Reasons and impact of assembly variations on machine tools

Abstract

Exchangeability of machine tools requires identical results of identical machining operation. For estimating the behaviour of machine tools, which are highly integrated mechatronic systems, the interaction of electromechanic system and controller must be observed together. Furthermore, it is possible to measure friction at start-up in serial production of machine tools and at the production plants of machine tool consumers without any additional measuring equipment. By these measurements of friction severe variances of behaviour of feed axes have been detected. These are mainly caused by assembly variances. Especially the alignments and preloads of bearings and ball screw have a strong effect because these components have a high stiffness. This means even smallest deviations result in high constraining forces, which directly affect the friction behaviour. Since it is impossible to determine the reasons of these variances in completely assembled machine tools, a test stand of a ball screw driven axis is examined. This test stand allows both detailed identification of components’ friction behaviour and definite variations of assembly conditions, namely alignment and preload, of bearings and ball screw nut. Hence a detailed investigation of effects of assembly variations becomes possible and has been conducted for alignment of ball screw nut and loose bearing. However, by this examination only the effects on the complete system can be examined but not the situation of constraining forces within the components. The knowledge of these internal forces is important because they directly affect the life expectancy of components and therewith feed drives. In order to improve the significance and to estimate the effect of assembly variances in detail, a modelling method based on multibody simulation has been developed and verified by comparison of measuring results of the test bed and simulation.

Keywords

• friction
• machine tool
• feeding drive
• assembly