Modeling and Analysis of Cross Feed Servo System of Heavy Duty Lathe Subjected to Friction

The performance of feed servo system has an important effect on the path tracking and positioning accuracy of computer numerical control (CNC)machine tool and the machined surface of the workpiece. The nonlinearity of friction can lead to stick-slip behavior of the feed servo system. A modified Stribeck friction model is established based on the ideas of the Karnopp friction model. The cross feed servo system of a heavy duty lathe with closed loop control is studied. The multiple degree of freedom mechanical and mathematical models of the cross feed servo system subjected to torsional stiffness of shaft, meshing stiffness of gears, screw pair contacting stiffness, axial stiffness of the bearing and the ballscrew, kinematical friction force between up and down guide way are established. Effects of stiffness on worktable are studied at low velocity. Also, the weak element of the mechanical transmission system is found out. In the field test, the critical stick slip feed velocity at different feed position is measured. And the relationship between the critical stick slip velocity and the axial stiffness of the ballscrew is obtained. The theoretical analysis and the experiment results match well. The research conclusions can provide theoretical support for the optimal design and performance prediction of feed servo systems of heavy duty lathe.