Computer-Aided Fixture Design Verification. Part 3. Stability Analysis

In fixture design, a workpiece is required to remain stable throughout the fixturing and machining processes in order to achieve safety and machining accuracy. This requirement is verified by a function of the computer-aided fixture design verification (CAFDV) system. This paper presents the methodologies of fixturing stability analysis in CAFDV. A kinetic fixture model is created to formulate the stability problem, and a fixture stiffness matrix (FSM) is derived to solve the problem. This approach not only verifies fixturing stability, but also finds the minimum clamping forces, fixture deformation, and fixture reaction forces. The clamping sequence can also be verified with this approach.     

An integrated fixture planning system for minimum tolerances

Fixture planning is an important aspect of process planning. The steps involved in automatic fixture planning are manufacturing feature recognition, setup planning and fixture configuration planning. In the present work, an integrated setup and fixture planning system is developed for minimum tolerances at critical regions using a data exchanged part model as an input. A platform-independent STEP-based automatic feature recognition system that can recognize both design and manufacturing features, including intersecting features is implemented. An automatic setup planning module is developed for generating setup plans for complete machining of a given component. A fixture planning module is developed applying the criteria of uniqueness, stability, accessibility and tolerance minimization. A case study is presented to demonstrate the capabilities and integration between the various modules of the system.     

Automated tolerance analysis for CAPP system

Various computer-aided process planning (CAPP) systems can produce different process plans, but few of them can deal with the specified design tolerances that have a significant influence on the process selection, machine selection, set-up and datum selection, operation sequencing, cutting condition decisions, and time and cost calculations; nor can they provide exact information on fixture/datum and the coordinates of the tool movements that are the necessary data for automated NC programming. The focus of this paper is on the discussion of an automated tolerance based analysis approach of CAPP systems.     

A model for evaluating the quality of the fixturing of the part from a machining process planning perspective

Achieving the required tolerance of positioning between two features is not obvious when features are produced in multiple set-ups where the fixture/workpiece interface plays a critical role. From a computer-aided process planning perspective, the suitability of a fixturing feature to achieve the required tolerances has to be evaluated. A model of an indicator for a locating quality is proposed here based upon the distribution of a small displacement of the workpiece compatible with the required tolerance onto the candidate fixturing surfaces.     

A Fast Interference Checking Algorithm for Automated Fixture Design Verification

Fixtures are tooling devices used to locate, support and hold workpieces during a manufacturing process. The major purpose of a computer-aided fixture design (CAFD) system is to provide a fixture design based on fixturing principles and workpiece information. Interference checking between the machining tool and fixture units, as well as between fixture units, is one of the important functions of automated fixture design. This paper presents a fast interference checking algorithm for automated modular fixture design validation. It is based on the study of the geometric characteristics of modular fixture components and the machining tool. The fixture component model is simplified into a 2D contour model with height information. The tool-path model is represented by a moving dot for 3-axis operations, or a moving line segment in 5-axis operations, as the fixture component model is different from the popular collision detection procedure using swept volume and is more efficient for fixturing verification. Application of this method will greatly reduce the computation complexity for fixturing interference checking. The method is implemented with a CAFD system. An example is given at the end of the paper.     

Dimensional and geometric tolerance design based on constraints

The paper presents a computer-aided approach of dimensional and geometric tolerance design. The method allows a designer to specify synthetically dimensional and geometric tolerances, including tolerance types and values. Firstly, tolerances are classified as self- and cross-referenced tolerances, and the rules for tolerance types design are presented. Secondly, the stack-up of 3D feature variation is formulated as a set of stack-up constraints (equation constraints), and the variation specified by tolerance forms tolerance constraints (inequality constraints). Tolerance value design is represented as the combinatorial optimization problem. The application of the variation and tolerance constraints to specify tolerance values is studied. Finally, a tolerance design example is used to illustrate the method.     

Computer-Aided Fixture Design Verification. Part 1. The Framework and Modelling

Computer-aided fixture design (CAFD) techniques have been advanced rapidly so that fixture configurations can be generated automatically, for both modular fixtures and dedicated fixtures. Computer-aided fixture design verification (CAFDV) is the technique for verifying and improving existing fixture designs. In this paper, the framework of CAFDV is introduced based on two models, i.e., geometric and kinematic models. The fixturing tolerance and stability verification will be presented in separate papers.     

Integrated tolerance optimisation with simulated annealing

Tolerance is one of the most important parameters in design and manufacturing. The allocation of design and machining tolerances has a significant impact on manufacturing cost and product quality. This article presents an analytical model for simultaneously allocating design and machining tolerances based on the least-manufacturing-cost criterion. In this study, tolerance allocation is formulated as a non-linear optimisation model based on the cost-tolerance relationship. A new global optimisation algorithm, simulated annealing, is employed to solve the non-linear programming problem. An example for illustrating the optimisation model and the solution procedure is provided.     

