Geometric dimensioning, tolerancing and inspection

This paper introduces the novel concept of fixture capability measure to determine fixture layout for the best assembly process yield by optimizing position of locators and reference clamps to compensate stochastic product variations and part deformation. This allows reducing the risk of product failures caused by product and process variation. The method is based on three main steps: (i) physics-based modelling of parts and fixtures, (ii) stochastic polynomial chaos expansion to calculate fixture capability, and (iii) fixture capability optimisation using surrogate modelling. The methodology is demonstrated and validated using the results of an aerospace wing sub-assembly joined by riveting technique.

Keywords: Fixture Capability , Fixture Layout Optimisation , Polynomial Chaos , Surrogate Model

One of the most important parameters to be controlled during the production of textile yarns obtained by mixing pre-colored fibers, is the color correspondence between the manufactured yarn and a given reference, usually provided by a designer or a customer. Obtaining yarns from raw pre-colored fibers is a complex manufacturing process entailing a number of steps such as laboratory sampling, color recipe corrections, blowing, carding and spinning. Carding process is the one devoted to transform a “fuzzy mass” of tufted fibers into a regular mass of untwisted fibers, named “tow”. During this process, unfortunately, the correspondence between the color of the tow and the target one cannot be assured, thus leading to yarns whose color differs from the one used for reference. To solve this issue, the main aim of this work is to provide a system able to perform a spectral camera-based real-time measurement of a carded tow, to assess its color correspondence with a reference carded fabric and, at the same time, to monitor the overall quality of the tow during the carding process. Tested against a number of differently colored carded fabrics, the proposed system proved its effectiveness in reliably assessing color correspondence in real-time.

Keywords: Carding process , Color assessment , Spectral camera sensor , Spectrophotometry

Reverse Engineering (RE), also known as "CAD reconstruction", aims at the reconstruction of 3D geometric models of objects/mechanical parts, starting from 3D measured data (points/mesh). In recent years, considerable developments in RE were achieved thanks to both academic and industrial research (e.g. RE software packages). The aim of this work is to provide an overview of state of the art techniques and approaches presented in recent years (considering at the same time tools and methods provided by commercial CAD software and RE systems). In particular, this article focuses on the "constrained fitting" approach, which considers geometrical constraints between the generated surfaces, improving the reconstruction result. On the basis of the overview, possible theoretical principles are drafted with the aim of suggest new strategies to make the CAD reconstruction process more effective in order to obtain more ready/usable CAD models. Finally, a new RE framework is briefly outlined: the proposed approach hypothesizes a tool built within the environment of an existing CAD system and considers the fitting of a custom-built archetypal model, defined with all the a-priori known dimensions and constraints, to the scanned data.

This paper presents the advancements of an automatic segmentation procedure based on the concept of Hierarchical Space Partitioning. It is aimed at tolerance inspection of electromechanical parts produced by injection moulding and acquired by laser scanning. After a general overview of the procedure, its application for recognising cylindrical surfaces is presented and discussed through a specific industrial test case.

Keywords: Computer aided tolerancing & inspection , Hierarchical space partitioning , Injection moulding , Reverse engineering

In this paper we analyze the capabilities of a routine, based on Fuzzy logic, for elaborating a data set coming from a CMM (Coordinate Measuring Machine). We will show how to obtain, during holes measuring, the best measure, so that the approximation error is minimized. Moreover the CMM on-board software can elaborate these data and select the mathematical representation of the stored data, by identifying quotes, measures, axes, diameters, tolerances and so on. Information on measured parts is usually elaborated by an algorithm based on the least square squared error method, in order to evaluate the good shape of the hole; our purpose is to propose a new kind of approach, based on the Inferential Fuzzy system method, both to reduce the number of measured points, and to obtain the same accuracy. Our approach enables to measure the holes with a number of points lower than those usually needed for the CMM software. Thus time spent for obtaining a good measure is significantly reduced.

