (i.e. SKD-11). Traditionally, to achieve high dimensional accuracy and good surface roughness on a mould, the brass wire-electrode must be subjectedto multiple compensating passes on the machined surfaces, which are fine-tuned on a pass-by-pass basis by experienced operators. To minimize the error of human operation and quickly achieve high dimensional accuracy machining, an"Equi-Energy Density Erosion Technology" with "one-cut & one-skim" processes is proposed in this study. The experiment is planned to create a series of precise micro-step differences by the first rough cutting (one-cut) process, and then the second finishing (one-skim) process is used to find out the discharge energy corresponding to the finishing of each micro-step difference, whereby precision remove the residual amount after the first rough cutting process. The experiment was conducted with ahigh-frequency signal acquisition device and a multi-channel oscilloscope for high-frequency pulse train detection and average discharge current measurement. The analysed items included processing time, working coordinates, normal discharge frequency, arc discharge frequency, short-circuit discharge frequency, working voltage and wire-electrode feed-rate. The"In-situ Measuring Device" is designed to measure the dimensional accuracy of the workpiece on-line directly by using a digital electronic gauge. This reduces calibration errors and process time of the workpiece. Experimental results show that the working voltage has no significant relationship with the average machining current, discharge frequency, material removal amount, and surface roughness of the machined workpiece; while the wire-electrode feed-rate has a positive relationship with the average machining current, discharge frequency and material removal amount. According to the regression analysis of the experimental data, the"equal energy density ablation technology" can accurately remove the residual amount of each micro-step difference after the rough cutting process without changing the pulse on-time and the pulse off-time. The second pass finishing (one-skim) of both straight and circular arcs can improve the existing accuracy by 50% (< 3 μm), which is a breakthrough from the traditional processing method that requires a change in the discharge energy, so that the surface roughness of the machined surfaces can be maintained at a good level, and the target of"one cut, one repair" processing with high accuracy and efficiency can be realised. The results of this research are comparable to the accuracy results obtained by artificial intelligence algorithms, and are of great commercial value.