Influence of the ball screw stiffness on the positioning accuracy of the CNC machine tools

Abstract

This work is researching the ball screw stiffness influence on the positioning accuracy of the feed kinematic linkage of a CNC machine tool fitted with an indirect type measuring system for positioning the moving element. Because the machine tool is equipped with an indirect measuring system, backlashes and elastic deformations in the kinematic linkage cannot be offset by the position loop, consequently these are directly affecting the positioning accuracy. In this context, methods are used for diminishing their influence as well as compensation methods with a view to improve the positioning accuracy. Ways to preloading the ball screws and bearings of the ball screw are presented as well, by focussing on the optimisation, through calculations, of the preloading force value. An important component, analysed in this work, consists of the research of the ball screw–nut stiffness that depends on the strategy being used on the axial bearing of the two ends of the ball screw. The experimental tests have been performed at idle running on the CNC milling machine (CPFH 500), fitted with three controlled axes (X, Y and Z). Based on the experimental results, it may be noticed that, once increased the ball screw stiffness, within the limits stated by the ball screw supplier, the positioning accuracy will increase as well.