Keywords: Accuracy , Coordinate Measuring Machine , Fuzzy inference , Holes measurement , Precision

In the last decade, sandwich composite materials have had an increasing use in design of racing boats. The main reasons are: higher strength-weight ratio, low density, excellent durability and versatility. The knowledge of impact response is very important to design racing boats. The aim of the present study is the investigation of absorbing impact energy ability of a sandwich composite material used for offshore vessels in UIM (Unione Internationale Motonautique) Championship. The material analysed in this study is a sandwich manufactured with hand lay-up technique. In the first phase, the damage assessment of single impact has been studied with an optical measurement technique. In a second phase, the damage evaluation due to repeated impacts has been analysed with the similar technique.

Keywords: Damage accumulation , Lightweight composites , Low energy impact , Nautical applications , Optical mesaurements , Safety design

The present work collects some results of the three-years Research Program "BioForming", funded by the Italian Ministry of Education (MIUR) and aimed to investigate the possibility of using flexible sheet forming processes, i.e. Super Plastic Forming (SPF) and Single Point Incremental Forming (SPIF), for the manufacturing of patient-oriented titanium prostheses. The prosthetic implants used as case studies were from the skull; in particular, two different Ti alloys and geometries were considered: one to be produced in Ti-Gr23 by SPF and one to be produced in Ti-Gr2 by SPIF. Numerical simulations implementing material behaviours evaluated by characterization tests were conducted in order to design both the manufacturing processes. Subsequently, experimental tests were carried out implementing numerical results in terms of: (i) gas pressure profile able to determine a constant (and optimal) strain rate during the SPF process; (ii) tool path able to avoid rupture during the SPIF process. Post forming characteristics of the prostheses in terms of thickness distributions were measured and compared to data from simulations for validation purposes. A good correlation between numerical and experimental thickness distributions has been obtained; in addition, the possibility of successfully adopting both the SPF and the SPIF processes for the manufacturing of prostheses has been demonstrated.

Cranioplasty is a surgery in which a prosthesis must be anchored on skull bone to repair a defect. One of the most used materials is the titanium. However, titanium prostheses could be made using the incremental sheet forming (ISF). Since titanium and bone are characterized by different Young modules, a detailed design of anchoring system is required to avoid cranial rupture. Aim of this study was to present a design procedure in order to identify the optimal anchoring system in case of craniofacial prostheses made with ISF. In detail, an optimization process and a predictive model for bone stress were used, choosing the numerical outputs of different FEM analyses as input data. The results indicate that our predictive and optimization models are accurate and, so, that this procedure could be very helpful for the prosthesis design, as demonstrated by the application of the procedure to a real case study.

Keywords: Anchoring system , Design procedure , Incremental sheet forming , Titanium prosthesis

Pier Paolo Valentini, PhD in Design of Mechanical Systems, is Associate Professor of Applied Mechanics. He is research fellow of the National Laboratories of Frascati (Rome) of the Italian Institute for the Nuclear Physics and member of the Steering Committee of the Italian Association of Machine Design. He teaches Virtual Prototyping and Simulation of Mechanical Systems and Bio-prosthesis. He has been advisor of more than 130 master and PhD dissertations. He has been principal investigator of more than 40 research projects funded by institutions and industrial partners. He has authored more than 130 papers in international journals and conference proceedings. Eugenio Pezzuti is Associate Professor of Machine Drawing. He is member of the advisory board of the Italian Ministry for Education, University and Research. He has been the principal investigator of three projects funded by national institutions. His fields of interests are the design of machine components, study on optimal tolerance allocations, reverse engineering methodologies and biomechanics. He has authored more than 80 papers in international journals and conference proceedings. The allocation of the right tolerances is one of the most important phases in the design process of mechanical devices and systems. In general, the choice has to take into account the manufacturing costs and limitations, looking at the specific functionality of the device and the assemblability of parts. In the case of flexible components, the designer has to include the evaluation of the compliance effects and the structural resistance. The paper discusses a numerical methodology for addressing the problem of the geometrical tolerance allocation for the compliant ortho-planar spring. The methodology is based on the definition of the functionality and resistance objective functions by means of finite element models. Numerical constrained optimisation is then applied to find the suitable combination of tolerance parameters acting on the main dimensions.

