Belt-MRF For Large Aperture Mirrors

With high-determinacy and no subsurface damage, MagnetorheologicalFinishing (MRF) has become an important tool in fabricating high-precision optics.But now the applications of MRF is concentrated mainly on the small opticsfabrication. For large aperture optical elements, the existing MRF device still needsfurther improvement in material removal rate. As we know higher removal rateneeds larger polishing wheel which is extremely demanding in terms of precisionmachining and assembly. In view of the above, to bypass the puzzle of using largerwheel, we provided Belt-MRF using a belt wrapping a permanent magnet box withlarge radius instead of a large polishing wheel in the traditional MRF. The effect isequivalent to applying a polishing wheel with the same radius of curvature.Compared with the existing magnetorheological equipment, it has the followingadvantages:1. Belt-MRF has higher volume removal rate (VRR) because of largerpolishing length. We provided Belt-MRF using a belt wrapping a permanent magnetbox with large radius instead of a large polishing wheel in the traditional MRF. Theeffect is equivalent to applying a polishing wheel with the same radius of curvature.2. With higher relative speed Belt-MRF has higher volume removal rate (VRR)with higher relative speed. We improve the relative speed between the workpieceand the ribbon, by increasing the line speed of belt to achieve the goal of increasingmaterial removal rate.3. It is easier for us to control Belt-MRF than traditional MRF device. Polishing wheel used in traditional MRF device is usually made of metal. It becomes difficultto control polishing wheel when it is too large. The Belt-MRF device is much lighterbecause of its structure and material.A series of experimental results on Silicon carbide (SiC) and BK7specimensand fabrication simulation verified that the SiC removal function is nearly5timeslarger comparing with conditional MRF in our lab. Removal function experimentson BK7validates the stability and dwell time linearity of belt-MRF. Compared to avariety of processing methods currently used, Belt-MRF has advantages of highremoval rate and deterministic. Thus, it is not only suitable for large diameter opticalprocessing in fine polishing stage, but also provides a promising way for largeoptical processing in rough polishing stage.