Normal Force and Drag Force in Magnetorheological Finishing

Transcription

1 Normal Force and Drag Force in Magnetorheological Finishing C. Miao, S. N. Shafrir, J. C. Lambropoulos, and S. D. Jacobs University of Rochester Laboratory for Laser Energetics Optical Manufacturing and Testing VIII SPIE Optics and Photonics San Diego, CA 4 5 August 2009

2 Summary A frictional investigation of MRF for optical glasses and hard ceramics is carried out A dual-load cell is used for simultaneous, in-situ measurement of frictional forces on optical glasses and hard ceramics Drag force decreases linearly with increasing material hardness, while normal force saturates at high hardness The measured ratio F D /F N, corresponding to the coefficient of friction, is closely correlated in MRF with the hardness values of materials We demonstrate that the measured ratio F D /F N, taking into account both drag and normal forces, is a useful measure of material removal in MRF for both glasses and ceramics. G8620a

3 Objectives Employ a dual-load cell for real time, simultaneous measurement of both drag force and normal force on materials during MRF Determine how drag force (F D ) and normal force (F N ) respond to material mechanical properties Study the relationship between coefficient of friction (COF F D /F N ) and material removal rate G8621a

4 Material removal in MRF is known to be based on drag force and shear stress Preston (1927): F MRR N = CpPV = Cp V Ac Shorey et al. (2000): nf F C N V C A D MRF = lp MRF _ F V C V Ni = lp MRF _ F s Di = l A p, MRF ^xh : x : s Miao et al. (2009): E MRR MRF = Clp, MRF ^x, FOM h V Kc HV : x : MRR = material removal rate n = friction coefficient = F D /F N C p = Preston s coefficient P = normal pressure F N = normal force F D = drag force A c = nominal contact area A s = spot area V = relative velocity x = shear stress = F D /A s E = Young s modulus K c = fracture toughness = Vickers hardness H V G8622a Cl p,mrf(fn ) = modified Preston s coefficient in terms of F N Cl p,mrf(fd ) = modified Preston s coefficient in terms of F D Cl p,mrf(x) = modified Preston s coefficient in terms of shear stress Cl p,mrf(x,fom) = modified Preston s coefficient in terms of shear stress and FOM FOM = figure of merit, defined as 2 E K c H v

5 Materials of interest include optical glasses and hard ceramics Materials in each group are listed by the order of increasing Vickers hardness Materials Mat. ID Grain Size (nm) Young s Modulus E (GPa) Vickers Hardness H v (GPa) Fracture Toughness K c (MPa m 1/2 ) Source Optical glasses (100-g load, literature values) Phosphate LHG Hoya Borosilicate N-BK Schott Fused silica Corning Hard ceramics (500-g load, Shafrir 2007) Magnesium aluminum Spinel TA&T oxide Aluminum oxynitride ALON Surmet Polycrystalline alumina PCA ~ CeraNova Silicon carbide CVC SiC Trex G8623

6 MRF spots are taken on a research platform: a spot-taking machine (STM) Part mounted on a nonrotating z-axis slide MRF spot created by lowering nonrotating part into the rotating MR fluid ribbon Volumetric removal rate (VRR) calculated from spot volume and spot time STM machine parameters held constant for this work All parts pre-polished flats Interferometric map of polishing spot on part surface Part Ribbon Flow direction Z axis Z axis G8624 Nozzle STM

7 The drag force (F D ) and normal force (F N ) are measured in situ on the STM using piezoelectric sensors Z axis Sensors measure dynamic, normal, and shear forces Sensors respond to changes in substrate type substrate surface condition STM machine settings MR fluid composition All spots taken on machine z axis Normal force sensor Drag force sensor Ribbon F N Part Wheel rotating clockwise F D G8625 Fluid direction Single-axis slim line shear (K9143B21) and compressive (K9133B21) load-cell measuring system (Kistler Instrument Corp., Amherst, NY).