Keywords: Compliant mechanism , Flexible component , Ortho-planar spring , Tolerance allocation

Melt rheology in injection molded metastable supercooled liquid metal of Zr44-Ti11-Cu10-Ni10-Be25 alloy may induce selective crystallizations. High mobility Be, Cu and Ni atoms have been observed to differently crystallize in bulk metal glassy supercooled liquids. Here, we analyze the result of morphological microscopic observation conduct on Bulk Metallic Glass (BMG) with composition of a commercial liquid metal alloy (LM001B). The injection molded plate has been supplied by “Liquid Metals Technologies Inc, Ca USA” and manufactured using an Engel injection molding machine operating at 1050-1100°C; the observed sample then has been cut by water jet. FEI Scios Dual-Beam has carried out the microscopic observation. Particularly, through a cross section, we observe the presence of crystalline phases on the short-range order. We investigate the presence of short-range order clusters, their distribution and the effect that they could cause on the alloys' behaviors and properties.

Keywords: Bulk glass metals , Chemorheological model , Cold crystallization , Rheology , Segregation

With a view to enabling additive manufacturing (AM) processes, today, open-source, low-cost 3D printers are systems with great potential. However there is a significant lack of scientific data on the performance of open-source 3D systems and on the selection of adequate process parameters that can help to improve the quality of the parts. The purpose of this paper is to assess the effects of the main process parameters on the dimensional accuracy of a specific open-source 3D printer, the RepRap Prusa-Mendel I2. This study consisted of a benchmarking part, involving elementary shapes representing a series of different features. By means of a full factorial DoE (Design of Experiments), with three factors (layer thickness, deposition speed, and flow rate), three levels, and three replications, 81 parts were obtained. Subsequently, a laser scanner (D700 Laser Scanner-3Shape, Denmark) was used as high resolution reverse engineering system in order to evaluate the variation between real parts and nominal geometry. The impact of the main process parameters was evaluated and optimal combinations were analyzed. On the basis of the results obtained in the experiments, practical suggestions for the settings of common process parameters were formulated. Test results serve to improve the quality of AM parts through the most appropriate selection of the main process parameters.

Remote Laser Welding (RLW) is a joining technique more recent than the most known and widely adopted (mainly in the automotive field) resistance spot welding technique (RSW). While RLW offers much more benefits than RSW (higherprocess speed, less number of operating robots, decreased factory floor footprint, reduced overall process costs), it also adds issues related to process planning and quality control of welded joints. The European Commission, under the ICT-Factories of the Future programme of FP7, funded a project, starting from January 2012 and just ended on June 2015, titled "RLW Navigator - Remote Laser Welding System Navigator for Eco Resilient Automotive Factories" which aims at developing an innovative Process Navigator for planning, integrating, testing and validating applications on the use of RLW in the automotive assembly addressing today's critical needs for frequently changing operating conditions and product-mix provisions. The project results are a set of toolboxes able to offer very useful tools to plan and control the RLW joining process. The key word that mainly describes the provided ICT-based solutions is Optimisation: from the optimisation of the working cell, by analysing several layout configurations, to the optimisation of robot path and weld sequencing for assigned product and process constraints; from the fixture optimisation (involving the best clamp configuration) starting from real/non-ideal shape parts, to the optimisation of process laser parameters to guarantee higher welded join quality. The applicability of these software tools goes beyond the automotive field and this makes the provided solutions much more interesting for the industry.

