Skin Model Shapes Research Articles

Assembly Tolerance Design Based on Skin Model Shapes Considering Processing Feature Degradation

To increase the reliability and accuracy of tolerance design, more and more research works are considering not only orientation and position deviations; they are also forming errors in tolerance modeling. As a direct cause of form errors in industrial mass production, the processing features of the machining system degrade over time. Under the Industry 4.0 paradigm, an assembly tolerance design method based on Skin Model Shape is proposed to take the effect of degrading processing features into consideration. A continuous-time multi-dimensional Markov process is trained through maximum likelihood estimation based on the nodal sampling point set on the machined surface. Degradation of the machined surface is modeled based on the joint probability distribution of nodal displacements. Assembly force constraints and assembly entity constraints are applied to spatial assembly simulations. Tolerance synthesis takes the manufacturing cost and assembling probability as design objectives. A design example of the rotary feed component in a five-axis machine tool is proposed for explanation and verification.

Anomaly detection in Skin Model Shapes using machine learning classifiers

The concept of Skin Model Shapes has been proposed as a method to generate digital twins of manufactured parts and is a new paradigm in the design and manufacturing industry. Skin Model Shapes use discrete surface representation schemes, such as meshes and point clouds, to represent surfaces, which makes them enablers to perform an accurate tolerance analysis and surface inspection. However, online inspection of manufactured parts through use of Skin Model Shapes has not been extensively studied. Moreover, the existing geometric variation inspection techniques do not detect unfamiliar changes within tolerance, which could be the precursors to the onset of the manufacturing of out of tolerance part. To detect the unfamiliar changes, as anomalies, and categorize them as systematic and random variations, some unique surface characteristics can be extracted and studied. Random surface deviations exhibit narrow normal distributions, and systematic deviations, on the other hand, exhibit wide, skewed, and multimodal distributions. Using those surface characteristics as key traits, machine learning classifiers can be used to classify deviations into systematic and random variations. To illustrate the method, multiple samples from a truck component manufacturing line were scanned and the collected 3D point cloud data was used to extract features. A prediction score of 97–100% can be achieved by decision tree, k-nearest neighbor, support vector machines, and ensemble classifiers. The purposed approach is expected to extend the existing online inspection approaches and applications of Skin Model Shapes in quality control.

Assembly tolerance analysis based on the Jacobian model and skin model shapes

Purpose The purpose of this paper is to carry out an assembly tolerance analysis by means of a combined Jacobian model and skin model shape. The former is based on small displacements modeling of points using 6 × 6 transformation matrices of open kinematic chains in robotics. The latter easily models toleranced features with all kinds of geometric deviations. Design/methodology/approach This paper presents the procedure of performing tolerance analysis by means of the Jacobian model and skin model shape for assemblies. The point cloud-based discrete representative is able to model the actual toleranced surfaces instead of the ideal or associated ones in an assembly, which brings the simulation tools closer to reality. Findings The proposed method has the advantage of skin model shape which is suitable for geometric tolerances management along the product life cycle and contact analysis of kinematic small variations, as well as, with the Jacobian, enabling transformation of locally expressed parts deviations to globally expressed functional requirements. The result of the case study shows the accuracy of the method. Research limitations/implications The proposed approach has not been developed fully; other functional features such as the pyramid are still ongoing challenges. Practical implications It is an effective method for supporting design, manufacturing and inspection by providing a quantitative analysis of the effects of multi-tolerances on the final functional key characteristics and for predicting the quality level. Originality/value The paper is original in taking advantages of both Jacobian model and skin model shape to consider all geometric tolerances in assembly.

