Simultaneous optimal selection of design and manufacturing tolerances with different stack-up conditions using genetic algorithms

Tolerance design is one of the most critical aspects of product design and development process as it affects both the product's functional requirements and manufacturing cost. Unnecessarily tight tolerances lead to increased manufacturing cost, while loose tolerances may lead to malfunctioning of the product. Traditionally, this important phase of product development is accomplished intuitively to satisfy design constraints, based on handbooks' data and/or skill and experience of the designers. Tolerance design carried out in this manner does not necessarily lead to an optimum design. Research in this area indicates that, in general, tolerance design is carried out sequentially in two steps; (1) tolerance design in CAD to obtain design or functional tolerances and (2) tolerance design in CAPP to obtain manufacturing tolerances. Such a sequential approach to tolerance design suffers from several drawbacks, such as more time consumption, suboptimality and unhealthy working atmosphere. This paper reports on an integrated approach for simultaneous selection of design and manufacturing tolerances based on the minimization of the total manufacturing cost. The nonlinear multivariable optimization problem formulated in this manner may result in a noisy solution surface, which can effectively be solved with the help of a global optimization technique. A solution methodology using genetic algorithms and applying penalty function approach with proper normalization of the penalty terms for handling the constraints is proposed. The application of the proposed methodology is demonstrated on a simple mechanical assembly with different tolerance stack-up conditions.     

Generalization and evaluation of vectorial tolerances

In recent years, vectorial tolerancing has emerged as a new alternative for representing workpiece tolerances. In contrast to conventional geometric tolerances which originated from hard gauging practices, vectorial tolerancing follows the working principle of coordinate measuring machines and CAD/ CAM systems. Moreover it provides feedback from measurement directly to manufacturing process control. Many believe it is a better tolerancing method to tie design, manufacuturing, and measurement together. However, the current proposal of vectorial tolerancing has some limitations. First, the currently adopted orientation vector is not sufficient for representing true 3D orientations. As a result, the orientation of a free form surface cannot be properly established. Second, there is lacking a unified and consistent method for the evaluation of vectorial tolerances. This paper proposes a new orientation vector which provides a more general mathematical basis for representing vectorial tolerances. It enables true 3D orientation representation and relates tolerances to the functional requirement. With the improved mathematical definition, a systematic tolerance evaluation approach becomes possible for both analytical geometric elements and free-form surfaces. Computer simulations and real-world applications are studied to validate this new approach.     

Integration and application of feature recognition and mould manufacturing planning navigating process

Plastic injection mould manufacturing is diverse and complex. Traditional mould manufacturing planning often relied on experience and techniques. Therefore, in order not to affect the operation or even cause some loss of the enterprises, geometry information of computer-aided design (CAD) should be converted into manufacturing information of computer-aided process planning (CAPP) and CAM for automatic feature recognition to shorten the lead time. This study is the secondary development in a CAD environment creating a network plastic injection mould manufacturing navigating system and CAD/CAPP for integration and application. As part design is feature-based, each processing step can be regarded as a feature. By applying the hybrid recognition technology of the graph-based approach, the rule-based approach and hint-based approach in the analysis of part feature appearances, it can automatically categorise all the manufacturing features of the part and convert them into corresponding processes. The CAPP developed in this study can convert the part manufacturing features into corresponding processes and plan the part process sequence according to the process priority constraint relations. As proved by the case studies, the use of CAPP in this study can reduce planning time by 87%. It can provide users with a reference regarding process sequence, and speed up the part planning progress. Coupled with the mould manufacturing navigating process, it can achieve automatic design and manufacturing.     

Representation of geometric variations using matrix transforms for statistical tolerance analysis in assemblies

The goal of this article is to develop a tolerance representation for assemblies compatible with tolerance analysis based on a closed-form algorithm used in robotic applications. A methodology is described that represents standard Y14.5M-1982 tolerances using homogeneous 4×4 matrix transforms. Transforms represent both the nominal relations between parts and the variations caused by geometric deviations allowed by the tolerances. The analysis calculates a statistical estimate of the location of theNth part in an assembly starting from the first part or a fixture. Except forform tolerances, most types of tolerance specifications are compatible with the proposed representation. This approach is well suited to integration with CAD systems and feature-based design. Since assembly apparatus errors can be calculated using the same methodology, one can predict the relative position and angle errors between two parts about to be mated. This permits useful evaluation of assembly equipment errors, comparison of different product tolerance assignments, and calculations of assembly process capability.     

