Establishing conformity to tolerances by Coordinate Measuring Machines: a first step for a statistical approach

Conformity to tolerances is currently assessed taking into account the Maximum Material Condition (MMC). Coordinate Measuring Machines (CMM) do not however operate accordingly, a small sample only of points on workpiece surface being actually contacted, with a fairly low chance of hitting the outmost point. That problem should therefore be properly addressed using statistical methods, as currently considered also by the ISO Technical Committee on “Geometrical Product Specifications and Verification”. This paper proposes to associate maximum material with the statistical distribution of maximum; typical examples of saw-tooth and round grooved surface profiles are theoretically examined. An actual surface with a good finish, representing the common case of normal distribution, was experimentally evaluated, showing that the method proposed produces consistent results. Furthermore, the method proposed readily associates results obtained with a statistical confidence interval; the uncertainty of result obtained can thus be assessed at any given confidence level.