Precision polishing employs normal loads that tend to embed polishing abrasives in the surfaces of work pieces and/or leave residual pits and scratches just below part surfaces. MRF does not leave pits or scratches, because removal of material with MRF is driven by shear stresses delivered to the part surface by abrasive particles flowing in a converging gap. The normal forces on the abrasive particles are extremely low, and the resulting surfaces are very clean and free of damage.

On the Mohs scale for resistance to scratching, optical polymers as a class are ~4 times less robust than fused silica. For this reason, polymers are rarely polished with abrasives, and are instead either molded for large quantity applications or diamond turned for prototyping. Success depends on the skill of the manufacturer for a given polymer, and problems are known to exist in the achievement of precise aspheric shapes. Scatter and flare in imaging systems as a result of residual diamond turning marks on polymer lenses prototypes is also a problem.

MRF could be an ideal finishing technology for soft, optical polymers. We are conducting experiments to study the best methods for MRF of optical polymers using the Q22-Y, a new MRF machine manufactured by QED Technologies. In several experiments with diamond turned pucks composed of Acrylic or Zeonex, we have demonstrated significant improvements to diamond turned surface quality from MRF. Surface roughness levels below 0.5 nm rms have been achieved with a novel MR fluid containing zirconia abrasives. Additional good results have been observed on Cleargard (a polyester manufactured by Simula Safety Systems, Tucson, AZ), Lexan polycarbonate, and polystyrene using an alumina-based MR fluid.

MRF Removes Diamond Turning Marks
	Spatial frequency of diamond turning marks is denoted by a peak in the PSD plot at 300 (1/mm) which correlates to a period of 3 µm.
	After a MRF polishing run, the peak at 300 (1/mm) was gone showing that MRF successfully removed the diamond turning marks from the surface.

S. D. Jacobs, “Manipulating mechanics and chemistry in precision optics finishing,” Science and Technology of Advanced Materials   8 153-157 (2007).

J. E. DeGroote, H. J. Romanofsky, I. A. Kozhinova, J. M. Schoen, and S. D. Jacobs, "Polishing of PMMA and other Optical Polymers with Magnetorheological Finishing," in Optical Manufacturing and Testing V , edited by H. P. Stahl (SPIE, Bellingham, WA, 2003), Vol. 5180, pp. 123-134..

J. E. DeGroote, S. D. Jacobs, and J. M. Schoen, "Experiments on Magnetorheological Finishing of Optical Polymers," in Optical Fabrication and Testing Digest (Optical Society of America, Washington, DC, 2002), pp. 6-9.