Form Defects Consideration in Polytope-Based Tolerance Analysis

The polytope-based tolerance analysis in design process uses a finite set of constraints to represent specifications and propagates these constraints to any objective point in the Euclidean space. The operations of Minkowski sum and intersection on polytopes are well suited to serial and parallel assemblies. The polytope model has been applied to complex assemblies which contain a large number of joints and geometrical tolerances. However, the previous studies on this model consider toleranced features as surfaces of perfect form. The ignorance of form defects in tolerance analysis would result in a significant loss in accuracy and reliability. In this paper, an extension of the polytope model for tolerance analysis considering form defects is described in which the skin model shape representing the physical shape of the product is adopted to simulate the actual toleranced feature in place of the substitute one used conventionally. The combination of polytope model and skin model shape is expected to inherit many of the advantages of each model, combining easy-to-use tolerance propagation and form defects representation with accuracy guarantees. To demonstrate the method and its respective application, a case study of an assembly is illustrated in detail. The proposed method further enhances the capability of the polytope model in handling form defects and provides more realistic assembly results that approximate the actual assembly conditions for design evaluation.

Octree-Based Generation and Variation Analysis of Skin Model Shapes

The concept of Skin Model Shape has been introduced as a method for a close representation of manufactured parts using a discrete geometry representation scheme. However, discretized surfaces make irregular polyhedra, which are computationally demanding to model and process using the traditional implicit surface and boundary representation techniques. Moreover, there are still some research challenges related to the geometrical variation modelling of manufactured products; specifically, methods for geometrical data processing, the mapping of manufacturing variation sources to a geometric model, and the improvement of variation visualization techniques. To provide steps towards addressing these challenges this work uses Octree, a 3D space partitioning technique, as an aid for geometrical data processing, variation visualization, variation modelling and propagation, and tolerance analysis. Further, Skin Model Shapes are generated either by manufacturing a simulation using a non-ideal toolpath on solid models of Skin Model Shapes that are assembled to non-ideal fixtures or from measurement data. Octrees are then used in a variation envelope extraction from the simulated or measurement data, which becomes a basis for further simulation and tolerance analysis. To illustrate the method, an industrial two-stage truck component manufacturing line was studied. Simulation results show that the predicted Skin Model Shapes closely match to the measurement data from the manufacturing line, which could also be used to map to manufacturing error sources. This approach contributes towards the application of Octrees in many Skin Model Shape related operations and processes.

A generic approach for analysis of mechanical assembly

Assembly tolerance analysis is increasingly becoming an innovative method for checking whether specified tolerances satisfy assembly functional requirements. Skin model shapes that employ discrete geometry methods can be addressed efficiently when covering geometric tolerances, especially form defects over the product's lifecycle. The point cloud–based discrete geometry uses the actual surfaces to replace the ideal or substitute surfaces in conventional methods, which brings a significant improvement in analysis accuracy and reliability. However, the level of detail and the ordinal simulations restrict their use to simple assemblies. To overcome the issue of the skin model shape in representing deviation propagation, integration of the Jacobian model is proposed; the resulting skin model shapes-Jacobian model combines easy-to-use tolerance propagation and tolerance representation with accuracy guarantees. Besides, Relative positioning method enables calculation of the small displacement torsors for serial or parallel joints, which makes the proposed method a generic approach applicable to both serial and partial parallel kinematic chains. The obtained torsors in the Jacobian model are then used to calculate the final deviations of the functional requirement. The approach is finally illustrated by a practical example.

Tolerance analysis using skin model shapes and linear complementarity conditions

The aim of tolerance analysis is to find a balance between manufacturing cost and product geometric quality. Earlier research usually considered ideal surface-based deviation models to conduct simulations, which ignore form defects. With the increasing demands on quality control, a skin model shape that includes form defects has its advantages. First, the effects of the chosen geometrical model on the assembly simulation are analyzed; the geometrical models considered are: nominal model, ideal surface-based deviation model, and skin model shape. The importance of form defects and assembly load conditions is highlighted. However, it is not easy to integrate all these factors into tolerance analysis. Difficulties in assembly simulation are discussed, such as the mating between non-ideal surfaces, model balance under external and internal loads, the consistency of simulation results, etc. Based on the analysis, a skin model shape-based tolerance analysis method, which also considers the load and displacement boundary conditions, is proposed. The assembly of the skin model shape is transformed into the objective function of a quadratic optimization problem, links between physical properties and optimization constraints are established. Corresponding simulation results are generated by conducting the optimization iteratively. To illustrate the method and validate its simulation result, various models are used as examples. Lastly, a simplified hand saw model is used as a case study.