Advances in numerical computation based mechanical system design and simulation

This paper highlights the available numerical computation techniques in mechanical engineering field with focus on application of modeling and simulation within renewable energy conversion machines as a particular research case.The study makes special focus on simulation approaches based on finite elements and multibody dynamics that are currently focused within the research community.Developing the simulation model in a computer-aided design(CAD)tool is a precondition for a successful simulation-based mechanical system research.The article discusses and elaborates the methods implemented to integrate design data with other computer-aided engineering functions.The motivation is the fact that simulation-based research is useful for design optimization of mechanical systems that operate in harsh and unfriendly environment where conducting physical testing of prototypes is difficult.In addition,the manufacturing environment is benefiting from the developments in numerical computation techniques through machining simulation that significantly reduce the time-to-market and thus increase competitiveness.The study is also intended to explore the existing gaps of capabilities in current approaches in application of computational techniques in order to draw attention to future challenges and trends.     

Concurrent optimization of assembly tolerances for quality with position control using scatter search approach

Tolerance allocation to individual parts in any assembly should be a vital design function with which both the design and manufacturing engineers are concerned. Generally design engineers prefer to have tighter tolerances to ensure the quality of their design, whereas manufacturing engineers prefer loose tolerances for ease of production and the need to be economical. This paper introduces a concurrent tolerance approach, which determines optimal product tolerances and minimizes combined manufacturing and quality related costs in the early stages of design. A non-linear multivariable optimization model is formulated here for assembly. A combinatorial optimization problem by treating cost minimization as the objective function and stack-up conditions as the constraints are solved using scatter search algorithm. In order to further explore the influence of geometric tolerances in quality as well as in the manufacturing cost, position control is included in the model. The results show how position control enhances quality and reduces cost.     

A methodology for evaluation of metal forming system design and performance via CAE simulation

In the current metal forming product development paradigm, product cost, time-to-market and product quality are three overriding issues, which determine the competitiveness of the developed products. In the up front design process, the first 20% of design activities commits to about 80% of product development cost and product quality issues. How to conduct ‘right the first time’ design is critical to ensure low development cost, high product quality and short time-to-market. To address these issues, state of the art technologies are needed. Traditionally, computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies provide solutions for representation of design intent and realization of design solution physically. However, it is difficult to address some critical issues in the design of forming process, tooling structure, material selection and properties configuration, and finally the product quality control and assurance. Computer-aided engineering (CAE) technology fills this gap as it helps practitioners generate, verify, validate and optimize design solutions before they are practically implemented and physically realized. In this paper, a methodology for evaluation of metal forming system design and performance via CAE simulation is developed. The concept of metal forming system and its design is first articulated and how the CAE technology helps design and design solution evaluation is then presented. To assess and evaluate the performance of metal forming systems, quantitative design evaluation criteria are developed. Through industrial case study, how the developed methodology helps metal forming system design and evaluation is illustrated and its efficiency and validity is finally verified.     

Fixture design information support for integrated design and manufacturing

Although a vast amount of research has been conducted on developing computer-aided fixture design systems, the need for information exchange between the fixture design domain and other manufacturing domains has not been thoroughly dealt with. This paper addresses this gap in fixture design research through the development of appropriate information models for computer-aided fixture design systems to support integrated design and manufacturing. A fixture design activity model is presented that relates fixture design to other design and manufacturing activities. The implementation of the information models in XML and the exchange of the information models based on an XML messaging approach are also discussed.     

CAD／CAM系统中几何参数公差的辅助确定

本文从机械精加工中常用的外圆磨削着手，寻求圆柱形零件的尺寸误差、圆度误差、表面粗糙度之间的内在联系；同时考虑零件的功能要求、加工方法和加工成本，建立计算机辅助确定几何参数公差的数学模型，为ＣＡＤ／ＣＡＭ系统及其它机械设计中合理选择零件几何参数公差提供依据。     

Tolerance analysis and synthesis for cam mechanisms

Tolerance is one of the most important parameters in product and process design, so tolerancing plays a key role in design and manufacturing. Tolerance synthesis is in a period of extensive study due both to increased demands for quality products and to increasing automation of machining and assembly. Optimum tolerance design and synthesis ensures good quality product at low cost. This paper presents an analytical methodology for tolerance analysis and synthesis for a disk cam-translating follower system. Both dimensional ( size) and geometric tolerances ( position and profile ) on the components are considered. Tolerance analysis is performed on individual tolerances as well as on total tolerance accumulation. With the lowest manufacturing cost as its objective function a nonlinear optimization model is formulated for tolerance synthesis and solved by a sequential quadratic programming ( SQP) algorithm. An example is provided to illustrate the optimization model and solution procedure.     

