Abstract: Based on the FOCAS function set for the FANUC numerical control system， a real-time acquisition method of worktable position and servo motor torque current for a feed drive system is proposed. The continuous functions of the worktable position and the servo motor torque current for time are determined by virtue of inserting their discrete data with the cubic spline function， respectively. Then， an algorithm is proposed to identify the friction of the feed system. The characteristics of average value and spectrum for the friction are analyzed by using the experimental data. The results demonstrate that the local average frictions take on a catenary form because the lead screw-nut pair and the bearings at both ends are not concentric. The average friction over the experimental feed length is larger when the feed speed is low， and it decreases rapidly with the increase of the feed speed. However， when the feed speed reaches a certain value， the friction force increases slightly with the increase of the feed speed. In the operating process of the feed drive system， the periodic motion of rolling elements inside the screw groove， screw waviness， balls going in and out of the screw-nut pair and circular rails in the guides can motivate the corresponding fundamental and harmonic frequencies of the friction.

Key words: friction, CNC machine tool, feed drive system, spectrum, position-dependent characteristics, velocity-dependent characteristics