Abstract: Virtual assembly has become a popular trend in recent years and is used for various purposes, including selective assembly and adaptive tooling. Monte Carlo approaches based on Finite Element Method (FEM) simulations are commonly used for production applications. However, during the design phase, when testing different configurations and design options, a variational method is more suitable. This paper aims to test different implementations of the Method of System Moments applied to the second-order tolerance analysis method when actual distributions, which are non-centered and non-normal, are used as input for the simulation. The study reveals that the simulation results can significantly vary depending on the simulation settings in some cases. As a result, a series of best practices are highlighted to improve the accuracy and reliability of the simulation outcomes.

Keywords: Assembly simulation | Computer-aided tolerancing | Tolerance stack-up | Virtual assembly

Abstract: In tolerancing activities focusing on the allocation of geometrical tolerances, many critical issues originate from the non-optimal assignment of responsibilities among the organization units involved. This paper aims to depict relations between different tolerancing activities and relevant specifications, assigning them to the proper actor and, therefore, expanding the ISO 8015:2011 “responsibility principle”. A classification among tolerancing activities, specifications, and media is proposed; a horizontal hierarchical framework among functional, manufacturing, and verification specifications and a vertical hierarchical framework along the supply chain are discussed. Examples of both hierarchical structures are presented.

Keywords: functional specification | functional tolerancing | geometrical product specification | ISO/TS 21619:2018 | manufacturing specification | manufacturing tolerancing | tolerance specification | verification specification | verification tolerancing

Abstract: This paper presents a possible functional geometric specification for a lifting airfoil including the definition of functional tolerance limits (tolerance synthesis) and an associated inspection procedure. The proposed specification scheme is derived from the analogy between the mating of the airfoil with a fluid field and the consolidated example of the mating of a prismatic element in its site. The airfoil thickness is defined as a non-constant size with non-constant tolerances and the airfoil shape is prescribed with a non-constant profile of a line tolerance applied to the median airfoil line. The tolerance synthesis is based on XFLR5 software and Computational Fluid Dynamics (CFD) simulations. The inspection procedure uses the data acquired with a laser probe elaborated in Geomagic Wrap, GOM inspect and MATLAB. The overall process has been applied to a case study allowing to define limits and proposing a set of possible improvements regarding, particularly, the geometric specification of the leading and trailing edges of the airfoil.

Keywords: Airfoil | Geometric Inspection | Geometric Specification | Tolerance Synthesis

Abstract: This paper proposes a tool to analyze the diffusion and knowledge of the ISO GPS language in both industry and academia. A survey has been designed based on the maturity model concept to achieve this goal. Six Key Performance Indicators (KPI) arguments have been defined: general concept, datum systems, geometric tolerances, dimensional tolerances, modifiers and indications, and tolerance stack-up. Per each of these, three assessments are proposed, and a rating is given based both on self-assessment and unbiased check questions. The result is a survey that takes between 10 to 15 min to be filled out. The assessment is based on both knowledge and usage. The defined survey, through testing, proved to be a simple and usable tool to test the actual diffusion and knowledge of the ISO GPS language thanks to its shortness and the different levels of analysis it allows.

Keywords: Geometric Specification | ISO GPS | Maturity Model | Survey

Abstract: This paper proposes a methodology to compare the trajectories from different articulators - both physical and digital - during lateral and protusive movements. In the case of digital articulators, the articulated models are digitally moved and exported in position; in the case of mechanical articulators, the models are locked into position and 3D scanned. The digital models in position, both digital and scanned, are aligned to a common reference system and a maxilla-based reference system is tracked. The trajectories are defined as interpolating splines through the maxilla-based reference system origins. A Gerber mechanical articulator and an “Artex CR adjustable” virtual articulator were compared. The repeatability of the mechanical trajectory is found to be less than 184 microns. The resting position of the two articulators is found significantly different meaning that a bias is introduced by the operator in the analogic protocol. The trajectories have significantly different shapes as expected coming from two different articulator models. The proposed methodology proved to be a valid means to compare different articulators.