One of most demanding tasks in the manufacturing field is controlling the variability of parts as it may affect strongly the deliverability of key characteristics defined at the final (product) assembly level. Current CAT systems offer a good solution to the tolerance analysis/synthesis task, but to handle flexible objects with shape errors more effort is needed to include methods able to capture the elastic behaviour of parts that adds more variability on the final assembly. Usually, sheet metal assemblies require dedicated fixtures and clamps layout to control the gap between parts in the specific location where a join must be placed. Due to the variability of parts the position of clamps can also be varied. The paper describes a FEM-based method able take into account part flexibility and shape error to parametrically analyse sheet metal assemblies by acting on some key parameters to look for the optimal clamp layout that guarantee the gap between parts to be under control after joining parts together. This method offers, with respect to commercial solutions, the ability to model fixtures, clamps and different joint types with no matter on the node positions of the mesh. Locations of such elements are based on the shape functions defined at element (shell) mesh level and modelled as local constraints. So the user can generate a mesh without a previous knowledge of the exact positions of clamps, for example. This allows to conduit a parametric analysis without remeshing the surfaces and with no need to physically model the clamps. An aeronautic case study is described with a four-part assembly riveted on a quite complex fixture by using several clamps.

Keywords: Clamp layout optimisation , Compliant assemblies , Free shape variability , Sheet metal parts , Shell.

The production of prostheses is still not completely optimized, especially for those districts where both functional and aesthetic requirements have to be combined with the urgency of intervention. The prostheses manufactured by machining using CAD/CAM techniques represent the conventional way to obtain a "custom-made" part. However, the above-mentioned solutions are penalized by the too long manufacturing time. This limit can be overcome by using an innovative metal-forming process, i.e. the Incremental Sheet Forming (ISF), which also allows to obtain complex patient-specific geometries even if characterized by a lower precision compared to the conventional process. In this paper, alternative approaches to manufacture a skull prosthesis (i.e. conventional milling and ISF) are compared from technological and economical points of view.

Keywords: Anchoring system design , Prostheses modelling , Skull manufacturing

Purpose: The study aims to evaluate three-dimensionally (3D) the accuracy of implant impressions using a new resin splinting material, "Smart Dentin Replacement" (SDR). Materials and Methods: A titanium model of an edentulous mandible with six implant analogues was used as a master model and its dimensions measured with a coordinate measuring machine. Before the total 60 impressions were taken (open tray, screw-retained abutments, vinyl polysiloxane), they were divided in four groups: A (test): copings pick-up splinted with dental floss and fotopolymerizing SDR; B (test): see A, additionally sectioned and splinted again with SDR; C (control): copings pick-up splinted with dental floss and autopolymerizing Duralay® (Reliance Dental Mfg. Co., Alsip, IL, USA) acrylic resin; and D (control): see C, additionally sectioned and splinted again with Duralay. The impressions were measured directly with an optomechanical coordinate measuring machine and analyzed with a computer-aided design (CAD) geometric modeling software. The Wilcoxon matched-pair signed-rank test was used to compare groups. Results: While there was no difference (p=430) between the mean 3D deviations of the test groups A (17.5μm) and B (17.4μm), they both showed statistically significant differences (p<.003) compared with both control groups (C 25.0μm, D 19.1μm). Conclusions: Conventional impression techniques for edentulous jaws with multiple implants are highly accurate using the new fotopolymerizing splinting material SDR. Sectioning and rejoining of the SDR splinting had no impact on the impression accuracy.

Keywords: Accuracy , Edentulous jaw , Implant impression technique , Impression copings , Passive fit , Splinting material

We present a sequential, two-step procedure based on machine vision for detecting and characterizing impurities in paper. The method is based on a preliminary classification step to differentiate defective paper patches (i.e., with impurities) from non-defective ones (i.e., with no impurities), followed by a thresholding step to separate the impurities from the background. This approach permits to avoid the artifacts which occur when thresholding is applied to paper samples that contain no impurities. We discuss and compare different solutions and methods to implement the procedure and experimentally validate it on a datasets of 11 paper classes. The results show that a marked increase in detection accuracy can be obtained with the two-step procedure in comparison with thresholding alone. © 2013 Elsevier B.V. All rights reserved.

