Methodology for tolerance design using quality loss function

In this paper, we develop a methodology for tolerance design that focuses on total variability from the target value. A general optimization model is developed to allocate tolerances to assembly parts. The constraint for the optimization model is the requirment for assembly tolerances or variability and cost is related to the total cost to reduce variance for the components and loss due to variation from target for the assembly. Different quality loss functions are also discussed.     

Self-organizing migration algorithm applied to machining allocation of clutch assembly

Tolerancing is an important issue in product and manufacturing process designs. The allocation of design tolerances between the components of a mechanical assembly and manufacturing tolerances in the intermediate machining steps of component fabrication can significantly affect the quality, robustness and life-cycle of a product. Stimulated by the growing demand for improving the reliability and performance of manufacturing process designs, the tolerance design optimization has been receiving significant attention from researchers in the field. In recent years, a broad class of meta-heuristics algorithms has been developed for tolerance optimization. Recently, a new class of stochastic optimization algorithm called self-organizing migrating algorithm (SOMA) was proposed in literature. SOMA works on a population of potential solutions called specimen and it is based on the self-organizing behavior of groups of individuals in a “social environment”. This paper introduces a modified SOMA approach based on Gaussian operator (GSOMA) to solve the machining tolerance allocation of an overrunning clutch assembly. The objective is to obtain optimum tolerances of the individual components for the minimum cost of manufacturing. Simulation results obtained by the SOMA and GSOMA approaches are compared with results presented in recent literature using geometric programming, genetic algorithm, and particle swarm optimization.     

Stacked tolerance analysis and allocation using assembly models

Allocation of appropriate tolerances is critical to ensure that components fit right and function satisfactorily in an assembly involving stacked components. There are numerous techniques available today to model assemblies on a computer. What is lacking is a common platform to make use of these computer models in order to perform tolerance analysis and allocation. This paper describes a technique to automate tolerance analysis and allocation of an assembly involving components stacked one on another represented in the boundary form. An algorithm is developed to track dimension loops in the stacked assembly. Statistical tolerance analysis and allocation is then performed on these interrelating dimensions and tolerances encompassed by a dimension loop. Advantages and limitations of this technique are compared against the manual method to conduct tolerance analysis and allocation.     

A generic deviation-based approach for synthesis of tolerances

In this paper, a generic model for the synthesis of tolerances for manufactured parts is presented. The model uses a method of transforming traditional tolerance specifications (as defined in ASME Y14.5M) to a generalized coordinate system (hereinafter referred to as deviation space). Small displacement torsors (SDTs) have been used for representing the deviations. The tolerance synthesis method is formulated as a constrained nonlinear optimization process. Three different types of constraints have been considered for the optimization process: 1) representation of assemblability; 2) mapping of tolerance specification to deviations; and 3) functional requirements. A new deviation-based cost of manufacturing model has been proposed. A working module of the scheme has been implemented and the process has been elaborated with two examples. The possibility of extension of the model and scope for further generalization have been discussed. Note to Practitioners-This paper presents an optimization method for finding tolerance values for different features of an assembly of manufactured parts. Determination of different types of tolerances and their exact values for critical features of any part has been an ad-hoc process; it has been mostly experience-based till recent time. In this paper, efforts have been made to establish a strong mathematically oriented method for synthesis of tolerances. The procedure attempts to minimize cost of manufacturing while the functionality and assemblability are satisfied. It is a generalized method and could be applied for designing rigid manufacture parts. For practical usage, this method should be integrated with three-dimensional computer-aided design packages as a tolerance synthesis module for integrated tolerance design.     

基于特征的装配尺寸链自动生成及分析的研究

讨论了尺寸和公差的工程语义的表达，提出零件邻接表和尺寸邻接表的装置体分级建模方式，并由封闭环信息优化调整装置体零件邻接表，在此基础上提出了根据产品的装配特征，形状特征，精度特征和指定的封闭环信息自动生成装置尺寸／公差链的算法。     

Generation of functional tolerancing based on positioning features

The aim behind applying functional tolerancing to a mechanism is to widen the tolerances used in parts manufacturing according to the effective functional properties of the product. This step may be performed using CAD systems and geometrical specifications defined by ISO standards. The present paper will describe the complete process involved in functional tolerancing. The CLIC tolerancing method has been implemented within an Excel software environment. CAD models for parts have been imported via a STEP interface. According to this approach, the designer describes the assembly process; the CLIC system then determines the functional requirements corresponding to the joints between parts and generates all datum reference frames and tolerancing of set-up surfaces in compliance with ISO standards. CLIC also determines both the geometrical conditions of minimum distances in order to avoid interference between parts and the conditions for assembling small standard components. The designer next adds other functional requirements. For each such requirement, a tolerancing process creates location and orientation specifications for influential parts using datum reference frames derived during the previous stage. Excel formulae focusing on the sum of tolerances are generated using a three-dimensional statistical approach. Moreover, the tolerance database allows optimizing the tolerances and nominal dimensions of parts.     

