Thermal influence on the positioning accuracy of the CNC machine tool rotary tables

Abstract

This work analyses the perturbing thermal sources and design solutions to avoiding the thermal effects. The analysis is focused, as main factors of the thermal dilatation, on the rotary tables, the guiding systems, the radial one and, especially, the axial one (that takes over the weight of both table and work piece), as well as on the positioning of the driving servomotor that is a thermal source in the vicinity of the rotary table. The new solutions presented in this work refer to the rotary tables with direct measuring whose number of indexing positions is 360000 that means a minimum increment of 0.0001degrees. It is to be mentioned that this structure allows the kinematic linkage of the rotary table to take part to machining. If the rotary table provides positioning only, the table will have a clamping system that confers a high stiffness, needed for heavy duty cutting. The experimental trials have been carried out on a machining centre of the Micro-production workshop of the University in Bacau. The size of the rotary table is 630x630 mm. Repeated trials have been performed for determining the positioning accuracy through the modification of the thermal conditions of the worm-wormed gear mechanism. The results have proven an enhancement of the positioning accuracy of approximately 7%.