Investigations on the Development of a New Magnetorheological Grinding Tool for Finishing of Cylindrical Blind Holes Surface

Abstract

Fine finishing of the inner surfaces of cylindrical blind hole (CBH) components is currently in high demand in the manufacturing industry as a means of reducing friction and improving functional efficiency. The industries like automobile, medical, and mould manufacturing are currently looking for fine finished CBH-type components with increased functional performance. Fine finished CBH-type industrial components like moulds are used to make plastic parts such as drosophila vials, bottle caps, light covers, etc. This increases the service life of finished products while also improving their aesthetic appearance and dimensional precision. The other CBH-type components like femoral head-neck taper junction and brake master cylinder also require fine finishing for increasing their functional performance. For the finishing of such CBH-type workpieces in industries, the most common method is to use a traditional handheld grinding technique. In this process, the bonded abrasive wheel is utilized for finishing the CBH workpiece surfaces which result in tensile residual stresses, heat-affected zones, microcracks, and many other defects. Surface fatigue, contact stiffness, and corrosion can all increase if these faults are not removed. Beyond that, there is no control over the finishing forces that act on the workpiece surface during the traditional finishing process. To achieve efficient and defect-free finished surfaces, a permanent magnet-based technique with two tools was recently developed for the MR finishing of longitudinal and bottom flat surfaces of the CBH type workpieces. The use of permanent magnets restricts the in-process control over the finishing forces. Also, this process can be utilized for the finishing of workpieces having a particular diameter size. Therefore, a tool is required for fine finishing of straight or tapered longitudinal and flat end surfaces of the CBH-type workpieces of variable sizes. This is possible only by keeping the tool eccentric from the central axis of the workpiece. Further, in-process control is required for fine finishing of the CBH-type workpiece made up of different materials. To address these issues, a magnetorheological polishing fluid-based technique with a new electromagnetically controlled single tool has been designed and developed for fine finishing both the longitudinal and flat end surfaces of CBH-type workpieces in a manner similar to internal cylindrical grinding. For the design and development of the magnetorheological grinding (MRG) tool, magnetostatic finite element analysis (FEA) is performed. The FEA result reveals that the initially proposed single MRG tool without grooves can be used to finish both the surfaces of the CBH-type workpieces. Also, the best dimensions of the MRG tool are obtained using FEA. Further, FEA is performed to achieve a higher and uniform magnetic flux density (MFD) over the MRG tool with grooves. After the FEA, the MRG tool with grooves is found more effective in fine finishing both the surfaces of the CBH-type workpieces. The MFD is also analyzed experimentally. It is found that the MRG tool with grooves provides higher and uniform MFD over its longitudinal and flat end finishing surfaces. Further, theoretical analysis is performed to understand the mechanism during finishing of the straight or tapered longitudinal and flat end surfaces of the cylindrical blind hole type workpiece using the newly developed MRG tool. The influence of the MFD, path followed and the number of active abrasive particles have been explored in-depth using theoretical analysis. Also, in the theoretical analysis, a surface roughness model is developed to predict finishing with the present MRG tool. From the theoretical model, the surface roughness achieved over the longitudinal surface is 99 nm and on the flat end surface is 56 nm using the present developed MRG tool. Further, these values of the theoretical surface roughness values are compared with the experimentally obtained values. On comparing, the experimentally obtained values with the theoretically calculated value, the percentage error is found in the range of 4.67 to 10.33 on the longitudinal surface and 5 to 12 on the flat end surface of the CBH type workpiece. Thus, from the results of the theoretical analysis, it is found that the MRG tool is capable of fine finishing the tapered longitudinal as well as flat end surfaces of the CBH-type workpiece. After the efficacy through the theoretical analysis, the performance of the developed MRG tool is validated for the finishing of tapered longitudinal as well as flat end surfaces of the CBH type workpieces, the detailed experimentation is performed. For finishing over the tapered longitudinal surface, a swivel rotating vise is used. This rotating vise provides a uniform working gap between the longitudinal MRG tool surface and the tapered longitudinal CBH-type workpiece surface. First, the magnetorheological finishing is performed over the typical EN31 CBH type mould cavity utilized for the manufacturing of spray bottle cap hoods. To perform the fine finishing, the composition of the magnetorheological polishing (MRP) fluid and optimum process parameters is predicted using the response surface methodology technique. After fine finishing using the predicted optimum process parameters, the reduction in surface roughness value is found as 80 nm from 350 nm over the tapered longitudinal surface and 50 nm from 320 nm over the flat end surface of the typical EN31 CBH type workpiece. In addition, a performance analysis of the current MRG tool for the finishing of the typical H13 CBH type mould cavity is carried out. After utilizing the predicted optimum process parameters for the finishing over the typical H13 CBH type mould cavity surfaces, the performance of the MRG tool is analysed. The results of the roughness value over tapered longitudinal surface reduces to 70 nm from 450 nm and 60 nm on flat end surface from 320 nm validates the performance capability of the present developed MRG tool. The surface characteristics like scanning electron microscope micrographs, optical microscope, and mirror images are improved significantly along with the reduction in waviness on both surfaces. Thus, the present process can help to achieve fine finished CBH type mould cavity surfaces for making clear polycarbonate light covers. The overall results reveal that the present MRG tool-based process is capable of fine finishing the tapered longitudinal and flat end surfaces of the CBH type mould cavities used in different industries.