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Magnetorheological Finishing (MRF) is a novel process for deterministic figure correction and polishing of optical materials that utilizes a sub-aperture lap created by moving a magnetic field-stiffened magnetorheological (MR) fluid ribbon against an optical surface. MRF has been successfully applied to a wide range of optical materials. A new research platform has been designed and built that is used to generate sub-aperture polishing profiles, i.e., polishing "spots," on optical flats under wellcontrolled conditions. This platform uses the same fluid circulation and conditioning system as the commercial computer numerically controlled MRF machine, thereby allowing fluid performance issues to be investigated. This new machine complements the capabilities of the original MRF research platform that has been in continuous use for over six years. These two machines have been used to generate polishing spots on a variety of optical materials. The spot profiles were measured to calculate material removal rates and the quality of the polished surfaces characterized by measuring the microroughness within the polishing spots. Examples are presented which illustrate how the evaluation of polishing spots was used to develop MR fluids and operating conditions for calcium fluoride, CaF2, and potassium dihydrogen phosphate, KDP.

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In general terms, the magnetorheological finishing (MRF)1 process occurs as follows (refer to Fig.1). A workpiece is installed at some fixed distance from a moving wall, so that the workpiece surface and the wall form a converging gap. An electromagnet, placed below the moving wall, generates a non-uniform magnetic field in the vicinity of the gap. The magnetic field gradient is normal to the wall. The MR polishing fluid is delivered to the wall just above the electromagnet pole pieces. The MR polishing fluid is pressed against the wall by the magnetic field gradient, acquires the wall velocity, and becomes a plastic Bingham medium before it enters the gap. Thereafter, a shear flow of plastic MR suspension occurs through the gap, resulting in the development of high stresses in the interface zone and thus, material removal over a portion of the workpiece surface. This area is designated as the “polishing spot” or removal function.

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