Investigation of contact of metrological probe tips with rough engineering surfaces
This investigation explores the behaviour of contact interaction between probe tips of indicators (displacement gauges) and rough engineering surfaces, in order to gain new insights into uncertainty in industrial precision gauging. The conditions of interest are contact of a mm-scale hard sphere at 0.1 to 5 N force with metallic surfaces, Rq < 2 mm. The motivation arises from the growing demand of modern industries for higher machining processes and for finer inspection capabilities. In the near future, such a demand is likely to require careful analyses of the measurement error budget of these instruments with a greater consideration of the potential errors induced by the complex behaviour of contact, which is currently assumed to be negligible. A preliminary experimental study was carried out on an existing test-rig of limited accuracy. It raised several concerns related to the effects of surface roughness and material on the resulting contact deflection at the load regimes of indicators’ probes. It stimulated the need for more reliable data. So, a new test-rig of advanced capabilities was designed to perform a comprehensive study of a wider range of contact parameters to simulate real situations of industrial gauging processes. This advanced study confirms the consistent effects of roughness, material and probe tip size not only on the surface deflections but also on the probe rotations. Roughness variability across the same surface caused some inconsistencies in the deflection behaviour and repeatability, and surface contamination gave additional unsystematic effects on these relations. Re-establishing contact on the same position of a contaminated surface appeared to contribute errors of the order of those of a clean surface. Based on the results of this investigation, systematic errors of contact can exceed the 1 mm level with uncertainty up to at least 0.4 mm in many measurement processes with precision indicators. Such figures reveal that the contact cannot any longer be considered a negligible source of errors, and precision metrology must account for these induced errors in the budget of displacement gauges.