Feed Drive System on Large NC Machine Tools

Summary

The object of this paper is to give designers some principles to analyse, choose and calculate the feed drive system and the D.C. motor of a NC machine tool feed axis.

After the theoretic treatment of contouring errors in plane interpolation and their causes, an effective and simple measuring and inspection system is reported for the determination of error amount and successful location of periodic error causes.

Among contouring error causes there are torque disturbances due to the D.C. motor and Coulomb’s friction torque disturbances.

From the analysis of the feed drive system transfer function, significant expressions are deduced, allowing to value the effects of the aforesaid torque disturbances.

Moreover the analysis of the main problems relevant to this feed drive system and feed axis characteristics are reported. The capacity of the feed drive motor to supply the necessary torque in all speed ranges and in acceleration is calculated and checked, as well as the amount of the contouring errors due to motor torque and friction torque disturbances.

Follows a description of the wide feed range regulation that can be obtained without the Numerical Control System, by means of a special low speed device called Position Loop and its applications. This device is a mixed analog-digital position feed-back system, allowing the control of a D.C. motor as a closed loop stepper motor and to carry out “zero velocity” that means motionless axis, but under electric control. It enables the use of other unconventional devices, such as: electronic threading, electronic handle, electric shafts, servo-clamp, positioning.