Uncertainty of coordinate measurements on sheet-metal parts in the automotive industry

Abstract In the automotive industry, steadily increasing demands on the accuracy of the size and form of drawn sheet metals and the rising complexity of measurement tasks have meant that more and more measurements have to be made on the shopfloor with CNC coordinate measuring machines for workpiece inspection and process control. It is usually assumed that the uncertainty of CMMs has a sufficiently small value for reliable inspection of tolerances within a common range of about 1 mm. The characteristic values like E 3 for length measurement uncertainties or R for the probing uncertainty, as specified by the manufacturers, only describe which uncertainties can be expected for specific measurement tasks under ideal conditions. However, these measurement tasks are only performed very rarely during the normal use of the instrument. Statements about uncertainties of other measurements cannot be derived directly from these values. Using the example of measurements on car-body parts, it is shown what uncertainty contributions can really be expected from the measuring device, workpiece, environment, and operator. Not only the device-specific uncertainty but also other influences play a major role. The effects of environmental conditions, especially temperature and operator-specific actions can make at least the same contribution to the measurement uncertainty.