| dc.description.abstract | 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). | en_US |