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
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.
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.
Removes Diamond Turning Marks
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.
a MRF polishing run, the peak at 300 (1/mm) was gone showing
that MRF successfully removed the diamond turning marks from
D. Jacobs, “Manipulating mechanics and chemistry in precision optics
finishing,” Science and Technology of Advanced Materials
8 153-157 (2007).
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..
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.