Impact of the Threshold on the Performance Verification of Computerized Tomography Scanners

Computerized tomography is an emerging technology for geometric inspection. Its capability of easily scanning internal and undercut surfaces, as well as micro components, makes it the only possible choice for several measurement tasks. However, traceability is still a relevant issue, due to the lack of well-established procedures for testing CT scanners: several international standards about the application of computerized tomography for geometric inspection are still under development. In this work, we will propose the results we obtained in the application of the VDI/VDE 2617 part 13 standard on two computerized tomography scanners. In particular, we will show the impact of the choice of the threshold on the results of the test

Optimal inspection strategy planning for geometric tolerance verification

"Two features characterize a good inspection system: it is accurate, and compared to the manufacturing cost, it is not expensive. Unfortunately, few measuring systems posses both these characteristics, i.e. low uncertainty comes with a cost. But also high uncertainty comes with a cost, because measuring systems with high uncertainty tend to generate more inspection errors, which come with a cost. In the case of geometric inspection, the geometric deviation is evaluated from a cloud of points sampled on a part. Therefore, not only the measuring device has to be selected, but also the sampling strategy has to be planned, i.e. the sampling point cloud size and where points should be located on the feature to inspect have to be decided. When the measuring device is already available, as it often happens in geometric measurement, where most instruments are flexible, an unwise strategy planning can be the largest uncertainty contributor. In this work, a model for the evaluation of the overall inspection cost is proposed. The optimization of the model can lead to an optimal inspection strategy in economic sense. However, the model itself is based on uncertainty evaluation, in order to assess the impact of measurement error on inspection cost. Therefore, two methodologies for evaluating the uncertainty will be proposed. These methodologies will be focused on the evaluation of the contribution of the sampling strategy to the uncertainty. Finally, few case studies dealing with the inspection planning for a Coordinate Measuring Machine will be proposed

3D Tolerance Analysis with Manufacturing Signature and Operating Conditions

The present work shows a method to integrate the manufacturing signature and the operating conditions into a model for 3D tolerance analysis of rigid parts. The paper presents an easy way to manage the actual surfaces due to a manufacturing process and the operating conditions, such as gravity and friction, inside the variational model for a 3D tolerance analysis. The used 3D case study is deliberately simple in order to develop a conceptual demonstration. The obtained results have been compared with those due to a geometrical model that reproduces what happens during assembly. It has been considered as reference case

Artifact-based calibration and performance verification of the MScMS-II

Large scale measuring systems, i.e. measuring systems characterized by a measurement volume from some meters up to some hundreds of meters, are gaining importance in industry to check large parts or track the position of automated vehicles. In contrast with classical monolithic measuring systems, modern large scale measuring systems are constituted by constellations of sensors able to track the position of objects by triangulation or trilateration. This new design allows a greater system flexibility, scalability, and portability, together with a general reduction of costs. The MScMS-II is a large scale measuring system based on infrared triangulation. It has been designed to guarantee the maximum flexibility and reconfigurability, so every set-up procedure has been reduced as much as possible, so that its deployment and calibration requires a short time. However, its accuracy could benefit of a more complete volumetric calibration through the definition of a model of the volumetric error to be compensated. This work continues the one proposed at the CAT2012 conference [1]. An artifact has been developed which is constituted by a series of infrared reflective spheres, thus being well visible by the MScMS-II system. It has been calibrated with a ~1 µm uncertainty. It carries two series of balls. A pair of spheres with a reciprocal distance equal to 800 mm can be used for system calibration. A series of couples of balls with reciprocal distances equal to 200, 400, 600, 800, and 1000 mm respectively can be adopted for performance verification similarly to what is suggested in the ISO 10360 series of standards for CMMs. Experimental results are proposed for the calibration and performance verification procedure of the MScMS-II system

Geometrical product specification and verification in additive manufacturing

The geometric freedom associated with additive manufacturing (AM) processes create new challenges in defining, communicating, and assessing the dimensional and geometric accuracy of parts. Starting from a review of the ASME-GD&T and ISO-GPS current practices, a new approach is proposed in this paper. The new approach combines current tolerancing practices with an enriched voxel-based volumetric representation scheme to overcome the limitations of standard methods. Moreover, the new approach enables a linkage between product design optimization and product verification with respect to the AM process chain. A case study is considered to demonstrate the concept

Geometric tolerance evaluation: A discussion on minimum zone fitting algorithms

Estimating any geometric tolerance requires to sample a cloud of points on the feature to check, and then to fit an ideal substitute geometry respect to which to evaluate errors. Usually, points will be sampled by means of a CMM, and then a suitable software algorithm will be applied to them. Algorithms that can be found in literature show some limitations: they may be very slow, the solution they propose is just an approximated one, or some degenerate situations cannot be managed. We will propose a categorization of algorithms found in literature, highlighting merits and defects for them. Moreover, a new algorithm will be proposed. This algorithm will be tested against classical algorithms to show its advantages

Inspection strategies and multiple geometric tolerances

Recent years have seen an increase in the adoption of geometric tolerances. It is often possible to find several geometric tolerances defined on a single part. However, this poses inspection issues: the values of the geometric error may be interrelated; therefore, the presence of multiple tolerances should be considered in inspection design. In this work, a methodology is proposed for planning CMM sampling strategies for multiple tolerances based on the minimization of inspection costs. A model for inspection costs is proposed, which takes into account the influence of the inspection strategy on measurement and inspection errors costs, both directly and through its impact on measurement uncertainty. The cost is then minimized by means of a suitable optimization algorithm, thus defining an optimal sampling strategy. The approach can be adopted both to optimize generic, uniform, sampling strategies, and to generate manufacturing specific strategies, which consider the manufacturing signature, i.e., the part shape deviation from design nominal inherent to a specific manufacturing process. The latter kind of strategies is shown to be the most effective to minimize costs. A case study which illustrates the methodology is presented