Story of the Week  
Magnetorheological Abrasive Flow Finishing (MRAFF)
 

Magnetorheological Abrasive Flow Finishing (MRAFF) is a novel precision finishing process developed at I.I.T. Kanpur for nanofinishing of complex internal geometries using smart magnetorheological polishing fluid. MRAFF process provides better in-process control over rheological properties of abrasive laden magnetorheological finishing medium. Magnetorheological (MR) polishing fluid comprises of carbonyl iron particles (CIPs) and silicon carbide abrasives dispersed in the viscoplastic base of grease and mineral oil, exhibits change in rheological behaviour in presence of external magnetic field.  This smart behaviour of MR-Polishing Fluid is utilized to precisely control the finishing forces, hence final surface finish. In MRP-fluid, on the application of external magnetic field, the iron particles acquire dipole moments proportional to the magnetic field strength and when the dipolar interaction between the particles exceeds their thermal energy, the particles aggregate into chains of dipoles aligned in the field direction. Figures 1a and 1b shows actual photographs taken by optical microscope of CIPs interconnected chain structure. Figure 1c illustrates a model of the structure formed when no magnetic field is applied while Figure 1d shows the structure formed with abrasives trapped and embedded between iron chains, in the presence of finite magnetic field.

The mechanism of MRAFF action is illustrated in Fig. 2. The abrasive particles under the action of precisely controlled normal magnetic force due to CIP chains and tangential hydraulic extrusion force remove irregularities from the workpiece surface to finish them in nanometer range. A hydraulically powered experimental setup as shown in Fig. 3 is designed to study the process characteristics and performance.

Fig. 3: MRAFF Experimental setup

Experiments were conducted on stainless steel workpieces at different magnetic field strength to observe its effect on final surface finish. No measurable change in surface roughness is observed after finishing at zero magnetic fields. However, for the same number of cycles the roughness reduces gradually with the increase of magnetic field. This validates the role of rheological behaviour of magnetorheological polishing fluid in performing finishing action. The results on stainless steel and silicon nitride workpieces are quite encouraging and surface roughness value, Ra upto 30 nm is already obtained using this process.

Following research group is working on this topic :

Professor V. K. Jain & Professor P. S. Ghoshdastidar
Messers : Sunil Jha and Manas Das
Department of Mechanical Engineering
Indian Institute of Technology Kanpur
    eMail: vkjain@iitk.ac.in; psg@iitk.ac.in
    Web: http://home.iitk.ac.in/~vkjain/ ; http://home.iitk.ac.in/~psg/

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