finishing did not exist for most optics until COM and its industrial
collaborators invented, patented, and commercialized the magnetorheological
finishing (MRF) process. As exemplified by a line of MRF machines
sold by QED Technologies, Inc.,
of Rochester, NY, the MRF process is capable of rapidly polishing
out and figuring a variety of materials from a few mm to over 1
m in diameter. Plano, spherical, aspherical, and cylindrical optics
with round or non-round apertures may be finished to better than
0.1 µm p-v form accuracy in minutes with a resulting surface
micro-roughness of <1 nm rms.
Q22-Y MRF (magnetorheological finishing) machine for CNC
polishing. Over 50 machines have been sold to industry for the
precision finishing of optics used in high-end cameras, in semiconductor
photolithography, and in other complex optical systems applications.
key to MRF is a ribbon of abrasive-doped magnetic fluid that moves
over a wheel and into contact with the surface of a spindle-mounted,
rotating part. The fluid stiffens by four orders of magnitude in
the contact zone, due to the presence of a magnetic field, turning
from the consistency of honey to that of clay. Shear stresses between
polishing abrasives in the fluid and the part surface cause material
removal. The removal mechanism in MRF is unlike any other, and it
results in a pit-free and scratch-free surface with high resistance
to laser-induced damage.
Quality Conventional Polish
field microscopy for acid-etched fused silica surfaces. A
collaboration among Lawrence Livermore National Laboratory,
Zygo Corp., and QED Technologies found that clean surfaces
resulting from MRF are critical to enhanced UV laser damage
removal spot is characterized with interferometric precision, and
sophisticated computer algorithms allow a machine operator to define
the MRF process goal. This may be the uniform removal of material
from the micro-ground work piece surface to eliminate sub-surface
damage, or the preferential removal of material to eliminate global
surface figure errors. COM, ARL, and QED Technologies, Inc. won
the DoD Defense Manufacturing Technologies Achievement Award in
2000 for MRF, and the R & D 100 Award for the Q22-Y MRF (Magnetorheological
Finishing) System in 2001. [The Q22-Y, a new machine for finishing
prisms, square and rectangular flats, and cylinders was introduced
at the Optotech Trade Show and Exhibition in Fraunhofer, Germany
in June, 2001.]
are two magnetorheological (MR) fluids currently in widespread industrial
use. One composition consists of cerium oxide in an aqueous suspension
of magnetic carbonyl iron (CI) powder, and it has been found appropriate
for almost all soft and hard optical glasses and low expansion glass-ceramics.
The second composition uses nanodiamond powder as the polishing
abrasive, and it is better suited to calcium fluoride, IR glasses,
hard single crystals like silicon and sapphire, and very hard polycrystalline
have recently been made toward understanding the mechanism of removal
with MRF, based in part on the hardness of the CI particles, the
tribochemical interaction of cerium oxide or other abrasives with
the work piece surface, and the type of slurry. We have found indirect
evidence that, upon coming into contact with the work piece surface
in the region of high magnetic field, the converging ribbon of MR
fluid separates into layers. The top layer seems to be composed
of the carrier liquid and the polishing abrasives, while the bottom
layer consists of compacted chains of magnetic particles attached
to the rotating wheel. In this condition, fluid flow in the polishing
interface for the two commercially available MR fluids described
above is optimal for promoting a high removal rate and smooth surfaces.
maintain an active research program to address various issues associated
with MRF. We concentrate on how MRF may be improved through experiments
to better understand the relationship between MR fluid composition
and the following: rheology in and out of a magnetic field, stability,
material removal, the polishing process, and the quality of the
finished part. Materials of interest are defined to be anything
with relevance to research and industrial applications, and are
not limited to "optical" materials. Specific areas of
research are currently identified as follows:
- Colloid science-based
strategy for MR fluid formulation, including magnetic particle
surface modification, surfactant additives as coatings, 3D-gel
formation, and dispersion/blending protocols;
- MR fluid
formulation for different materials to be polished, including
magnetic abrasive selection, polishing abrasive selection, carrier
fluid selection, and stabilizer selection;
- MR fluid
property measurement, including structure within the ribbon, sedimentation
stability, chemical stability, base viscosity, and shear stress
- MR fluid
performance evaluation, including material removal rate and lowering
the rms surface micro-roughness to below 0.5 nm rms.
depiction of MRF circulation system. The low viscosity MR
fluid is pumped through a shaping nozzle onto a vertical,
rotating wheel. At the apex of the wheel, the fluid stiffens
into a ribbon ,under the influence of a dc magnetic field.
The workpiece is placed into the ribbon and forms a converging
gap. Material is removed by the shearing action of the nonmagnetic
abrasives in the MR fluid. Shaping and smoothing are accomplished
simultaneously as the rotating workpiece is moved through
the ribbon under computer control.
are interested in addressing issues that may arise from the use
of MRF machines in industrial settings. User observations can offer
additional insights toward our mission to improve the MRF process.
To this end, we perform experiments to study user-related issues
brought to our attention.
monitoring the progress of removal with MRF on a 266 mm diameter,
fused silica parabola intended for the OMEGA EP laser at LLE.
of the Q22-Y after being set up by Ed Fess (LLE ME Department)
to polish a 300 mm diameter, convex fused siica asphere for
the OMEGA laser system.
Chunlin Miao, "Frictional Forces in Material Removal for Glasses and Ceramics using Magnetorheological Finishing", Materials Science Program, University of Rochester, Rochester, NY, Dec. 2009.
Jessica E. DeGroote, "Surface Interactions Between Nanodiamonds
and Glass in Magnetorheological Finishing (MRF)," The Institute
of Optics, University of Rochester, Rochester, NY, June 2007.
N. Shafrir, "Surface Finish and Subsurface Damage in Polycrystalline
Optical Materials," Materials Science Program, University of
Rochester, Rochester, NY, June 2007.
S. N. Shafrir, H. J. Romanofsky, C. Miao, M. Wang, R. Shen, H. Yang, J. C. Lambropoulos and S. D. Jacobs, "Zirconia-coated carbonyl-iron particle-based magnetorheological fluid for polishing optical glasses and ceramics", Applied Optics 48, 6797-6810 (2009)
R. Shen, S. Shafrir, C. Miao, M. Wang, J. Lambropoulos, S. Jacobs and H. Yang, "Synthesis and Corrosion Study of Zirconia Coated Carbonyl Iron Particles", J. Colloid Interface Sci. (2009), doi:10.1016/j.jcis.2009.09.033
C. Miao, S. N. Shafrir, J. C. Lambropoulos, J. Mici, and Stephen D. Jacobs, "Shear stress in magnetorheological finishing for glasses", Appl. Opt. 48, 2585-2594 (2009).
S.N. Shafrir, J. C. Lambropoulos, and S. D. Jacobs “Subsurface damage (SSD) and microstructure development in precision microground hard ceramics using MRF spots”, Appl. Opt. 46, pp. 5500-5515 (2007).
J. E. DeGroote, A. E. Marino, J. P. Wilson, A. L. Bishop J. C. Lambropoulos and S. D. Jacobs, “Removal rate model for magnetorheological finishing (MRF) of glass,” Appl Opt. 46, pp. 7927-7941 (2007).
S. N. Shafrir, J.C. Lambropoulos, S.D. Jacobs, 2007, “Toward magnetorheological finishing of magnetic materials,” J. of Manuf. Sci. and Eng., Vol. 129 5, pp.961-964 (2007).