Assessment of the volumetric accuracy of a machine with geometric compensation

Abstract

In a production environment it is important to be able to quantify the accuracy of all machines in order to estimate the production capability of the company as a whole. A measure of machine production capability is volumetric accuracy, which is effectively a combination of all sources of geometric error. The requirement for ever tighter tolerances on manufactured parts has led to the development of several systems capable of compensating for the effects of geometric errors inherent in machine tools and CMMs. This is usually acheved by making linear correction actuations to the orthogonal axes at each point throughout the working volume, dependent upon the magnitude of the sum of the errors in that axis direction. This paper discusses the problems involved in assessing the volumetric performance of a machine tool operating with such a volumetric error compensation system. It explains the need for measurement of the effect of angular errors, rather than the error source themselves when quantifying the performance of a compensated machine. It further considers the additional requirement for measuring the angular error sources directly if tool offsets of different lengths are to be considered. This is of particular relevance when long tool or probe offsets are commonly used, thus machining at a distance from the position of measurement when calibrating the machine. The need for a uniform error compensation grid when using any indirect volumetric assessment technique is also noted. It has been found that such a regimen is not necessarily enforced, particularly in the CMM field.