This document discusses a study measuring drag forces during magnetorheological finishing (MRF) of optical glasses. The researchers used a spotting technique to measure in-situ drag forces on glass substrates during MRF. They found that drag force increases linearly with substrate penetration depth into the MRF fluid ribbon. Higher initial surface roughness of the substrate also correlated with higher drag forces. Additionally, higher drag forces produced through greater penetration depths resulted in higher material removal rates during MRF.

1. S. Salzman†1, H. Romanofsky*, S. N. Shafrir*, J. C. Lambropoulos*,2, and S. D. Jacobs*,3S. Salzman†1, H. Romanofsky*, S. N. Shafrir*, J. C. Lambropoulos*,2, and S. D. Jacobs*,3
In-Situ Drag Force Measurements
in MRF of Optical Glasses
In-Situ Drag Force Measurements
in MRF of Optical Glasses
University of Rochester, Laboratory for Laser Energetics
†Corresponding author e-mail: sisalzm@me.rochester.edu, 1ORT Braude College of Engineering, Karmiel, Israel, 2Department of Mechanical Engineering, 3The Institute of Optics University of Rochester, Rochester, NY
Abstract
MRF is a deterministic process developed at the University of Rochester for
polishing optical materials.1 The properties of the surface polished with MRF
are a function of the MR fluid used, process (or machine) parameters, and the
interactions with the sample.The removal mechanism is governed by a local
shear stress, created at the interface between the ribbon of MRF fluid and the
substrate.2 This parameter is defined as the ratio between the tangential drag
force, FD (1-5 N, from Ref. 3), and the contact area, A (this is the MRF contact
area,denoted as the MRF spot).In this work an MRF spotting technique is used
to study material removal for optical glasses.The capability for measuring in-
situ drag force permits us to study, for the first time, drag force as a function
of substrate surface roughness.
1S. D. Jacobs et al., “Magnetorheological finishing: A deterministic process for optics
manufacturing,” in Optical Fabrication and Testing (SPIE,Tokyo, Japan, 1995), Vol. 2576,
pp. 372–382.
2A. B. Shorey, “Mechanisms of material removal in magnetorheological finishing (MRF)
of glass,” Ph.D. dissertation, Department of Mechancial Engineering, Materials Science
Program, University of Rochester, 2000.
3J. E. DeGroote et al., “Removal rate model for magnetorheological finishing of glass,”
Applied Optics 46, 7927–7941 (2007).
Magnetorheological finishing (MRF) with a compliant,
magnetic fluid lap was invented at the University
of Rochester in 1993–1995
G7168d
Wheel and
ribbon
Carbonyl iron (CI) and CeO2 in water
2 nm
Field “on”
Magnetic-field
gradient
Unsheared
fluid
Abrasive particles
Iron particles
Sheared
fluid
Pressure
distribution
Spindle
rotation
Lens
sweeps
through
MR fluid
Removal by tangential force
Wheel rotation
Four University of Rochester Ph.D.Theses:
A. Shorey (’00), J. Randi (’04), J. DeGroote (’07), S. Shafrir (’07)
LHG-8, BK7, and FS optical glasses were studied
G8599
• Nd: phosphate composition LHG8 is used in all high-peak-power glass lasers
• Developed at LLE in 1978 in collaboration with Hoya Corporation1
• High-gain, low-nonlinear refractive index, athermal OMEGA laser rods and
disks are made entirely from LHG8
• One-half of the LLNL NIF laser slabs are composed of LHG8 (>200 tons)
• Moderate chemical durability, better than most other phosphates
• Other optical glass substrates (BK7, FS) were chosen based on their
economic importance
1 S. D. Jacobs, in Proceedings of the Technical Program, Electro-Optics/
Laser 1978 Conference and Exposition (Industrial and Scientific
Conference Management, Chicago, 1978), pp. 24–31.