Characteristic extraction of machined surface using wavelet transformation

In the development and design of machinery products, product performance and tolerance stack-up analysis are generally calculated using computer software. In these calculations, the product shape is modeled as a set of primitives, which are simple geometrical shapes, such as plane, cylinder and sphere. However, the shape of an actual product is different from these shapes because machining error cannot be avoided. Actual shapes usually exhibit deviations in scales, such as geometric deviations, waviness and surface roughness. Therefore, the error between the result of computer calculation and the actual behavior of the product is caused by the shape simplification. A method of for generation of a pseudo shape is required to reduce the calculation error. In this study, a new method for random generation of skin model shapes, which maintain the surface characteristics in multiscale levels, is proposed. In the method, first, an optimal mother wavelet should be decided because they depend on extracting the surface characteristics. In this paper, eight representative mother wavelets are used in wavelet transformation for comparison. The results are discussed based on the results obtained through a case study, and latent problems are also observed.

Integrating form errors and local surface deformations into tolerance analysis based on skin model shapes and a boundary element method

Tolerance analysis aims to ensure the performance of mechanical products. However, traditional analysis methods largely ignore the effects of form errors and local surface deformations, which are inevitably introduced during manufacturing and assembly phases. Therefore, this paper proposes a novel tolerance analysis framework integrating form errors and local surface deformations based on skin model shapes and a Boundary Element Method (BEM) for rectangular surfaces. First, it introduces a method for modeling skin model shapes considering form errors for rectangular surfaces. Next, it presents the calculation of local surface deformations based on the Conjugate Gradient-Fast Fourier Transform (CG-FFT) method, a classic BEM method for solving contact problems. Together, these can be used to obtain the relative positioning of mating surfaces considering the effects of form errors and local surface deformations. Case studies revealed the considerable effects of form errors and local surface deformations on assembly deviations, and demonstrated the efficiency of the proposed tolerance analysis process. Because the proposed tolerance analysis framework couples geometric deviations introduced during the manufacturing phase with mechanical deformations introduced during the assembly phase, it is expected to help control the geometric deviations of mechanical assemblies more accurately and also ensure product performance.

Novel approaches for the assembly simulation of rigid Skin Model Shapes in tolerance analysis

While the modelling of nominal assemblies is sufficiently solved in modern computer-aided design tools, the assembly simulation for parts considering geometrical deviations is still an important research issue. Particularly in tolerance analysis, the assessment of the effects of geometrical part deviations on the part assembly behaviour is of central importance. Though established assembly simulation approaches for parts with geometrical deviations cover a moderate range of real assembly problems, they are not suitable for all assembly simulation problems. To overcome this shortcoming, a general framework and new approaches for the assembly simulation of Skin Model Shapes for the use in tolerance analysis are presented. The application of these assembly simulation approaches is highlighted in generalized case studies and recommendations for their use in tolerance simulations are derived.

FEA integration in the tolerance analysis using Skin Model Shapes

Many research works on tolerance analysis have been carried out in the last thirty years. In this paper, a new idea is proposed, aiming to investigate the effect of form error and mechanical behavior of parts on the stack-up result by combining the recent novelties on the tolerance analysis of rigid and flexible bodies. The real parts are simulated considering the non-nominal Skin Model Shape and the mechanical properties in order to simulate the assembly of a realistic case study. A manufacturing signature model to generate the features with geometric deviations and Finite Element Analysis (FEA) are used.

