In order to improve NC machining precision this article analyzes the effects of cutting force on NC machining precision by means of simulation, and builds up the mathematical models of CK7815 feed drive system and radial action force of cutting feed, in order to study the effects of cutting load torque and radial cutting force on work pieces processing precision. The simulation results illustrate that cutting load torque and radial cutting force have respectively effects on axial precision and radial precision of work piece. The bigger is cutting load torque, the bigger is axial error. The smaller is cutting process coefficient and the higher is converting spring stiffness of lathe and tool, the smaller is radial error. In practice NC machining precision is improved by trying to decrease the cutting load torque and cutting process coefficient and increase converting spring stiffness.