UNCERTAINTY OF COORDINATE MEASUREMENTS ON SHEET METAL PARTS IN AUTOMOTIVE INDUSTRY

A. Weckenmann, M. Knauer, T. Killmaier
Department Quality Management and Manufacturing Metrology, University Erlangen-Nuremberg, Germany

Steadily increasing demands on the accuracy of size and form of drawn sheet metals and the rising complexity of measurement tasks caused in automotive industry more and more measurements for workpiece inspection and process control to be made at shop floor with CNC coordinate measuring machines.
Usually it is assumed that the uncertainty of CMMs has a sufficient small value for a certain inspection of tolerances in a common range of about 1 mm. The characteristic values like E3 for length measurement uncertainties or R for the probing uncertainty as specified by the manufacturers only describe which uncertainties can be expected for special measurement tasks under ideal conditions. However these measurement tasks happen to appear very rarely during the normal usage 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 uncertainties contributions from measuring device, workpiece, environment, and operator, can be expected for real. Beneath the device specific uncertainty other influences also play a major role. The effects of environmental conditions, especially temperature and operator specific actions can cause at least the same contribution to the measurement uncertainty.

Keywords: measurement uncertainty, coordinate measuring machine, car-body