Assembly simulation of Skin Model Shapes: a comparison of two methods taking into account external forces

Assembly simulation of parts with form defects using Skin Model Shapes (SMS) is much more difficult than the usual assembly of ideal models in a CAD system. Some research studies try to tackle this issue, but very few authors focus on form defect, multibody system and external forces and torques, all at once. Yet external forces are preponderant when considering the relative positioning of parts with form defects.The aim of the paper is to compare two different assembly simulation methods based on: (1) Linear Complementarity Condition (LCC), (2) elimination of contact configurations which are not in mechanical equilibrium. First, the two methods are introduced, and a theoretical comparison shows their differences. Next, several assembly cases are studied. Simulation results are compared to evaluate the proposed methods. The influences of form defect and assembly loads are emphasized through these cases.

Tolerance analysis of annular surfaces considering form errors and local surface deformations

This paper proposes a tolerance analysis method of annular surfaces considering form errors and local surface deformations. Specifically, form errors are properly considered based on skin model shapes. Local surface deformations are calculated based on a Conjugate Gradient-Fast Fourier Transform (CG-FFT) method, which is a classic Boundary Element Method (BEM). Based thereon, tolerance analysis of annular surfaces is conducted based on Monte Carlo simulation, and efficiency of the process is also demonstrated.

An Approach to the Sensitivity Analysis in Variation Simulations considering Form Deviations

In tolerance and variation simulations, sensitivity analyses are used to assess the influences of tolerance specifications and geometrical deviations on the assembly key characteristics and to support product and process developers in deriving more suitable tolerancing decisions and in optimizing manufacturing and assembly processes. However, established methods to the sensitivity analysis in the context of variation simulations disregard important statistical dependencies, which occur when considering form deviations. To overcome these shortcomings, an approach to the density-based sensitivity analysis in variation simulations considering form deviations based on Skin Model Shapes is presented. The method is highlighted and discussed employing a typical tolerance analysis case study.

Generation of consistent skin model shape based on FEA method

Controlling product geometric quality is an important issue, because real parts deviate from their nominal value (e.g., in form, orientation, and position error of features, size of part, etc.). To analyze the influence of these deviations on final product, one solution is to consider the nonnominal Skin Model Shape to simulate assembly, manufacturing, or metrology. The modeling of nonnominal parts is still in its initial phases. First, methods of generating a single feature with deviations are reviewed and classified. With the combination of the single nonideal features to obtain the complete nonideal model of the part, geometrical issues appear, such as gaps and self-intersections. These can be influenced by acute and obtuse angles and the ratio between mesh size and deviation value. From an analysis of these issues, two deviation combination methods are proposed to preserve the manufacturing deviation of features and consistency of the model. These methods are qualified as local and global methods. The local method is based on the iterative calculation of mesh regularization. The global method is based on finite element analysis, with manufacturing deviations added to the nominal model by the penalty function approach. The effectiveness and efficiency of both kinds of method are compared on a trial geometry. The global method is preferred as it needs no iterative calculation, no stop criteria and gives better results. Finally, the proposed method is validated on a more complex mechanical part: a cutter body.

Hybrid Tolerance Representation of Systems in Motion

Dimensional management deals with the fact that the real geometry of every manufactured part deviates from its ideal shape. To evaluate the effects of these deviations tolerance analysis, which are often based on vectorial models, are carried out. Nevertheless the use of vectorial models has one major disadvantage – they cannot adequately represent form deviations. As a consequence new concepts of representations based on the GPS’ Skin Model have been established. Since the use of Skin Model Shapes (SMS) is time-consuming and does not always offer advantages over vectorial models, the Hybrid Tolerance Representation (HTR) which combines the advantages of vectorial and discretely represented tolerances is introduced in this paper. The HTR is based on a classification of the contact situation of the cinematic chain into lower and higher kinematic pairs. Based in this classification all higher kinematic pairs are going to be represented by SMS whereas lower pairs, are represented by vectors. Besides the simulation of the contact situation of all higher kinematic pairs the coupling of vectorial and discrete geometry representations is a challenge. The practical implementation of the presented method is shown on an X-ray shutter.

