Coordinate measuring machine verification from parametric calibration to artefact based techniques. A review of calibration procedures - advantages and disadvantages

摘要

Ever since the publication in 1987 of BS6808, Coordinate Measuring Machine (CMM) performance assessment has been the subject of much discussion and many different viewpoints have been developed. This paper attempts to describe some of the ways in which CMM performance may be evaluated. Parametric calibration of CMM's is described initially as this method of test has been used by CMM manufacturers particularly for over 20 years now. Increasingly though many users of CMM's now have the measurement equipment to perform parametric calibrations themselves and this can serve as part of the annual calibration routine. The British Standard BS6808 Parts 1 - 3 are then described. The standard, written particularly for the CMM user, gives a verification as to whether a CMM meets the manufacturer's specification for both unidirectional repeatability and the three-dimensional uncertainty of length measurement. These artefact based techniques are usually performed on machine installation and then subsequently as a planned routine maintenance calibration exercise. Some CMM users now prefer to combine the above two tests and compare volumetric accuracy of the CMM from the two sets of results obtained. This method is also described in this paper along with a worked example. Finally, the advantages and disadvantages of the various international standard methods of test are assessed, particularly in their application to large CMM's. Inconsistencies in these calibration methods are highlighted and a new way forward in standardisation is proposed.