Development of high speed servo-spindle for NC gear grinding machines

Abstract

A new servomechanism for high speed servo-spindle of NC gear grinding machines is presented in this paper. The NC gear grinding machine requires high speed servo-spindles with extremely high accuracy. However, it is not easy to keep the rotary accuracy high enough at a high speed rotation. Usually, the resolution of a rotary encoder decreases with the increase of rotational speed by reason of response-frequency, and it becomes extremely low at speeds of more than 1000 rpm. Since the rotary accuracy changes in proportion to the resolution, the authors used a magnetic scale of high accuracy and tried to raise the resolution by means of two-phase type PLL proposed by Emura. The servomotor driving the servo-spindle is an A.C. induction motor controlled by the vector control method, and its rated power is 7.5 kW. To increase the servo-stiffness of the spindle, an inertial damper was used. This damper is very effective and can increase the position-loop-gain stably and extremely. Two high speed servo-spindles of the same structure were used as a grinding spindle and a work spindle of a newly designed NC gear grinding machine, and some grinding experiments were carried out. As a result of these experiments, it became clear that the servo-spindle developed by the authors is very suitable for the NC gear grinding machines.