The Calibration Of Co-ordinate Measuring MachinesAnd Touch Trigger Probes

Abstract

The paper describes the verification of a co-ordinate measurement machine (CMM) with its associated touch trigger probe (TTP). The preferred measuring methods use involved techniques but meaningful results were achieved with standard gauges. The CMM was a DEA Mistral, fitted with a Renishaw TP2 TTP. The verification method employing standard hemispheres, 'short' and 'long' gauge blocks. The objective was to detect periodic variations and to compensate where necessary. Previous work had shown that TTPs have a significant effect on gauge block measurements. To demonstrate this the TP2's characteristics were deduced from forces and deflections at the stylus and verified by hemisphere measurements. This established the TP2's 3 lobe characteristics, which were used to predict the cyclical pattern occurring when gauge blocks were measured. This pattern was confirmed by making TP2 measurements as a gauge block was rotated through 180'. The variations were in the region of 3pm and their importance was discussed in relation to standard permissible errors. Having established the linear variations the squareness accuracy was determined using paired gauge blocks, which resulted in compensation for the lobing errors. Finally comparisons were made between compensation methods and their effect on random behaviour.