Modeling methods for a flexible computer-aided embodiment design system

The investigation and implementation of product modeling methods are two of the most important research objectives in the field of computer-aided engineering design. Commercial product modeling systems go beyond geometric modeling systems in that they are adapted to user requirements through the addition of systems that aid in the generation of engineering models and methods. The embodiment design of a mechanical energy storage system serves as an example of future continuous and flexible computer-aided design (CAD). This example is implemented with GEKO, a user system for the layout of design elements. GEKO is coupled with a commercial geometric modeler (CATIA) and uses several programming interfaces.     

Identified research directions for using manufacturing knowledge earlier in the product life cycle

Design for manufacturing (DFM), especially the use of manufacturing knowledge to support design decisions, has received attention in the academic domain. However, industry practice has not been studied enough to provide solutions that are mature for industry. The current state of the art for DFM is often rule-based functionality within computer-aided design (CAD) systems that enforce specific design requirements. That rule-based functionality may or may not dynamically affect geometry definition. And, if rule-based functionality exists in the CAD system, it is typically a customisation on a case-by-case basis. Manufacturing knowledge is a phrase with vast meanings, which may include knowledge on the effects of material properties decisions, machine and process capabilities or understanding the unintended consequences of design decisions on manufacturing. One of the DFM questions to answer is: How can manufacturing knowledge, depending on its definition, be used earlier in the product life cycle to enable a more collaborative development environment? This paper will discuss the results of a workshop on manufacturing knowledge that highlights several research questions needing more study. This paper proposes recommendations for investigating the relationship of manufacturing knowledge with shape, behaviour and context characteristics of a product to produce a better understanding of what knowledge is most important. In addition, the proposal includes recommendations for investigating the system-level barriers to reusing manufacturing knowledge and how model-based manufacturing may ease the burden of knowledge sharing. Lastly, the proposal addresses the direction of future research for holistic solutions of using manufacturing knowledge earlier in the product life cycle.     

Tolerance optimization of a mobile phone camera lens system

In the manufacturing process for the lens system of a mobile phone camera, various types of assembly and manufacturing tolerances, such as tilt and decenter, should be appropriately allocated. Because these tolerances affect manufacturing cost and the expected optical performance, it is necessary to choose a systematic design methodology for determining optimal tolerances. In order to determine the tolerances that minimize production cost while satisfying the reliability constraints on important optical performance indices, we propose a tolerance design procedure for a lens system. A tolerance analysis is carried out using Latin hypercube sampling for evaluating the expected optical performance. The tolerance optimization is carried out using a function-based sequential approximate optimization technique that can reduce the computational burden and smooth numerical noise occurring in the optimization process. Using the proposed design approach, the optimal production cost was decreased by 28.3% compared to the initial cost while satisfying all the constraints on the expected optical performance. We believe that the tolerance analysis and design procedure presented in this study can be applied to the tolerance optimization of other systems.     

Concurrent process planning for finish milling and dimensional inspection of sculptured surfaces in die and mould manufacturing

With the introduction of computer-aided tools, traditional manufacturing tasks such as design, machining and inspection are now highly automated. However, due to the complexity and enormous knowledge involved in each process, most of these activities are still dealt with separately. Recent development of concurrent engineering emphasizes the importance of bringing manufacturing knowledge into the early design stage for optimum product and process design. In this paper, a knowledge-based CAD/CAM system which integrates process planning for finish milling and dimensional inspection of sculptured surfaces in die and mould manufacturing is presented. Optimum production plans are determined by minimizing the integral cost of machining and inspection. NC path generation and inspection planning are then verified by dynamic geometric simulations which provide the designer with the evaluations of machinability and inspectability. The implied significance is that strong inter-dependency may exist among various design life-cycle activities and that optimum solutions can be obtained by taking into account the interactions of the life-cycle events.     

