The material removal in magnetorheological finishing (MRF) is known to be controlled by shear stress, λ, which equals
drag force, Fd, divided by spot area, As. However, it is unclear how the normal force, Fn, affects the material removal in
MRF and how the measured ratio of drag force to normal force Fd/Fn, equivalent to coefficient of friction, is related to
material removal. This work studies, for the first time for MRF, the normal force and the measured ratio Fd/Fn as a function
of material mechanical properties. Experimental data were obtained by taking spots on a variety of materials including
optical glasses and hard ceramics with a spot-taking machine (STM). Drag force and normal force were measured with a
dual load cell. Drag force decreases linearly with increasing material hardness. In contrast, normal force increases with
hardness for glasses, saturating at high hardness values for ceramics. Volumetric removal rate decreases with normal force
across all materials. The measured ratio Fd/Fn shows a strong negative linear correlation with material hardness. Hard
materials exhibit a low "coefficient of friction". The volumetric removal rate increases with the measured ratio Fd/Fn which
is also correlated with shear stress, indicating that the measured ratio Fd/Fn is a useful measure of material removal in MRF.
Magnetorheological finishing (MRF) is a sub-aperture deterministic process for fabricating high-precision optics by
removing material and smoothing the surface. The goal of this work is to study the relative contribution of
nanodiamonds and water in material removal for MRF of aluminum oxynitride ceramic (ALON) based upon a
nonaqueous magnetorheological (MR) fluid. Removal was enhanced by a high carbonyl iron concentration and the
addition of nanodiamond abrasives. Small amounts of deionized (DI) water were introduced into the nonaqueous MR
fluid to further influence the material removal process. Material removal data were collected with a spot-taking machine.
Drag force (Fd) and normal force (Fn) before and after adding nanodiamonds or DI water were measured with a dual load
cell. Both drag force and normal force were insensitive to the addition of nanodiamonds but increased with DI water
content in the nonaqueous MR fluid. Shear stress (i.e., drag force divided by spot area) was calculated, and examined as
a function of nanodiamond concentration and DI water concentration. Volumetric removal rate increased with increasing
shear stress, which was shown to be a result of increasing viscosity after adding nanodiamonds and DI water. This work
demonstrates that removal rate for a hard ceramic with MRF can be enhanced by adding DI water into a nonaqueous MR fluid.
Aqueous magnetorheological (MR) polishing fluids used in magnetorheological finishing (MRF) have a high solids
concentration consisting of magnetic carbonyl iron particles and nonmagnetic polishing abrasives. The properties of MR
polishing fluids are affected over time by corrosion of CI particles. Here we report on MRF spotting experiments
performed on optical glasses using a zirconia coated carbonyl iron (CI) particle-based MR fluid. The zirconia coated
magnetic CI particles were prepared via sol-gel synthesis in kg quantities. The coating layer was ~50-100 nm thick,
faceted in surface structure, and well adhered. Coated particles showed long term stability against aqueous corrosion.
"Free" nano-crystalline zirconia polishing abrasives were co-generated in the coating process, resulting in an abrasivecharged
powder for MRF. A viable MR fluid was prepared simply by adding water. Spot polishing tests were performed
on a variety of optical glasses over a period of 3 weeks with no signs of MR fluid degradation or corrosion. Stable
material removal rates and smooth surfaces inside spots were obtained.
We developed a new magnetorheological (MR) fluid for studying the relative contributions of mechanics and chemistry
in polishing hard materials. The base carrier fluid is a mixture of two non-aqueous liquids. At conventional carbonyl iron
(CI) magnetic particle concentrations, removal rates with this formulation were unacceptably low for the polycrystalline
optical ceramic aluminum oxynitride (ALON). We overcame this problem by creating a high magnetic solids
concentration suspension consisting of a blend of large and small CI particles. Our test bed for experiments was a
magnetorheological finishing (MRF) spot-taking machine (STM) that can only polish spots into a non-rotating part. We
demonstrated that, using this new MR fluid formulation, we could substantially increase peak removal rates on ALON
with small additions of nonmagnetic, nanodiamond abrasives. Material removal with this fluid was assumed to be
predominately driven by mechanics. With the addition of small amounts of DI water to the base fluid containing
nanodiamonds, the peak removal rate showed an additional increase, presumably due to the altered fluid rheology and
possibly chemical interactions. It is possible, however, that this result is due to increased fluid viscosity as well.
Interestingly, the microtexture on the surfaces of the ALON grains (albeit-two different ALON parts) showed distinctly
different features when spotted with nanodiamonds or with nanodiamonds and water, and an understanding of this
phenomenon is the goal of future work. In this paper we describe the difficult fluid viscosity issues that were addressed
in creating a viable, high removal rate, non-aqueous MR fluid template that could be pumped in the STM for several
days of experiments.
Conference Committee Involvement (3)
Optical Manufacturing and Testing X
26 August 2013 | San Diego, California, United States
Optical Manufacturing and Testing IX
22 August 2011 | San Diego, California, United States
Optical Manufacturing and Testing VIII
4 August 2009 | San Diego, California, United States
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