Keywords: 3D Scanners | Articulators | Complete Dentures | Trajectory Reconstruction

Abstract: Multi-material additive manufacturing enables the opportunity to combine multiple materials within the same part, allowing for an expanded range of properties that can gradually change inside the design space. This category of materials is commonly referred to as functionally graded materials (FGMs). However, FGMs currently face several limitations and challenges in terms of design and manufacturing, such as compatibility, distribution design, and prediction of mechanical properties. Furthermore, when dealing with parts possessing complex micro/meso-structures, finite element simulation often becomes a costly and time-consuming process. Among various additive manufacturing technologies, fused filament fabrication allows the combination of multiple thermoplastic materials within the same nozzle during the deposition process, thereby creating FGMs. This process, known as coextrusion, enables the gradual deposition of materials adjacent to each other while changing their fractions. Moreover, the deposition direction shapes the distribution of materials within each deposited layer, influencing the material structure and the resulting mechanical properties. A recent study proposes a preliminary model describing the deposition mechanism, which has been confirmed by experimental tests. This model delineates the section of the material deposited based on the tool path and process parameters, such as layer thickness and hatching space. To expand upon these findings, this paper applies a homogenization approach based on finite element analysis to the deposition model. This approach enables the description of material mechanical properties based on the material fractions, tool path, and other process parameters. Additionally, this study presents a methodology to tailor the mechanical properties according to the printed part’s orientation around the print bed.

Keywords: FFF | FGM | Functionally graded materials | Fused Filament Fabrication | Homogenization | Multi-material coextrusion | Representative Volume Element | RVE

Abstract: The manufacturing process may lead non-rigid parts to endure large deformations which could be reduced during assembly. The manufacturing specifications of the single parts should refer to their free state or “as manufactured” state; the functional specifications should instead address the “as assembled” state. Therefore, a functional geometrical inspection requires dedicated fixtures to bring the parts in “as assembled” state. In this paper, through a linearized model that considers fixturing and elastic spring-back, we aim to correlate the functional specification to the manufacturing specifications. The model suggests a hybrid approach called “restricted skin model” that allows to reduce the degrees of freedom considering the form error when relevant. Firstly, the framework is verified in a mono-dimensional test case. Subsequently, it is verified including FEM simulation and actual measurement for two simple assemblies. The results show that the model can correctly interpret the theoretical assembly behaviour for actual applications. The use of the “restricted skin model” approach shows a negligible difference when compared to full FEM simulation with differences of 2.1 · 10−7 mm for traslations and 6.0 · 10−3 deg for rotations. The comparison with actual measurement values showed an error of ±0.2 mm at the assembly features. Furthermore, the linearized model allows a possible real-time application during production that enables to adjust manufacturing specification limits in case of process drifting.

Keywords: Compliant assemblies | Deformable assemblies | Geometrical Product Specification | Linearized model | Restricted skin model | Skin model | Tolerancing

Abstract: PURPOSE. The aim of this study is to evaluate the accuracy of removable partial denture (RPD) frameworks produced using different digital protocols. MATERIALS AND METHODS. 80 frameworks for RPDs were produced using CAD-CAM technology and divided into four groups of twenty (n = 20): Group 1, Titanium frameworks manufactured by digital metal laser sintering (DMLS); Group 2, Co-Cr frameworks manufactured by DMLS; Group 3, Polyamide PA12 castable resin manufactured by multi-jet fusion (MJF); and Group 4, Metal (Co-Cr) casting by using lost-wax technique. After the digital acquisition, eight specific areas were selected in order to measure the Δ-error value at the intaglio surface of RPD. The minimum value required for point sampling density (0.4 mm) was derived from the sensitivity analysis. The obtained Δ-error mean value was used for comparisons: 1. between different manufacturing processes; 2. between different manufacturing techniques in the same area of interest (AOI); and 3. between different AOI of the same group. RESULTS. The Δ-error mean value of each group ranged between -0.002 (Ti) and 0.041 (Co-Cr) mm. The Pearson’s Chi-squared test revealed significant differences considering all groups paired two by two, except for group 3 and 4. The multiple comparison test documented a significant difference for each AOI among group 1, 3, and 4. The multiple comparison test showed significant differences among almost all different AOIs of each group. CONCLUSION. All Δ-mean error values of all digital protocols for manufacturing RPD frameworks optimally fit within the clinical tolerance limit of trueness and precision.