Feature-based design: Four hypotheses for future CAD systems

Features are meaningful abstractions of geometry that engineers use to reason about components, products, and processes. For design activity, features are design primitives, serve as the basis for product representations, and can incorporate information relevant to life-cycle activities such as manufacturing. Research on feature-based design has matured to the point that results are being incorporated into commercial CAD systems. The intent of this paper is to look forward. Hence, this paper does not play the role of a standard survey, which necessarily only reviews the past. Nevertheless, an appropriate historical context is provided as a basis for these particular futuristic projections. Applications of feature-based design and technologies of feature representations are reviewed, then open research issues are identified and put in the context of past and current work. Features are prominent as catalysts for computer-based design tools, but the vision presented expands upon this basis, delving beyond features. For that future design environment, four hypotheses are proposed as research challenges: two on the existence of fundamental subfeature elements and relationships for features, one that presents a new definition of design features, and one that argues for the successful development of concurrent engineering languages. Evidence for these hypotheses is provided from recent research results and from speculation about the future of feature-based design.     

Simultaneous tolerancing for design and manufacturing

This paper presents a new tolerance design theory—simultaneous tolerancing— which works in the concurrent engineering context. After stating the need to develop a simultaneous tolerancing theory by showing the shortcomings of conventional tolerancing technique, the concept of simultaneous tolerancing is given, and its elements are briefly presented. Then we focus our attention on the development of a general mathematical model of optimal tolerancing supporting concurrent engineering. Two commonly used models, worst-case and statistical, are discussed in detail. Next, a method of ‘interim tolerances’, which help to determine an appropriate machining process without using functional tolerances, is proposed. The simultaneous tolerancing theory presented in this paper permits of determining directly optimal machining tolerances in product design, reducing the manufacturing cost and improving the quality of products. Finally, an example is given, showing that the proposed theory is feasible in practice.     

基于UG的平面尺寸链的计算机辅助飞机公差设计

随着CAD 技术的普及,飞机三维外型计算机辅助设计基本实现数字化,但目前飞机公差设计尚未实现计算机辅助设计。公差设计仍然需要大量手工计算,导致飞机公差设计与产品总体设计不协调,制约飞机数字化设计、制造技术的发展。探讨如何以现有CAD 三维造型系统为技术平台,开发计算机辅助飞机公差设计模块,实现平面尺寸链的飞机公差分析和分配的计算机辅助设计。给出在UG 平台上实现计算机辅助飞机公差设计模块的开发方案,并有利用该模块进行飞机装配件公差设计的运行实例。     

Tolerance specification and comparative analysis for computer-integrated dimensional inspection

Two important issues in the development of a computer-integrated dimensional inspection environment for manufactured parts are described, namely tolerance specification and comparative analysis. These two issues are related directly and therefore, should be addressed together. For supporting the computer-integrated dimensional inspection, a geometric dimensioning and tolerancing (GD&T) specification module and a comparative analysis module are developed and integrated with CATTA of the IBM CAD/CAM system. The proposed specification module supports ISO and ANSI geometric tolerances and allows multiple tolerance assignments on each single feature as well as on a group of same pattern features. Using this specification module, various tolerance information can be directly specified to the 3D CAD model of a part and can be used to support the subsequent planning and operation for manfacturing and inspection. The comparative analysis module is created to work with the GD&T module for constructing datum reference frames and comparing the actual measurement data with nominal design. After specifying all necessary tolerance information, using discrete measurement data from coordinate measuring machines (CMM), one can evaluate the dimensional quality of an actual feature through the comparative analysis module.     

Environmental conscious product design using CAD and CAE

Environmental consciousness has been regarded as an important concept for survival in the contemporary scenario. Modern design engineers are in need of approaches for creating environmentally friendlier products. In this context, this article reports a case study carried out in an Indian rotary switches manufacturing organization. The existing components of rotary switch have been modelled using computer-aided design (CAD). Then, the sustainability analysis has been carried out for determining the environmental impact. This is followed by the optimisation of the components using computer-aided engineering (CAE). The environmental impact has been measured in terms of carbon footprint, energy consumption and air/water impacts. It has been found that the optimized components possess minimal environmental impact. The sustainability improvement after the implementation of the proposed approach has been computed. It could be inferred from the results of the case study that CAD and CAE could lead to the development of environmental conscious product designs with minimal impact to the environment.