MODELING AND ANALYSIS OF THE CROSSFEED SERVO SYSTEM OF A HEAVY-DUTY LATHE WITH FRICTION

摘要

The performance of feed servo system has an important effect on the path tracking and positioning accuracy of computer numerical control (CNC) machine tools. The nonlinearity of friction can lead to the occurrences of unstable behavior of the feed servo system, such as stick-slip motion and oscillation. The crossfeed servo system of a heavy-duty lathe with closed loop control subjected to friction is studied in this article. The multi-degree of freedom (MDOF) mechanical and mathematical models of the Cross-feed servo system are established. In the models, the torsional stiffness of shaft, meshing stiffness of gears, ballscrew-nut pair contacting stiffness, axial stiffness of the bearing, and the ballscrew and the kinematical friction force between up and down guideways are considered. The weakest link of the mechanical transmission system of the feed system is found by simulations. In the field test, the critical stick slip feed velocity at different feed position is measured by the Renishaw XL laser interferometer. And the relation curve of the critical stick slip velocity versus the axial stiffness of the ballscrew is obtained. The change tendency of the simulation results and that of the experimental results matches well. The research conclusions can provide a theoretical support for the optimal design and performance prediction of the feed servo system of heavy-duty lathes.