Keywords: Accuracy | CAD-CAM | Digital framework | Metrological measurements | Removable partial denture

Abstract: PURPOSE. Digital technology has enabled improvements in the fitting accuracy of denture bases via milling techniques. The aim of this study was to evaluate the trueness and precision of digital and analog techniques for manufacturing complete dentures (CDs). MATERIALS AND METHODS. Sixty identical CDs were manufactured using different production protocols. Digital and analog technologies were compared using the reference geometric approach, and the Δ-error values of eight areas of interest (AOI) were calculated. For each AOI, a precise number of measurement points was selected according to sensitivity analyses to compare the Δ-error of trueness and precision between the original model and manufactured prosthesis. Three types of statistical analysis were performed: to calculate the intergroup cumulative difference among the three protocols, the intergroup among the AOIs, and the intragroup difference among AOIs. RESULTS. There was a statistically significant difference between the dentures made using the oversize process and injection molding process (P <.001), but no significant difference between the other two manufacturing methods (P =.1227). There was also a statistically significant difference between the dentures made using the monolithic process and the other two processes for all AOIs (P =.0061), but there was no significant difference between the other two processes (P = 1). Within each group, significant differences among the AOIs were observed. CONCLUSION. The monolithic process yielded better results, in terms of accuracy (trueness and precision), than the other groups, although all three processes led to dentures with Δ-error values well within the clinical tolerance limit.

Keywords: CAD-CAM | Complete denture | Digital denture | Digital workflow | Reference geometry measurement

Abstract: Material extrusion additive manufacturing enables us to combine more materials in the same nozzle during the deposition process. This technology, called material coextrusion, generates an expanded range of material properties, which can gradually change in the design domain, ensuring blending or higher bonding/interlocking among the different materials. To exploit the opportunities offered by these technologies, it is necessary to know the behavior of the combined materials according to the materials fractions. In this work, two compatible pairs of materials, namely Polylactic Acid (PLA)-Thermoplastic Polyurethane (TPU) and Acrylonitrile Styrene Acrylate (ASA)-TPU, were investigated by changing the material fractions in the coextrusion process. An original model describing the distribution of the materials is proposed. Based on this, the mechanical properties were investigated by analytical and numerical approaches. The analytical model was developed on the simplified assumption that the coextruded materials are a set of rods, whereas the more realistic numerical model is based on homogenization theory, adopting the finite element analysis of a representative volume element. To verify the deposition model, a specific experimental test was developed, and the modeled material deposition was superimposed and qualitatively compared with the actual microscope images regarding the different deposition directions and material fractions. The analytical and numerical models show similar trends, and it can be assumed that the finite element model has a more realistic behavior due to the higher accuracy of the model description. The elastic moduli obtained by the models was verified in experimental tensile tests. The tensile tests show Young’s moduli of 3425 MPa for PLA, 1812 MPa for ASA, and 162 MPa for TPU. At the intermediate material fraction, the Young’s modulus shows an almost linear trend between PLA and TPU and between ASA and TPU. The ultimate tensile strength values are 63.9 MPa for PLA, 35.7 MPa for ASA, and 63.5 MPa for TPU, whereas at the intermediate material fraction, they assume lower values. In this initial work, the results show a good agreement between models and experiments, providing useful tools for designers and contributing to a new branch in additive manufacturing research.

