Investigations and Modeling on Advancement in Magnetorheological Fluid Based Honing Process

Abstract

The internal surface finishing of cylindrical components at nano-meter level with good surface characteristics is highly demanded in today’s industries for improving their operative functionalities. There are various cylindrical components which require internal surface finishing at a high precision level with good surface integrity. These cylindrical components are used for cylindrical molds and dies, hydraulic cylinders, cylindrical barrels for injection molding, air bearings and cylindrical tubes for the flow of high purity liquids etc. The highly finished internal surface of cylindrical components also results in resistance to wear, corrosion, pitting and oxidation. In industries, mostly traditional finishing processes like grinding or honing are used to finish the internal surface of the various cylindrical components because of their easy accessibility and low making cost. These traditional finishing processes make use of rigid tools and when the rigid tools perform surface finishing, they do not have control over the finishing forces. Owing to this, sometimes the defects get produced like pit holes, sharp edges, torn and folded metals etc. on the final finished surface. Moreover, the traditional finishing processes can finish the surface up to a certain level because of the use of the rigid abrasive tools and further to achieve a very fine level of surface finishing may not be possible. Therefore, there is a mandatory requirement of the advanced finishing processes to finish the internal surface of the cylindrical components for enhancing their operational life in real applications. The finished surface using the advanced finishing processes has several benefits like close tolerance design, reduction in wear, friction losses and increase in product service life etc. To attain the finishing over the inner surface of the cylindrical workpieces, the advanced finishing processes like magnetorheological abrasive flow finishing (MRAFF) and rotational-magnetorheological abrasive flow finishing (R-MRAFF) are developed which make use of magnetorheological polishing fluid for finishing the surface. But, these processes are found more significant to finish the internal surface of the non-ferromagnetic cylindrical workpieces. Recently, another advanced finishing process named as magnetorheological honing (MRH) process is developed for the internal surface finishing of the both ferromagnetic as well as non-ferromagnetic cylindrical workpieces. This existing MRH process makes use of an electromagnet finishing tool whose electromagnetic cores are fixed to its central rotating shaft. Therefore, the single electromagnetic MRH tool can likely able to finish a particular inner surface diameter only at a time of the cylindrical workpiece with keeping a working gap for the magnetorheological polishing fluid. In the present research work, to overcome the challenges of the existing electromagnet based MR honing process for finishing the different sizes of the internal surface of the different cylindrical workpieces with a single tool, further a novel magnetorheological (MR) honing tool is designed and fabricated. This new designed MR honing tool is found to be capable of nano-finishing the different sizes of the internal surface of the different cylindrical workpieces. The present design of the magnetorheological (MR) honing tool constitutes of four permanent magnet strips on its outer periphery. The present MR honing tool is made flexible to move its outer magnetic surface (permanent magnet strips) radially inwards or outwards about the central axis of the tool. Since the tool’s end surface of the permanent magnet strips can be adjusted either radially inwards or outwards as per the requirement of the internal surface diameter of the cylindrical workpiece to be finished, therefore the present designed MR honing tool is found to be capable for nano-finishing the inner surface of the cylindrical workpieces with different internal diameters. The present designed MR honing tool with stiffened MR polishing fluid over its magnetic surface rotates as well as reciprocates inside the cylindrical workpiece to finish its internal surface. The finishing tool moves inside the cylindrical workpiece, and the retained MR polishing fluid over the tool’s magnetic surface interacts with the inner surface of the cylindrical workpiece in working gap and performs the finishing. The present design of MR honing tool always possesses the higher magnitude of magnetic flux density on its outer magnetic surface as compared to the inner surface of the ferromagnetic cylindrical workpiece and there exists a gradient of magnetic flux density in the working gap. Due to the presence of a gradient of magnetic flux density in the working gap, the magnetic carbonyl iron particles move towards the tool’s outer magnetic surface (higher magnetic flux density) and non-magnetic abrasives move towards the inner surface (lower magnetic flux density) of the ferromagnetic cylindrical workpiece. This is the foremost requirement of any finishing process based on magnetorheological (MR) fluid to finish the ferromagnetic material surfaces. Due to the magnetic field induced by the tool’s permanent magnet strips, the carbonyl iron (CI) particles as present in the MR polishing fluid make chains in the working gap and grip the abrasives in between the chains of CI particles. Due to the relative motion between the active abrasives (gripped by the chains of CI particles) and the inner surface of the ferromagnetic cylindrical workpiece, the surface gets finished by the present MR honing tool. Thus, the developed finishing tool is found to be effective of nano-finish the inner surface of ferromagnetic cylindrical workpieces with different internal diameters. The magnetostatic finite element analysis for studying the distribution of magnetic flux density over the tool’s magnetic end surface helped to fulfill the design requirement of MR honing tool for finishing the internal surface of the different ferromagnetic cylindrical workpieces. Initially, the MR honing tool is designed with the four flat end magnetic strips. In this design, a