1 Copyright ©2005 by ASME EVALUATION OF RHEOLOGICAL PROPERTIES OF MAGNETORHEOLOGICAL POLISHING FLUID AND THEIR EFFECT ON SURFACE FINISH IN ULTRA PRECISION FINISHING PROCESSES Sunil Jha V.K.Jain ++ Mechanical Engineering Department, Indian Institute of Technology Kanpur - 208016 (India) ++ Corresponding Author; E-mail: vkjain@iitk.ac.in ; Voice: +91-512-2597916; Fax: +91-512-2597408/2590007 ABSTRACT Magnetorheological finishing (MRF) process for automated lens finishing and Magnetorheological abrasive flow finishing (MRAFF) for internal geometries rely on unique smart behavior of MRP-fluid. The rheological properties of MRP- fluid depend on carbonyl iron particle (CIP) and silicon carbide (SiC) particle size, their volume concentration, magnetic properties and applied magnetic field strength. To study the effect of particle size on rheological properties of MRP-fluid, a hydraulically driven specially designed capillary rheometer is fabricated. The best surface finish improvement was obtained with MRP-fluid containing approximately equal diameter of abrasive particles and CIPs. Least improvement was noticed with smaller CIPs and bigger abrasive combinations used. This is because the smaller size CIPs are incapable of providing the necessary finishing forces for bigger abrasive particles, which results in weak bonding strength. INTRODUCTION The available traditional and advanced finishing processes alone are incapable or uneconomical for producing desired surface characteristics on internal geometries. Abrasive flow machining (AFM) process [1] was developed to finish internal complex geometries by allowing abrasive mixed polymeric medium to flow over the surface under pressure. The abrading forces in the AFM process are a function of the viscosity of the viscoelastic polymeric base medium [2], which are difficult to control during operation. Magnetic field assisted manufacturing processes are relatively new finishing processes and they are becoming popular in finishing, cleaning, deburring and burnishing of metal and advanced engineering material parts. This process can produce surface finish of the order of a few nanometers [3-5]. The in-process control of MRP-fluid viscosity and yield shear stress using an external magnetic field adds determinism to the processes in which it is used. One such process is MRF [6] for lens finishing which works well for flat, spherical and aspherical surfaces [7]. To meet the finishing requirements of internal geometries and incorporating better in-process control of finishing forces, a variant of the MRF and AFM processes, MRAFF [8] was developed. The MRP-fluid comprises of carbonyl iron particles (CIPs) and very fine abrasives dispersed in a viscoplastic base medium of mineral oil and grease. It exhibits reversible change in its rheological properties on the application and removal of an external magnetic field. In the presence of a magnetic field, the CIPs acquire a magnetic dipole moment proportional to field strength and aggregate into a chain like structure aligned in the field direction [9], embedding non-magnetic abrasives in between. Depending on the size and volume concentration of abrasives and CIPs, the bonding strength gained by abrasives through surrounding CIPs chains varies. The objective of the present work is to evaluate the rheological properties of MRP- fluid with different particle size of its constituents and study their effect on change in surface roughness. Experimental Set-up: The finishing experiments were conducted on an MRAFF setup designed and developed by the authors [8]. The MRP-fluid is extruded through the workpiece passage to be finished utilizing two opposed cast iron cylinders under the presence of an external magnetic field. The transition in rheological properties of MRP-fluid takes place in the finishing zone immersed in a magnetic field produced by an electromagnet . Due to the high yield stress value, the presence of abrasive particles and the requirement of a magnetic field, no commercially available rheometer was found suitable for the measurement. Hence, a specially designed pressure driven capillary rheometer is fabricated, which consists of MRP-fluid reservoir, stainless steel capillary, hydraulic actuator, hydraulic system and electromagnet. Experimentation: Finishing experiments with MRP-fluid prepared with two different grades of CIP (HS & CS) and three mesh sizes of SiC (800, 1200, & 2000) were conducted on stainless steel workpieces. All experiments were conducted for 200 finishing cycles at 3.75 MPa extrusion pressure and 0.53 Tesla magnetic flux density. Each MRP-fluid was prepared by mixing 20 vol. % CIP and 20 vol. % SiC in 60 vol. % viscoplastic base medium of paraffin liquid and AP3 grease Proceedings of WTC2005 World Tribology Congress III September 12-16, 2005, Washington, D.C., USA WTC2005-64260