Multi-objective design and tolerance allocation for single- and multi-level systems

In this work we develop a method to perform simultaneous design and tolerance allocation for engineering problems with multiple objectives. Most studies in existing literature focus on either optimal design with constant tolerances or the optimal tolerance allocation for a given design setup. Simultaneously performing both design and tolerance allocation with multiple objectives for hierarchical systems increases problem dimensions and raises additional computational challenges. A design framework is proposed to obtain optimal design alternatives and to rank their performances when variations are present. An optimality influence range is developed to aid design alternatives selections with an influence signal-to-noise ratio that indicates the accordance of objective variations to the Pareto set and an influence area that quantifies the variations of a design . An additional tolerance design scheme is implemented to ensure that design alternatives meet the target tolerance regions. The proposed method is also extended to decomposed multi-level systems by integrating traditional sensitivity analysis for uncertainty propagation with analytical target cascading. This work enables decision-makers to select their best design alternatives on the Pareto set using three measures with different purposes. Examples demonstrate the effectiveness of the method on both single- and multi-level systems.     

Multiple-view feature modelling for integral product development

To allow a designer to focus on the information that is relevant for a particular product development phase, is an important aspect of integral product development. Unlike current modelling systems, multiple-view feature modelling can adequately support this, by providing an own view on a product for each phase. Each view contains a feature model of the product specific for the corresponding phase. An approach to multiple-view feature modelling is presented that supports conceptual design, assembly design, part detail design and part manufacturing planning. It does not only provide views with form features to model single parts, as previous approaches to multiple-view feature modelling did, but also a view with conceptual features, to model the product configuration with functional components and interfaces between these components, and a view with assembly features, to model the connections between components. The general concept of this multiple-view feature modelling approach, the functionality of the four views, and the way the views are kept consistent, are described.     

A method for computer-aided specification of geometric tolerances

The paper describes a method for the generation of tolerance specifications from product data. The problem is nontrivial due to the increasing adoption of geometric dimensioning criteria, which call for the use of many types of geometric tolerances to completely and unambiguously represent the design intent and the many constraints deriving from manufacturing, assembly and inspection processes. All these issues have to be modeled and explicitly provided to a generative specification procedure, which may thus need a large amount of input data. The proposed approach tries to avoid this difficulty by considering that most precision requirements to be defined relate to the assembly process, and can be automatically derived by analyzing the contact relations between parts and the assembly operations planned for the product. Along with possible user-defined additional requirements relating to function, assembly requirements are used in a rule-based geometric reasoning procedure to select datum reference frames for each part and to assign tolerance types to part features. A demonstrative software tool based on the developed procedure has allowed to verify its correctness and application scope on some product examples.     

Analysis of tolerances and process inaccuracies in discrete part manufacturing

This paper discusses basic problems of tolerancing mechanical parts. The problem of part design analysis (determination of consistency, determinacy and stability) is reduced to the analysis of systems of linear inequalities. It is shown that the calculation of resulting tolerances between components of parts can be easily solved by linear programming. A method is provided for the construction of a system of inequalities which the working dimensions and inaccuracies of machining operations must satisfy if a given tolerance specification is to be met.     

An algorithm for CAD tolerancing integration: Generation of assembly configurations according to dimensional and geometrical tolerances

For several years, Digital Mock-Up (DMU) has been improved by the integration of many tools as Finite Element (FE) Analysis, Computer Aided Manufacturing (CAM), and Computer Aided Tolerancing (CAT) in the Computer Aided Design (CAD) model. In the geometrical model, the tolerances, which specify the requirements for the proper functioning of mechanical systems, are formally represented. The nominal modeling of the parts and assemblies does not allow the prediction of the tolerance impacts on the simulation results as the optimization of mechanical system assemblability. So, improving the CAD model to be closer to the realistic model is a necessity to verify and validate the mechanical system assemblability. This paper proposes a new approach to integrate the tolerances in CAD model by the determination of the configurations with defects of a CAD part, used in a mechanical system. The realistic parts are computed according to the dimensional and geometrical tolerances. This approach provides an assembly result closer to the real assembly of the mechanical system. The Replacement of the nominal parts by the realistic ones requires the redefinition of the initially defined assembly mating constraints. The update of the mating constraints is performed by respecting an Objective Function of the Assembly (OFA). Integrating tolerances in CAD allows the visualization and simulation of the mechanical assemblies’ behavior in their real configuration and the detection of possible interference and collision effects between parts which are undetectable in the nominal state.     

