Characterization of Fixture-Workpiece Static Friction By Jose F. Hurtado Shreyes N. Melkote Precision Manufacturing Research Consortium Georgia Institute of Technology Sponsors: NSF, GM R&D Center
Background Critical Functions of a Machining Fixture: Accurately locate and hold a workpiece with respect to the cutting tool. Minimize workpiece movement and distortion due to clamping and machining. Fixture performance depends on contact forces and friction, e.g. mechanical vise. Locators Workpiece Hydraulic Clamps F y Workpiece F tan F n = F z F x F tan < µ s F n Base F tan = tangential force F n = normal force µ s = static coefficient of friction
Motivation Scientific understanding of the effects of workpiece, fixture and cutting process variables on workpiece-fixture contact forces and friction is limited. Factors of interest are those affecting the workpiece-fixture interface such as contact geometry, workpiece stiffness, surface finish and machining conditions. The understanding of these variables on workpiece-fixture contact force and friction, especially, under dynamic conditions is needed.
Objectives Experimentally study the effects of vibration and clamping load on the static coefficient of friction, µ s. Prediction of workpiece-fixture contact forces during machining based on an empirical model for dynamic friction.
Study of Dynamic Friction Objective: Understand the effect of oscillatory forces and vibration on the static coefficient of friction for a workpiece-fixture material pair. Materials Considered: Cast Aluminum 357 (Workpiece) Steel AISI 1144 with Black Oxide Finish (Locator) Working Principle of the Test Stand: Constant preload Oscillatory normal force Pulling force Oscillatory tangential force Workpiece material Fixture material
Experimental Set-Up 1. Linear stepper motor. Servo Systems 23A-6102A. 2. Strain gauge. Omegadyne LC-105. 3. Base plate. 4. Link. 5. Displacement probe. Kaman KD1106-1S, KD2300-2S. 6. Accelerometer. PCB Electronics 356A11. 7, 11, 18. Triaxial load cell. Kistler 9117A. 8. Preloading bolt. Kistler 9461. 9. Locator material holder. 10. Workpiece material sample. 12, 17. Piezoelectric pusher. Burleigh PZL-060. 13. Angle bracket. Carr Lane CL-13-GAB. 14. Thrust bearing. 1 in. O.D. 15. Stainless steel sliding plate. 2 16. Support. 1 4 6 F n 11 12 13 14 5 15 16 17 18 F tan 3 5 7 8 9 10
Experimental Design Experimental Design: Central Composite Design. 2 x 27 Test Runs. Factor A: Normal Preload Force. Low: 100 N Medium: 200 N High: 300 N Factor B: Vibration Amplitude in the Normal Direction. Low: 0 µm Medium: 2.5 µm High: 5.0 µm Factor C: Excitation Frequency in the Normal and Tangential Directions. Low: 100 Hz Medium: 150 Hz High: 200 Hz Factor D: Vibration Amplitude in the Tangential Direction. (Not controlled at fixed levels) Response: F = tan µ µ s,novib s,extvib µ s Fn
Results 300 80 250 F n = 242.75 N 60 Force (N) 200 150 100 50 Relative Displacement F tan = 43.45 N Onset of Motion 40 20 0-20 -40 Displacement (µm) 0 2000 samples/s -60 1 5001 10001 15001 20001 Sample Data from Test #16
Results Maximum reduction in µ s = 61% A second-order response surface was fit to the collected data. R 2 = 81.2 %. The following effects are significant for α = 5 %: Preload force Normal amplitude Normal/Tangential Frequency Preload Force * Normal Amplitude Preload Force * Frequency µ s, extvib µ s, novib % Change = ( ) 100% µ Normal Amplitude * Tangential amplitude Frequency * Tangential Amplitude s, novib Preload Force * Tangential Amplitude (only for α = 10 %)
Response Surface for µ s,extvib 0.270 µ s,extvib 0.25 0.20 0.257 0.274 0.242 0.142 0.273 0.229 0.267 0.15 100 100 200 Preload (N) 300 200 150 Tangential Frequency (Hz) Low-Intensity Machining Process. Normal Amplitude = 0 µm, Tangential Amplitude = 5.2 µm.
Response Surface for µ s,extvib 0.3 0.244 0.280 0.277 0.222 0.255 0.2 0.1 0.151 0.183 0.116 0.0 100 100 200 Preload (N) 300 0.010 200 150 Tangential and Normal Frequency (Hz) High-Intensity Machining Process. Normal Amplitude = 5 µm, Tangential Amplitude = 22.7 µm.
Explanations Increase in µ s,extvib with increasing excitation frequency at low normal preloads was attributed to breakdown of contaminant layers due to vibration. Decrease in µ s,extvib with increasing frequency at high normal preloads was attributed to decrease in duration of asperity contact and possible resonance. The increase in µ s,extvib with increasing preload was explained by the increase in the actual contact area with increasing normal preload.
Prediction of Reaction Forces Free Body Diagram of the Workpiece. +Z L1 µl1 +Y +X L2 µl2 AL2 AL1 L4 O µl4 AL4 µl5 L3 µl3 AL3 My Mz Mx C2 µc2 AC2 µl6 AL6 L6 W L5 AL5 µc1 AC1 C1 System Constraints. Force equilibrium. Moment equilibrium. Direction of the normal contact force. Maximum normal force.
Prediction of Reaction Forces Summary of Results: The model is in good agreement with reaction forces at a specific locator when only clamping forces are applied. The accuracy of the model can be improved by using estimated values of µ s under dynamic conditions similar to those present during machining. Relative Errors (%) Test Condition Average Maximum F tan F n F tan F n Clamping Only 4.2-3.9 10.6-6.5 Machining Using µ s,novib 16-9.8 42.4-11.2 Machining Using µ s,extvib -3.4-5.3 18.4-8.5
Conclusions There is a significant difference between the static coefficient of friction under dynamic and non-dynamic conditions. Preload force, amplitude of oscillation in the normal direction and frequency of externally applied loads are significant first-order factors affecting µ s,extvib. At low preload forces and for low- and medium-intensity machining processes, increasing the excitation frequency causes an increase in µ s,extvib. As the preload force and/or the intensity of the machining process increase, the effect of the excitation frequency is to lower µ s,extvib.
Future Work Study the effect of the following factors on the static coefficient of friction under dynamic conditions, µ s,extvib : Cutting fluids Higher normal preloads Phase of the normal and tangential excitation signals Other workpiece materials such as cast iron Spherical locators