| 摘要: | 摘 要 磁力研磨（MAF）是以磁性磨粒為切削刀具，並利用磁場控制磨粒之研磨壓力，再藉由磁性磨粒與工件表面間之相對運動，使工件表面材料微量移除至近似鏡面程度之精密拋光法；由於磁性磨粒受磁場作用而聚集成一束撓性的磁力刷，所以磁性磨粒可順著複雜曲面之外形，順勢起伏地進行拋光，而且機台結構之振動或顫動亦不會影響工件表面之拋光品質。本研究使用磁粒和磨粒自然混合而成之非結合式磁性磨料（UMA），首先探討圓柱面磁力研磨之拋光特性，並將其應用於放電加工表面之改善；由實驗結果得知UMA之拋光效果可媲美成本昂貴之燒結磁性磨料，在短短幾分鐘內，即可將硬度為HRC55之SKD11圓桿試件由0.25mm Ra拋光至0.042mm Ra；精進型磁力研磨將磁粒之研磨壓力經由一層不織布傳遞給工件表面之磨粒，更可以將試件之表面粗糙度再持續改善至0.017mm Ra；對於圓柱面上之放電加工表面，MAF亦可輕易地拋除其再凝固層和微裂紋，並獲得0.04mm Ra之精緻表面。為了提高拋光性能，本研究提出結合MAF和電解作用之電解磁力研磨複合拋光法（EMAF），先讓工件表面生成一層硬度比母金屬低之鈍化膜，再利用MAF予以拋除，本項研究分別針對SKD11之圓柱面以及AISI 304不銹鋼圓管之內表面探討其拋光特性，並與MAF作比較，由實驗結果証實EMAF之拋光特性明顯優於MAF，不論是圓柱面或圓管內表面拋光，EMAF不但有較高之拋光效率，同時亦可獲得極佳之表面粗糙度，尤其針對SKD11之圓柱面拋光，EMAF僅需5分鐘即可將0.178mm Ra之工件表面拋光至近乎鏡面之0.017mm Ra。本文除了詳述MAF和EMAF之工作原理，並探討其拋光特性及拋光機制之外，在EMAF拋光過程中，對於電解離子因受到洛侖茲力之作用導致運動軌跡變成曲線之理論以及鈍化膜之生成形態亦詳加說明。為了探討電解作用與MAF之協同效果（synergistic effect），本研究亦進行了多次純電解拋光實驗，由實驗結果得知，在適當的加工條件下，由於電解作用與MAF相互協同作用的結果，EMAF可獲得頗高之額外材料移除量。本研究亦利用田口法和L18直交表，經由實驗獲得EMAF圓柱面拋光之最佳表面粗糙度改善率和最大材料移除量之最佳加工參數組合，然後再利用變異數分析（ANOVA）探討各實驗參數影響表面粗糙度改善率和材料移除量之顯著程度。 Abstract Magnetic abrasive finishing (MAF) is a precise polishing method that the cutting tool is a group of magnetic abrasives, which the abrasion pressure is controlled by a magnetic filed. A limited amount of material will be removed by conducting a relative motion between the work surface and the abrasives, so as to obtain a mirrorlike finished surface. Owing to the magnetic field, the magnetic abrasives will gather to form a flexible magnetic brush. Thus the magnetic abrasives can move and polish along the profile of a complex surface, so the surface with complex shapes can be finished. Furthermore, the disturbances from the structure due to vibration or chatter will not affect the quality of the finished surface. The unbonded magnetic abrasive (UMA) used in this study is a mechanical mixture of ferromagnetic particles and abrasives. The finishing characteristics of UMA in cylindrical MAF are investigated, and then an application on improving the electrical discharge machined surfaces is performed. Experimental results demonstrate that the finishing characteristics of UMA are as good as those of sintered magnetic abrasives, which is much more expensive than UMA. The surface roughness of the SKD11 workpiece with HRC55 hardness can be improved from 0.25mm Ra to 0.042mm Ra after a few minutes of finishing. An improved MAF, which transfers the abrasion pressure to the abrasives through a sheet of unwoven cloth, can further improve the surface roughness of the workpiece to a level of 0.017mm Ra. Concerning the peripheral electrical discharge machined surfaces, MAF can remove the recast layer and the micro cracks easily, and a refined surface of 0.04mm Ra will be obtained. To elevate the finishing performance, an electrolytic magnetic abrasive finishing (EMAF), which is a compound polishing process by involving the traditional MAF and electrolysis, is developed. The passive film, whose hardness is lower than that of the original metal surface, is produced on the work surface, and is then removed by MAF during processing of EMAF. The finishing characteristics of EMAF are investigated, and then the results are compared with those of MAF concerning the cylindrical finishing of the SKD11 workpiece and the internal finishing of a circular pipe with AISI 304 stainless steel. Experimental results show that the finishing characteristics of EMAF are better than that of MAF. Despite what kind of surface is finished, EMAF yields a better surface roughness and higher material removal than MAF did. Especially in the case of the cylindrical finishing of SKD11, a mirrorlike finished surface of 0.017mm Ra can be produced from 0.178mm Ra after 5 minutes of finishing using EMAF. This study describes the principles of the process of MAF and EMAF, the finishing characteristics of surface roughness and material removal, and the associated mechanisms. Additionally, the theory, that the path of the electrolytic ions toward the anode surface is changed into a cycloid curve under the effect of the Lorentz force, and the forms of the passive film that is produced on the work surface in EMAF, are also described in detail. Some experiments on the only electrolytic process without MAF are performed to analyze the synergistic effect between MAF and electrolysis. Experimental results show that EMAF will produce rather large amount of extra material removal under proper process conditions due to the synergism of them. To determine the optimum process conditions for improving the surface finish and increasing the material removal in cylindrical EMAF, experiments using the Taguchi method and L18 orthogonal array are performed. Further, the significances of the control factors are identified with the assistance of analysis of variance (ANOVA). |
