DEFINITION OF A MEASURABILITY THRESHOLD OF GEOMETRIC TOLERANCES IN RELATION TO MEASUREMENT UNCERTAINTY AND DIMENSIONAL PARAMETERS

In order to avoid final product malfunction and to allow for assembly integration, geometric specifications and dimensional tolerances are commonly used in mechanical design. However the feasibility of geometric specification measurement and verification is often neglected and the influence of measurement uncertainty in geometric tolerances evaluation underestimated. The authors propose updated results of a mathematical and numerical model, based on Monte Carlo simulations, developed in order to define a measurability threshold of geometric tolerances in relation to measurement uncertainty and geometric parameters, such as feature dimensions, meant to help the designer to define measurable geometric specifications. Starting from EN ISO 14253-2:2011 and EN ISO 14253- 3:2011 standards, a perpendicularity tolerance between a cylindrical feature and a planar one has been simulated. A mathematical model has been defined for each feature, in order to assess both misalignment and its uncertainty when starting from the estimate of geometric entities obtained from point coordinates measured by a Coordinate Measurement Machine (CMM). Monte Carlo Analysis of these simulation underlined how geometric parameters, such as dimensions of the features involved, can act as magnifiers for measurement uncertainty when verifying a geometric specification: there could be cases where this magnification effect could lead to non-measurability of misalignment and non-verifiability of the geometrical specification requested.