Keywords: Image processing , Machine vision , Paper

In the aerospace sector, tolerance allocation and inspection of many components must provide not only for ease of manufacturing and cost reduction but also for reliability and safety requirements. Computer Aided Tolerancing and Inspection (CAT&I) methods based on Reverse Engineering techniques may enhance production quality assessment with significant reduction of the time-to-market. This paper presents an original reverse engineering methodology that is being developed to achieve the complete automation of the inspection process starting from the design requirements (dimensional and geometrical tolerances) as they generally appear in the component's official drawings. After a brief presentation of the proposed methodology and its possible implementation oriented to non-planar surface recognition, an aeronautical component made by beta-forging of Ti6Al4V Titanium Alloy powders is presented as test case. The discussion of the procedure allows to point out its benefits in terms of (a) more efficient management of the inspection process and data consolidation; (b) more thorough comprehension of the real component; and (c) a better understanding of possible feedbacks to be applied in design or manufacturing. Finally some remarks about the limits of the proposed methodology are shown and possible enhancements, that have been already planned to be applied, will be described. © 2014 IEEE.

Keywords: beta-forging , Computer Aided Tolerancing and Inspection , Reverse Engineering , Segmentation , Ti6Al4V Titanium Alloy powders

ABSTRACT: This paper presents a point cloud segmentation based on a spatial multiresolution discretisation that is derived from hierarchical space partitioning. Through part type recognition it aims to simplify Computer Aided Tolerance Inspection of electromechanical components avoiding cloud-CAD model registration. A voxel structure subdivides the point cloud. Then, through a suitable surface partitioning, it is linked to component volumes by means of the morphological components of the binary image that is derived from voxel attributes (‘true state' if points are included in a specific cluster or ‘false state' if they are not). The proposed approach is then applied on a din-rail clip of a breaker, made by injection moulding. This case study points out the suitability of the approach on box-shaped components or with normal protrusions, and its limits concerning the assumptions of the implementation.

Keywords: computer aided tolerance inspection , reverse engineering , segmentation

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.

Keywords: Dimensional parameters , Geometric tolerances , Measurability threshold , Measurement uncertainty

The increasing integration between electronics and mechanical engineering brings to the industrial market very hi-tech sensors, often non-linear, capable of more than a single input and single output. A problem more and more relevant for sensors like these is calibration. Classic linear calibration procedures, when applied to this extremely engineered sensors, lead to poor accuracy and are generally not satisfactory. The case study is the calibration of a bi laser based position sensor, in particular a positive sensitive detector, that is an optical position transducer based on series of photodiodes commonly used as multidimensional sensor. To perform the calibration a micrometric positioning table was used to test the whole photodiode active area in both directions. The sensor studied showed a very linear behaviour in the central region of the working range, and a limited nonlinearity closer to the range limits and was to be used to verify robot movement capabilities; to reduce uncertainty associated with nonlinearities, a set of nonstandard, non-linear, calibrations were performed, pointing out residual values in order to compare different algorithms. In a previous work, authors have already tested a linear model against an algorithm based on radial basis functions (RBF) and Nelder-Mead simplex method. Object of this paper is the definition of a procedure based on RBF and genetic algorithms for multi-dimensional interpolation of data cloud and a comparison between this updated procedure results and the ones of the previous studied algorithms. The reference model for calibration was a black box with two inputs, X and Y position of the laser spot, and two outputs, voltages Vx and Vy, while the calibration procedure was split in two separate layers, one for each output depending on both inputs. Given N data points in a M-dimensional environment and N values that represent the non linearity residual, purpose of the algorithm is to approximate a data cloud with a real function, that is represented as a sum of a polynomial (linear) part and I. radial basis functions, each associated with a different center (node) and weighted by an appropriate coefficient, that the procedure also allow to assess. When no starting guess for nodes are given in input, nodes coordinates are the output of a non-linear optimizer based on a genetic algorithm, whose goal is to locally minimize the objective function. The algorithm stops itself whenever it reaches a certain tolerance level, a user specified number of nodes or when the previous iteration has a better value of the objective function. This study has been performed for various RBF classes, and shows an increased accuracy, thus a better metrological behaviour, with respect to the standard linear (planar) calibration model traditionally used.

Keywords: Calibration , Genetic algorithm , Multi input , Radial basis function , Uncertainty