Contact-related uncertainty in high-precision displacement probes

Contact-based displacement gauging remains a preferred choice for many measurements of position, size and shape. So far, it has usually been acceptable to neglect the variability of the contact itself when determining uncertainty budgets, but further expected decreases in typical manufacturing tolerances are starting to draw attention to it. This paper discusses, selectively, a few of the issues that arise. It summarizes results of some recent studies into the variability of contact of typical displacement probes with moderately rough surfaces and of how stylus-based roughness instruments interact with local topography. Recognizing that they are no more than suggestions to prompt further work, it suggests working rules for dealing with uncertainty of contact. For displacement probing empirical rules based on assumed vales in the 0.1 to 1 in range may be adequate for the near future. For topography measurement, the situation might be better handled by using formal metrology procedures that minimize the practical impact of the uncertainty.