Measurement of nonrigid freeform surfaces by coordinate measuring machine

Nonrigid part could be subjected to significant distortion after the removal of manufacturing forces. This condition, known as free-state variation, is principally due to weight and flexibility of the part and the release of internal stresses resulting from fabrication. The present work deals with the inspection of freeform surfaces belonging to nonrigid parts. A manufactured aeronautic component, named Mid Cowling, is considered as case study. The design of an appropriate fixture equipment will be firstly presented: it enables both to simulate the mating part interface and to locate the part in coordinate measuring machines working volume. Then, a method for evaluation of a freeform surface with respect to the nominal one will be presented. This evaluation is based on Euclidean distance between actual and nominal surfaces. Finally, an analysis of the part deformation presented in order to evaluate the measurement process in terms of interaction of the measurement system with the inspected part will be proposed. The adopted method, based on a finite element analysis, was proposed in order to evaluate the interaction, due to the measuring force, between the touch probe and the inspected surface and thus its effect on the measurement result.