Tolerance analysis — Form defects modeling and simulation by modal decomposition and optimization

Tolerance analysis aims on checking whether specified tolerances enable functional and assembly requirements. The tolerance analysis approaches discussed in literature are generally assumed without the consideration of parts’ form defects. This paper presents a new model to consider the form defects in an assembly simulation. A Metric Modal Decomposition (MMD) method is henceforth, developed to model the form defects of various parts in a mechanism. The assemblies including form defects are further assessed using mathematical optimization. The optimization involves two models of surfaces: real model and difference surface-base method, and introduces the concept of signed distance. The optimization algorithms are then compared in terms of time consumption and accuracy. To illustrate the methods and their respective applications, a simplified over-constrained industrial mechanism in three dimensions is also used as a case study.     

Computerized tolerance techniques

The intense global competition to produce quality product at low cost has led many industrial nations to consider mechanical tolerances as a key factor to bring about cost savings as well as be competitive. In the last two decades, not enough research work has been done in the area of tolerance techniques. In this paper, a comprehensive summary of the state of the art and the projection of future trends in tolerance techniques is presented to help make a decision concerning the improvement techniques today and to aid in guiding research for the future. This paper reviews the status of theory and practice about how manufacturing and assembly processes are characterized for relating tolerances to process or production cost. The specification of tolerancing on the dimension of the manufactured part has a significant impact on the final production cost. Tight tolerances can result in excessive process cost, while loose tolerances may lead to increased waste and assembly problems. This paper systematically reviews the state of art by classifying the papers written so far into three categories. The categories are tolerance chain technique, tolerances based on analysis and synthesis, and tolerances based on cost-tolerance algorithms and design methods. Future areas of research and the unresolved issues have been presented that will serve as a guiding light for the researcher to investigate and bring about the solution in future.     

Concurrent optimal adjustment of nominal dimensions and selection of tolerances considering alternative machines

Traditional practice to tolerance design has been a part of a three-step sequential approach to the overall product design process involving (i) conceptual design, (ii) parameter design, and (iii) tolerance design, in isolation. This practice works well for linear assemblies, as the sensitivities of tolerances are fixed, i.e. independent of the nominal dimensions. However, for nonlinear assemblies after the second step, an integrated approach involving minor adjustment of nominal dimensions and selection of tolerances in the third step, can be better to control the variability in the assembly output characteristic. The latter case has been addressed in this study. Simultaneous selection of design and manufacturing tolerances, and choice of a machine from amongst the alternatives, frequently encountered in different stages of realization of individual dimensions, are important issues in product development. Optimal design problem with focus on these issues has been attempted here. The resulting optimization problem involving a combinatorial and nonlinear search space cannot be effectively solved for the global solution using conventional optimization techniques. The genetic algorithm, a nontraditional optimization technique, has been proposed in this research. The solution of the aforementioned concurrent design problem has been demonstrated with the help of a simple case study.     

Automatically generating assembly tolerance types with an ontology-based approach

In most cases, designers have to manually specify both assembly tolerance types and values when they design a mechanical product. Different designers will possibly specify different assembly tolerance types and values for the same nominal geometry. Furthermore, assembly tolerance specification design of a complex product is a highly collaborative process, in which semantic interoperability issues significantly arise. These situations will cause the uncertainty in assembly tolerance specification design and finally affect the quality of the product. In order to reduce the uncertainty and to support the semantic interoperability in assembly tolerance specification design, an ontology-based approach for automatically generating assembly tolerance types is proposed. First of all, an extended assembly tolerance representation model is constructed by introducing a spatial relation layer. The constructed model is hierarchically organized and consists of part layer, assembly feature surface layer, and spatial relation layer. All these layers are defined with Web Ontology Language (OWL) assertions. Next, a meta-ontology for assembly tolerance representations is constructed. With this meta-ontology, the domain-specific assembly tolerance representation knowledge can be derived by reusing or inheriting the classes or properties. Based on this, assembly tolerance representation knowledge is formalized using OWL. As a result, assembly tolerance representation knowledge has well-defined semantics due to the logic-based semantics of OWL, making it possible to automatically detect inconsistencies of assembly tolerance representation knowledge bases. The mapping relations between spatial relations and assembly tolerance types are represented in Semantic Web Rule Language (SWRL). Furthermore, actual generation processes of assembly tolerance types are carried out using Java Expert System Shell (JESS) by mapping OWL-based structure knowledge and SWRL-based constraint knowledge into JESS facts and JESS rules, respectively. Based on this, an approach for automatically generating assembly tolerance types is proposed. Finally, the effectiveness of the proposed approach is demonstrated by a practical example.     