Keywords: additive manufacturing | coextrusion | fused deposition modeling | material extrusion | material modeling

Abstract: Large parts produced by injection moulding are usually subjected to large deformations that may be reduced during assembly. The single parts manufacturing specification should refer to the as produced (free) state. On the other hand, the functional specification, derived from the assembly functional specification should address the “as assembled” state. Geometrical inspection, based on the functional specification requires dedicated fixtures to simulate the “as assembled” state. This contribution suggests a procedure, based on FEM simulation, to correlate the geometric specification at the “as assembled” state with the “as produced” (free) state, applied to an industrial case study. The result of the procedure are free state tolerance limits, e.g., manufacturing specification, that allows conformity of the part to the functional specification once assembled. The part may be inspected based on the manufacturing specification fixtureless during mass production. The result of the case study shows a significant reduction in position and orientation error due to the assembly process as it was expected.

Keywords: Compliant assemblies | Deformable assemblies | FEM simulation | Geometrical Product Specification | Tolerancing

Abstract: Shaft-hole pattern fits based on the Boundary Condition design criterion allows a 100% acceptability rate, but they may be not economically convenient. If the rejection rate needs to be statistically quantified and the pattern is itself the alignment feature, therefore promoted as datum feature (Intrinsic datum system), there is no trivial solution to create a tolerance stack-up: a unique assembly function cannot be determined. The focus of this contribution is “2x” patterns: different methodologies to create tolerance stack-up assessing assemblability are discussed and verified through Monte Carlo simulation. An equation to transform the variability seen from the Intrinsic datum system to the one seen from an external arbitrary reference system is given. The mutual distance between any two elements of an “nx” pattern is discussed and the implication of multiplicity and datum system is highlighted. A case, derived from an industrial case study, will be discussed by comparing the result from the simulated manual and automated assembly. A path towards “nx” patterns generalization is also presented.

Keywords: Boundary Condition | Rejection rate | Tolerance analysis | Tolerancing | Virtual condition

Abstract: The primary scope of tolerancing is to ensure functional requirements will be met by specifying geometrical tolerances. The duality principle, described within the ISO Geometrical Product Specification (GPS) standards, presents a perspective where the verification conceptually mirrors the specification. While this is a valuable model, further refinement is required to accommodate the different aims of inspection in support of the manufacturing process. This work aims to elaborate on the concept of a "geometrical verification specification" that is subordinate to the functional and/or manufacturing specifications. This verification specification may evolve as additional knowledge of the manufacturing process and inspection resources becomes available. A well-formed geometric verification specification should facilitate an appropriate inspection by any qualified operator, which in turn assures comparable measurement results across multiple instruments and facilities.

Keywords: Design for Metrology | Geometric Specififcation | ISO/TS 21619:2018

Abstract: To compare the reference geometry approach to the best-fit (or superimposition) approach in the estimation of geometric accuracy relevant to the digital and the analog workflow to fabricate a complete denture. Starting from a model of an edentulous maxilla, the two measuring methodologies were tested to estimate the geometric accuracy of the intaglio surface of the complete dentures fabricated by CNC milling and injection molding. Eight areas of interest were defined at the intaglio surface of the denture base; a sensitivity analysis determined the minimum number of measuring points to calculate a reliable Δ ¯ error value. A repeatability analysis was performed to assess the consistency of this experimental reference geometry approach with respect to the clinic acceptable requirements. For the analog workflow, the comparison of the reference geometry results to the best-fit results showed a − 76 (post-dam) ÷ 169 µm (right flange) range of the Δ ¯ mean value for the reference geometry approach, to be compared to − 15 (left crest) ÷ 146 µm (right tuberosity) range for the best-fit approach. For the digital workflow, the same comparison showed a − 21 (left crest) ÷ 51 µm (left flange) range for the reference geometry approach, compared to a − 20 (left crest) ÷ 23 µm (left flange) for the best-fit approach. The best-fit approach results in an underestimation of mean Δ ¯ error values and their distribution over the entire prosthesis. The reference geometry approach correctly estimates error values while focusing on the identification of sources of errors in the manufacturing process.

Keywords: Accuracy | Best fit | CAD–CAM | Complete dentures | Digital manufacturing | RPS