Accreditation for Dimensional Testing With Coordinate Measuring Machines

Abstract

This paper will address some of the issues surrounding the accreditation of laboratories for dimensional testing (and calibration) using Coordinate Measuring Machines (CMMs). The differences between calibrating instruments and using them for subsequent measurements will be explained. This is particularly important with CMMs because their calibration is made up of very specific measurements, while subsequent measurements can cover an enormous range of measurands (length, diameter, flatness, true position, run out, and many more). Given this difference in calibration and use, the next step is to discuss how measurement uncertainty for this discipline will appear on a scope of accreditation, and then the different ways that the uncertainty could, or should, appear on a test or calibration certificate. Although many CMMs are operated under direct computer control, with validated software to perform calculations, the interpretation of part drawings and the ability to transform the part specification into a measurement program to determine conformance relies heavily on the CMM programmer's knowledge of both GD&T and the CMM software implementation of GD&T. The next section of this paper will discuss the qualitative and quantitative evaluations that occur in the evaluation of a laboratory that performs CMM measurements, either for internal or external customers. While these subjects will be addressed in the context of laboratory accreditation, the underlying principles are important for anyone who has to justify the value of the CMM measurements that they perform.