8 The sensor output signals are stable throughout the measurement LabView interface records the drag and normal forces simultaneously Data collecting rate: 10 data points per second Force is averaged over the whole spotting time 15 Spinel Force in Newtons 10 5 F N : ~9 N F D : ~4 N F N : ~11 N F D : ~3.5 N 0 2 s 15 s G8626 Elapsed time (s)

9 Dual-load-cell results Both drag force (F D ) and normal force (F N ) strongly correlate to material hardness Drag force (F D ) Decreases linearly with increasing hardness (STM parameters fixed) F D (N) LHG8 BK7 BK7 LHG8 [ DeGroote ] 2007 ALON Spinel Sapphire Shorey [ 2001 ] y = 0.09x R 2 = 0.79 PCA SiC H v (GPa) G8627 Prior work done on the STM under different machine conditions Significant expansion of prior work on the STM

10 Dual-load-cell results Both drag force (F D ) and normal force (F N ) strongly correlate to material hardness Normal force (F N ) Increases linearly with increasing hardness Saturates at ~11 N for hard ceramics 15 Lines to guide eye Normal force, F N (N) 10 5 BK7 LHG8 Spinel ALON PCA SiC G8637a Vickers hardness, H v (GPa)

11 It is possible that neither normal force nor drag force can, by itself, adequately describe removal in MRF Volumetric removal rate, VRR (mm 3 /min) Lines to guide eye LHG8 Spinel SiC PCA ALON BK F D (N) Volumetric removal rate exhibits a negative correlation with normal force across all materials This inverse relationship is opposite to the plotted relationship between drag force and the removal rate Volumetric removal rate shows a tendency to increase with drag force There is no clear linear correlation Volumetric removal rate, VRR (mm 3 /min) LHG8 Line to guide eye BK7 SiC Spinel ALON PCA 15 F N (N) G8818

12 The measured ratio F D /F N shows an inverse correlation with material hardness The measured ratio F D /F N, taking into account both normal force and drag force, is equivalent to the coefficient of friction LHG8 y = 0.016x R 2 = 0.80 Confidence level: >99% F D /F N BK7 Spinel 0.2 ALON PCA SiC Vickers hardness, H v (GPa) Interactions between MR fluid particles and the part are dominated by mechanics. G8628a

13 The material removal rate shows a strong dependence on the measured ratio F D /F N 0.6 Volumetric removal rate, VRR (mm 3 /min) LHG8 0.4 Line to guide eye BK7 0.2 SiC Spinel 0.0 ALON PCA F D /F N G8629a Both drag and normal forces play an important role in material removal in MRF for glasses and ceramics.

14 The measured ratio F D /F N exhibits a strong correlation with shear stress Shear stress, x (MPa) y = 0.09x R 2 = 0.83 Confidence level: >99% Spinel PCA SiC ALON MRR MRF = Cl p, MRF(x) x V LHG8 BK F D /F N G8819 The measured ratio F D /F N, taking into account the contribution of both drag force and normal force, is an important alternative to shear stress for evaluating material removal for MRF

15 Acknowledgments Alex Maltsev and Mike Kaplan (LLE) for polishing parts Scott Russell (UR) for LabView software interface Spinel disks were provided by TA&T CVC SiC material was provided by Trex Polycrystalline alumina disks were provided by CeraNova Corporation. Development of this material by CeraNova is funded by NAVAIR through the U.S. Government SBIR program; SBIR data rights apply Continuous financial support Laboratory for Laser Energetics Horton Fellowship U.S. Army Armament, Research, Development, and Engineering Center U.S. Department of Energy Office of Inertial Confinement Fusion

16 Summary/Conclusions A frictional investigation of MRF for optical glasses and hard ceramics is carried out A dual-load cell is used for simultaneous, in-situ measurement of frictional forces on optical glasses and hard ceramics Drag force decreases linearly with increasing material hardness, while normal force saturates at high hardness The measured ratio F D /F N, corresponding to the coefficient of friction, is closely correlated in MRF with the hardness values of materials We demonstrate that the measured ratio F D /F N, taking into account both drag and normal forces, is a useful measure of material removal in MRF for both glasses and ceramics. G8620a