Magnetorheological polishing using a permanent magnetic yoke with straight air gap for ultra-smooth surface planarization

•The development of a permanent magnetic (PM) yoke with a straight air gap for magnetorheological (MR) planarization of flat surfaces was never reported previously.•The newly developed device enables a much larger working area than the finishing “spot” in the conventional MR polishing process.•The newly developed MR polishing process demonstrated great potential for planarization of ultra-smooth large surfaces. A permanent magnetic yoke with a straight air gap was developed as the magnetic excitation unit, in order to improve the efficiency of magnetorheological (MR) polishing for ultra-smooth surface planarization. Finite element modelling was used to simulate the magnetic performance of the newly developed yoke, which was found to agree well with the experimental measurement. With the developed MR polishing apparatus, the effects of trough speed, work and excitation gap width, particle concentration within the MR fluid and workpiece size on the material removal rate were systematically investigated. Final tests were performed on glass specimens to examine the polishing performance. The polishing area in the developed process was much larger than that of the conventional MR finishing process with a carrier wheel and ultra-smooth surface of 1nm in Ra was achieved.