Shape Deviation Analysis on Sheet-Metal Parts through Reverse Engineering Techniques

Generally speaking, shape measurement by digital image processing has a large field of applications in engineering research and industrial practice: it allows to verify manufacturing tolerances, acquiring geometry for reproducing dies or monitoring cinematic problems. In body-in-white manufacturing it may help in assessing final shape of stamped parts both during die set-up and production. In fact, it allows to check the occurrence of geometrical defects, such as spring-back deviation or small waviness buckling, which can cause problems during assembling or unpleasant aesthetic effects. Choosing the most appropriate analysis method and defining the comparison procedure among different acquisitions of the same part are the two most critical problems that have been faced in the present paper. First of all, the measurement device should fulfil several requirements regarding shape complexity, surface reflectance and environmental conditions (such as lighting and vibrations). A second issue concerns with the output data manipulation; in this specific application measuring the absolute shape does not represent the final goal to achieve, but the goal is to evaluate how the acquired data fit a target shape. The adopted approach is based on the so-called phase shift method. In the proposed set-up, it requires a fixed CCD camera and a LCD projector. A PC generates and projects a set of fringe patterns and also processes the pictures grabbed through the camera. The output consists of clouds of about 105 points. Through a reverse engineering procedure a surface reconstruction of part shape can be done in a CAD environment and this allows to extend the analysis of variations to the whole geometry of the sheet-metal components with an accuracy of about +/–0.1 mm.