Shaping the digital twin for design and production engineering

The digitalization of manufacturing fuels the application of sophisticated virtual product models, which are referred to as digital twins, throughout all stages of product realization. Particularly, more realistic virtual models of manufactured products are essential to bridge the gap between design and manufacturing and to mirror the real and virtual worlds. In this paper, we propose a comprehensive reference model based on the concept of Skin Model Shapes, which serves as a digital twin of the physical product in design and manufacturing. In this regard, model conceptualization, representation, and implementation as well as applications along the product life-cycle are addressed.

Manufacturing signature in jacobian and torsor models for tolerance analysis of rigid parts

Geometric deviations characterize manufactured workpieces and have great influences on the quality and function of mechanical products. Therefore, geometric variations management has to be performed to ensure the product function despite the presence of these deviations. Dimensional and Geometric Product Specification and Verification (GPS) are standards for the description of the workpiece. The Skin Model is an abstract concept of the physical interface between a workpiece and its environment. In contrast to this understanding, established models for computer-aided modelling and engineering simulations make severe assumptions about the workpiece surface. Therefore, this paper deals with operationalizing the Skin Model concept in discrete geometry for the use in geometric variations management. The Skin Model Shape has been connected with manufacturing processes, in order to bring closer the Computer Aided Tolerancing (CAT) simulation tools to reality. The pattern left by a turning process has been considered the discrete geometry framework of the skin model shape of the assembly components in a tolerance analysis approach. The obtained results are significantly different from those due to the traditional approaches. The aim of this paper is to connect the Skin Model to the manufacturing processes.Correlation among manufactured surface points constitutes the framework of the Skin Model.The manufacturing Skin Model was integrated into the jacobian and the torsor models.The new Jacobian and torsor models were applied to a three parts assembly.The obtained results were compared with those due to the jacobian and torsor models.

An Improved Tolerance Analysis Method Based on Skin Model Shapes of Planar Parts

Geometric deviations have huge influences on the functional behavior of product, which should be analyzed and properly controlled. Tolerance analysis, as a way to evaluate geometric deviations, is an essential part of product development. Current Computer Aided Tolerancing systems provide solutions for tolerance analysis but have limitations in the consideration of form deviations. The Skin Model theory, as a new research topic, represents part with non-ideal model that comprises geometric deviations, thus developing into a new computer aided tolerancing approach. In this paper, the related work with respect to the generation of Skin Model Shapes and its application in assembly simulation and tolerance analysis is briefly introduced. In order to overcome its shortcomings in the tolerance analysis employing SMSs, this paper proposes an improved method by taking advantage of the method adopted in a CAT system. The proposed method supports the analysis of position and orientation tolerances and has been proved to be valid through a case study.

Virtual Metrology Laboratory for e-Learning

Understanding geometrical specifications is becoming more and more difficult due to the latest developments in ISO GPS (Geometrical Product Specification) Standards, and at the same time, students’ learning habits are evolving and theoretical courses on standardized specifications are not attractive. Metrology laboratory work is much more appealing and highlights the difficulties encountered in interpreting specifications and the inherent method uncertainties. Nevertheless, metrology activities require an expensive metrology laboratory equipped with CMMs. In order to carry out real hands-on experiments, Bordeaux University is designing a virtual laboratory framework. It is integrated into Moodle (an L.M.S., Learning Management System) as a new activity to establish a link with other Moodle learning activities (courses, tests, etc.) and to ensure student tracking. A first prototype of the virtual laboratory is dedicated to dimensional and geometrical metrology with simulated traditional measuring devices (gauge, micrometer, dial indicator, etc.) and Coordinate Measuring Machines. Measurement simulation is in a three-dimensional environment and is based on models of parts with dimensions, orientation, position and form errors (skin model shapes) and on models of measuring devices with measurement uncertainties.