Improved algorithm for tolerance allocation based on Monte Carlo simulation and discrete optimization

The allocation of design and manufacturing tolerances has a significant effect on both manufacturing cost and quality. This paper considers nonlinearly constrained tolerance allocation problems. The purpose is to minimize the ratio between the sum of the manufacturing costs (tolerances costs) and the risk (probability of the respect of geometrical requirements). The techniques of Monte Carlo simulation and genetic algorithm are adopted to solve these problems. As the simplest and the popular method for non-linear statistical tolerance analysis, the Monte Carlo simulation is introduced into the frame. Moreover, in order to make the frame efficient, the genetic algorithm is improved according to the features of the Monte Carlo simulation. An illustrative example (hyperstatic mechanism) is given to demonstrate the efficiency of the proposed approach.     

High aspect ratio meso-scale parts enabled by wire micro-EDM

Micro-electro discharge machining (EDM) is a subtractive meso-scale machining process. The Agie Excellence 2F wire micro EDM is capable of machining with a 25 micron diameter wire electrode and positioning the work piece to within ±1.5 microns. The over-burn gap can be controlled to within 3 microns to obtain a minimum feature radius of about 16 microns while achieving submicron surface finish and an imperceptible recast layer. For example, meso-scale gears that require vertical sidewalls and contour tolerances to within 3 microns can be wire EDMed into a variety of conductive materials. Material instabilities can affect the dimensional precision of machined meso-scale parts by material relaxation during the machining process. A study is done to investigate the machining performance of the wire micro EDM process by machining a high aspect ratio meso-scale part into a variety of metals (e.g. 304L stainless steel, Nitronic 60 Austentic Stainless, Beryllium Copper, and Titanium). Machining performance parameters such as, profile tolerance, perpendicularity, and repeatability are compared for the different materials. Pertinent inspection methods desirable for meso-scale quality assurance tasks are also evaluated. Sandia National Laboratories is developing meso-scale electro-mechanical components and has an interest in the assembly implications of piece parts fabricated by various meso-scale manufacturing processes. Although the wire EDM process is typically used to fabricate 2½ dimensional features, these features can be machined into a 3 dimensional part having other features such as hubs and chamfers to facilitate assembly.     

Simultaneous optimal selection of design and manufacturing tolerances with alternative manufacturing process selection

Tolerance specification is an important part of mechanical design. Design tolerances strongly influence the functional performance and manufacturing cost of a mechanical product. Tighter tolerances normally produce superior components, better performing mechanical systems and good assemblability with assured exchangeability at the assembly line. However, unnecessarily tight tolerances lead to excessive manufacturing costs for a given application. The balancing of performance and manufacturing cost through identification of optimal design tolerances is a major concern in modern design. Traditionally, design tolerances are specified based on the designer’s experience. Computer-aided (or software-based) tolerance synthesis and alternative manufacturing process selection programs allow a designer to verify the relations between all design tolerances to produce a consistent and feasible design. In this paper, a general new methodology using intelligent algorithms viz., Elitist Non-dominated Sorting Genetic Algorithm (NSGA-II) and Multi Objective Particle Swarm Optimization (MOPSO) for simultaneous optimal selection of design and manufacturing tolerances with alternative manufacturing process selection is presented. The problem has a multi-criterion character in which 3 objective functions, 3 constraints and 5 variables are considered. The average fitness factor method and normalized weighted objective functions method are separately used to select the best optimal solution from Pareto optimal fronts. Two multi-objective performance measures namely solution spread measure and ratio of non-dominated individuals are used to evaluate the strength of Pareto optimal fronts. Two more multi-objective performance measures namely optimiser overhead and algorithm effort are used to find the computational effort of NSGA-II and MOPSO algorithms. The Pareto optimal fronts and results obtained from various techniques are compared and analysed.     

Tolerance Representation and Analysis in Industrial Inspection

In this work we address the problem of tolerance representation and analysis across the domains of industrialinspection using sensed data, CAD design, and manufacturing. Instead of using geometric primitives in CAD models todefine and represent tolerances, we propose the use of stronger methods that are completely based on the manufacturingknowledge for the objects to be inspected. We guide our sensing strategies based on the manufacturing process plans for theparts that are to be inspected and define, compute, and analyze the tolerance of the parts based on the uncertainty in thesensed data along the different toolpaths of the sensed part. We believe that our new approach is the best way to unifytolerances across sensing, CAD, and CAM, as it captures the manufacturing knowledge of the parts to be inspected, asopposed to just CAD geometric representations.     

基于模板的自顶向下并行装配设计

在分析产品设计系统的装配功能的基础上,提出了一个并行装配设计模式及装配模板与模板实例化的概念;在处理装配约束的虚拟表示与实例表示概念,使得系统具有了支持自顶向下和自底向上２种设计方法的能力。该方法已被成功地应用于参数化特征造型系统ＺＤ－ＭＣＡＤＩＩ中,证明了基于装配模板基础上的装配设计方法法能在零件设计级上有效地支持并行设计。