Surface-MR fluid–ribbon interactions are evaluated
by taking individual spots on stationary parts using
a spot-taking machine (STM)
G8600
• CNC control along only the z axis & precise penetration depth into ribbon
• Immersion times from 2 s to minutes & shallow/deep spot
• Laser interferometry & peak removal rates at depth of deepest penetration
(ddp) within spot, or stylus profilometry for spots deeper than 0.2 nm
• Long spotting times & accentuate abrasive–surface interactions; not
representative of the MRF process used to polish out the surface of a part
z
x
y
CollectorFluid-delivery system
Nonrotating
part
Nozzle
STM
Peak-to-valley
Leading edge ddp region Trailing edge
0.0
0.0
10 5
Distance (mm)
(nm)
0
Height(nm)
–0.3
0.2
MRF flow
Drag-force measurements are made in situ using
a single-axis, quartz dynamic-load cell
G8601
Load
cell
MR fluid ribbon Polishing zone
Mounting
device
Workpiece
System
at rest
Part depressed
into ribbon (3 s)
*
Dragforce(N)
Elapsed time
Output signal
drag force
• Rotation to a new location for each spot
without removing device from STM spindle
• High sensitivity (error less than !0.2 N)
permits detection of drag forces
corresponding to changes in
– substrate penetration depth
– substrate surface condition
– substrate type
– MR fluid composition
*An MRF force sensor program, LabView based, was written internally
The drag force linearly increases with the penetration
depth of the part into the MR fluid ribbon
G8602
• Noise as part is lowered to the ribbon
• Drag force increases at 0.5 N per 0.1-mm increment in depth to 0.5 mm
• Repetition of experiment gave reproducible results for up to 26 days
–2.0 –1.5 –1.0
0
2
Polished BK7 part
4
Dragforce(N)
6
8
Day 3
Day 4
Day 2
Day 1
Day 26
–0.5
Height above Into MR fluid ribbon
0.5 1.00.0
Probe position
MR fluid ribbon
~0.04
mm
#3 #2 #1
(Not to scale)
14 mm
0.0
Volumetricremovalrate
(mm3/min)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Polished surfaces
0.2 mm
0.1 mm
0.2 mm
0.3 mm
0.3 mm
0.1 mm
0.3 mm
0.5 1.0
Drag force (N)
1.5 2.0 2.5
BK7
FS
LHG8
0.2 mm
The drag force (FD) shows a positive linear
correlation to the volumetric removal rate
with part penetration into the MRF ribbon
G8604
• At a 0.3-mm depth (nominal working depth)
– 20% increase in FD corresponds to a 3# increase in removal rate
for BK7, FS
– 10# increase in removal rate for LHG8 with same drag force
A positive linear correlation is observed between initial
p–v surface roughness* and drag force for glasses
G8603 *Zygo NV5000, 5 samples in ddp, areal over 250 nm by 350 nm, unfiltered.
• Controlled stages of loose abrasive grinding used for sample preparation
• High initial p–v surface roughness results in a high drag force
• The drag force drops as progressively smoother surfaces are tested
• Stronger dependence for LHG8 (soft, less durable) and FS (hard, more
durable) compared to BK7
• Drag forces for polished parts span a range from 1.45 to 1.88 N
0
0.0
0.5
1.0
Averagedragforce(N)
1.5
2.0
2.5
3.0
5 10 15 20
Initial surface roughness p–v (nm)
25 30 35 40
y = 0.01x + 1.48
R2 = 0.98
y = 0.05x + 0.70
R2 = 0.83
y = 0.06x + 0.95
R2 = 0.97
0.3-mm depth into MR fluid ribbon
BK7
FS
LHG8
Drag forces in MRF of various glasses
has been measured
G8605
• Drag force measurements are very sensitive to part penetration into the
ribbon; drag force increases as the part substrate submerged deeper into
the ribbon.
• Drag force measurements are very sensitive to initial surface roughness
– high p–v " high drag force (FD), with larger contact area
– low p–v " low drag force (FD), with smaller contact area
• There is a strong linear relationship between volumetric material removal
rate and penetration depth.The higher the penetration depth, the higher
the volumetric removal rate.
Summary
Future work
G8606
• Expand to heterogeneous optical ceramics
– see Miao et al., “Frictional Investigation for Magnetorheological
Finishing (MRF) of Optical Ceramics and Hard Metals”
- Thursday, 23 October 2008; 11:30 am – 11:45 am
Acknowledgments
• Mechanical Engineering Department at Ort Braude College,
Karmiel, Israel
• Alex Maltsev and Mike Kaplun (LLE) — sample preparation
• Scott Russell (UR) — Load cell LabView software
• Continuing support
- Laboratory for Laser Energetics, University of Rochester,
Rochester, NY
- U.S. Army Armament, Research, Development,
and Engineering Center
- U.S. Department of Energy Office of Inertial Confinement Fusion
The removal rate in MRF depends
upon the shear stress
G8598
The drag force in MRF is measured in this work.
*A is the measured spot area in this work
C
A
F
V C
A
F
V C
A
V
F
C PV C VmRR N N D
p p pp p
n
x= = = = =l l
Preston (1927)
MRR = material removal rate
Cp = Preston’s coefficient
P = local pressure
V = relative velocity
FN = normal load
Shorey (2000)
A = contact area
Clp = modified Preston’s coefficient
n = friction coefficient = FD/FN
FD = drag force
x = shear stress = FD/A*
l