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The performance of the feed servo system has an important effect on CNC machine tools, including the tracking and position accuracy as well as the surface quality of the machined part. The nonlinear factors such as friction and backlash can induce unstable behaviors such as stick-slip and oscillation of the feed servo system. The mechanical and mathematical models containing friction and backlash were established for the motor direct-connected feed servo system with close loop controller. The effects of friction and backlash on the worktable under low feeding velocity were studied. A few of possible conditions for the stick-slip occurrence of the worktable were obtained. And some means for cancelling the stick-slip behavior of the feed servo system were presented at last. The research results provided a theoretical support for the optimum design and performance forecast of the feed servo system.