non-uniform working gap existed for the MR polishing fluid between the tool’s flat end magnetic surface and the inner surface of the cylindrical workpiece. This non-uniform working gap resulted in the non-uniform distribution of magnetic flux density in between the tool outer flat end magnetic surface and the inner surface of the cylindrical workpiece. This may result in non-uniformly strength of MR polishing fluid over the tool magnetic end surface due to the non-uniformly distribution of magnetic flux density over its magnetic end surface. This may further result in slow down the performance of the finishing process. Therefore, to further enhance the performance of the present finishing process, the limitations of the initially designed MR honing tool with the flat end magnetic surface have been removed by the redesigning of the MR honing tool with the four curved end magnetic strips. This redesign of the tool with the curved end magnetic surface has improved the uniformity in distribution of magnetic flux density over the tool’s magnetic end surface due to the accomplishment of the uniform working gap between the tool curved end magnetic surface and the inner surface of the cylindrical workpiece. This improved design of MR honing tool with curved end magnetic surface resulted in better finishing performance over the inner surface of the cylindrical workpieces as compared to the initial design of MR honing tool with the flat magnetic end surface. To examine the validity of the finishing performance of the initial and improved design of MR honing tool, the preliminary experimentations have been performed by both the designed tools over the inner surface of the ferromagnetic cylindrical mild steel workpieces. Experimental results with the MR honing tool having curved end magnetic surface has demonstrated better surface finishing over the inner surface of ferromagnetic cylindrical mild steel workpiece as compared to the MR honing tool with the flat end magnetic surface. The surface roughness parameters Ra, Rq and Rz get decreased to 0.088 μm, 0.115 μm and 0.626 μm from 0.371 μm, 0.493 μm and 2.525 μm respectively in 60 minutes of finishing with an MR honing tool having curved end magnetic surfaces. On the other hand, these parameters get reduced to 0.193 μm, 0.262 μm and 1.336 μm with MR honing tool having the flat end magnetic surface from the same initial surface roughness values in the same finishing time of 60 minutes. Thus, the improved MR honing tool design with the curved end magnetic surface is found more capable to nano-finish the inner surface of cylindrical workpieces as compared to the initially designed MR honing tool with the flat end magnetic surface. A comprehensive study using statistical design (plan of experiments) has been performed for finishing the internal cylindrical surfaces of EN-31 steel material by the present developed magnetorheological honing (MRH) tool with the curved end magnetic surface. The EN-31 steel is commonly used for manufacturing the punches and dies. The response surface methodology (RSM) has been used in “Design expert 10” software to plan and analyze the effects of the different process parameters on the percentage change in surface roughness (Ra) value. As present MR honing tool has been made flexible to move its outer magnetic finishing surface radially inward or outward to finish the different internal cylindrical diameters, therefore variation in working gap is also considered as one of the important process parameters. This validates the present design of the MR honing tool can be used for finishing the different internal cylindrical diameters. The analysis of experimental data showed that the percentage change in surface roughness value is mostly contributed by the working gap (between tool’s end surface and inner surface of the cylindrical workpiece) followed by the tool rotating speed, the percentage concentration of silicon carbide (SiC) abrasive particles, the tool reciprocating speed and the percentage concentration of carbonyl iron (CI) particles. Based on the results obtained from response surface methodology after regression analysis, the optimum process parameters for better process performance to finish the inner surface of the cylindrical EN-31 workpiece are found as 20 % volume concentration of SiC abrasive particles, 20 % volume concentration of carbonyl iron particles, the working gap of 2 mm, the tool rotating speed of 500 rpm and the tool reciprocating speed of 70 cm/min. At optimum process parameters, the least value of surface roughness of 95 nm is found from the initial value of 476 nm after 120 minutes of finishing over the inner surface of the cylindrical EN-31 workpiece. Further, a mathematical model for the change in surface roughness (Ra) value during the present MR honing process has also been developed. The developed mathematical model demonstrates the distribution of the induced magnetic flux density in the working gap included the contribution of the magnetic effect of iron particles as present in MR polishing fluid and the ferromagnetic cylindrical workpiece. From the evaluated magnitude of magnetic flux density at different points in the working gap, the indentation force acting on the actual tetrahedron shaped SiC abrasive particles has been evaluated mathematically. A theoretical expression is derived which predicts the change in surface roughness (Ra) value for the different number of finishing cycles during the present MR honing process. The developed mathematical model has been validated experimentally for the different number of finishing cycles. Results obtained from the theoretical mathematical model are found in close relation with the results obtained from the experimentations with an error in the range of 8.06 % to 1.03 %. This signifies the consistency of the developed mathematical model to predict the change in surface roughness Ra value while performed finishing with the present developed magnetorheological honing process. The developed mathematical model for the present MR honing process can be used to predict the change in surface roughness (Ra) value for finishing the different industrial cylindrical components and is further helpful in enhancing their operative functionality.