Accuracy testing of coordinate measuring machines

2013-02-25T14:02:03Z (GMT) by Jagpal Singh
The proliferation of the bridge type Coordinate Measuring Machine (CMM) within the manufacturing environment has resulted in it being a core and important tool in the manufacturing cycle. In many circumstances, CMM's provide excellent measurement services, but when manufacturing tolerances are small, then detailed understanding of CMM error sources is ideally required, to maximise the confidence in the measurement data Multi-axis measurement arms (MMAs) are a unique subset of Coordinate Measuring Machines (CMMs). They incorporate a large number of degrees of freedom and compact design based on multi-element arm construction, which provides tremendous versatility of movement and access around an object being measured. The consequences of the flexibility of MMAs is that the volumetric accuracy and repeatability specifications can be substantially inferior to those of more traditionally designed CMMs. Various CMM error sources have been previously identified, evaluated and quantified, on a machine by machine basis. Many of these sources (parametric errors for instance) are systematic and considered to be minimised from the measurement system via verification procedures and the generation of software based error maps (applying compensations). However a number of error elements still occur which need to be considered. The research reported here has considered variation between manual and automatic probe tip qualification and the influence on the confidence of the measurement data for a bridge type CMM. A range of probe orientation angles were examined, and the results presented focused on the orientations most readily used. For the MMA the manufacturers four probe tip calibration techniques recommended were executed and the repeatability tested with the arm in different resolves. The investigation was extended further to test the volumetric performance of a multi-axis measurement arm, with repeated systematic measurements of a calibrated reference sphere. The research has examined the influence of various styli on the confidence of the measurement data on a bridge type CMM. Taking into account a number of existing texts which consider different styli length, this work has extended the analysis to consider the influence of stylus design and stylus material. Specifically, styli from two different manufacturers were tested under the same conditions. Varying lengths of stylus were examined, manufactured from stainless steel, tungsten carbide, ceramic and carbon fibre. Further effects as a function of solid or hollow cross sectional geometry and styli made up of extension joints were considered and are presented here.