Development of a finite element analysis tool for fixture design integrity verification and optimisation

Machining fixtures are used to locate and constrain a workpiece during a machining operation. To ensure that the workpiece is manufactured according to specified dimensions and tolerances, it must be appropriately located and clamped. Minimising workpiece and fixture tooling deflections due to clamping and cutting forces in machining is critical to machining accuracy. An ideal fixture design maximises locating accuracy and workpiece stability, while minimising displacements.The purpose of this research is to develop a method for modelling workpiece boundary conditions and applied loads during a machining process, analyse modular fixture tool contact area deformation and optimise support locations, using finite element analysis (FEA). The workpiece boundary conditions are defined by locators and clamps. The locators are placed in a 3-2-1 fixture configuration, constraining all degrees of freedom of the workpiece and are modelled using linear spring-gap elements. The clamps are modelled as point loads. The workpiece is loaded to model cutting forces during drilling and milling machining operations. Fixture design integrity is verified. ANSYS parametric design language code is used to develop an algorithm to automatically optimise fixture support and clamp locations, and clamping forces, to minimise workpiece deformation, subsequently increasing machining accuracy. By implementing FEA in a computer-aided-fixture-design environment, unnecessary and uneconomical “trial and error” experimentation on the shop floor is eliminated.     

Computer-Aided Tolerance Charting for Products with Angular Features

This paper describes a process by which computer-aided design methods are used for the tolerance charting of products with angular features. If a product contains one or more angular features, such as chamfers and tapered surfaces, radial or normal machining of the features will result in axial dimension changes. In this paper, basic trigonometric formulae are first presented to explain the phenomenon of tolerance accumulation. In the process of tolerance charting, dummy cuts are included to reflect the corresponding dimensional changes due to indirect machining. With the assistance of flags and linked lists, the system proposed can automatically identify all dimensional chains which are associated with either regular cuts or dummy cuts. Moreover, optimisation techniques are recommended to allocate the allowable tolerances as specified by blueprints. In the search for an optimal design, the total manufacturing cost defined by the working tolerances is the objective function to be minimised.     

Surface error decomposition for fixture development

As an integrated element of the manufacturing system of components, the machining fixture is a major contributing factor of the profile and orientation errors of component features. An effective way to control the accuracy of components is to decompose error sources and evaluate individual influential factors. This paper proposed a systematic method of error identification and calculation, in which locating error and machining error were studied. The locating error, which is the surface error generated before machining, is obtained from the calculation of the surface error based on tolerances of the locating positions and the decomposition of clamping deformation using finite element analysis (FEA). The machining error, the surface error generated from machining operations, is gained mainly from the coordinate measurement machine’s (CMMs) measurements. The surface error of multi-machining operations is investigated and the resultant surface error is evaluated against tolerance. The analysis of a sample feature of a turbine blade is provided as an example.     

Algorithms for computer-aided generative process planning

Process planning is the function within a manufacturing facility that establishes the machining processes and parameters to be used so as to convert a piece-part from its initial form to the final form predetermined on an engineering drawing. Computer-aided process planning (CAPP) has become a major focus of manufacturing automation as it forms the interface between computer-aided design (CAD) and computer-aided manufacturing (CAM). Issues in CAPP include part representation, process selection, alternative process-plan generation, intermediate surface and tolerance determination, and operation sequencing. This paper focuses on quantitative models for determining cutting dimensions and tolerances for intermediate surfaces, and on a heuristic for sequencing cutting operations.     

夹具三维定位误差的计算机辅助分析

建立了三维定位误差影响加工精度的数学模型。运用矩阵计算法对工件的定位误差进行了计算，并分析了3种典型定位方式下夹具加工工件的定位误差。用实例分析了一面两销定位下工件的定位误差。此数学模型可用于计算机辅助夹具设计的后续精度评价。     

An intelligent fixture design method based on smart modular fixture unit

Fixture is an important assisting resource for manufacturing. Its design is under the combined influence of workpiece, machining methods, material performances, etc. and demands rich experience of the designer, which leads to significant increase in design cycle and costs. This paper researches on a new automatic fixture design methodology based on inheriting and reusing design knowledge. Traditional fixture model is changed to smart fixture model, which is not only a geometric model but also appending geometric knowledge, process planning knowledge, and performance knowledge. On the other hand, the ontology of workpiece’s machining requirement is established for driving smart fixture automatically. Based on the above ontology, the self-selection reasoning method is proposed, for machining planner, which can push unit selection scope and drive its size actively. To drive fixture unit assembly automatically, the self-assembly reasoning method is proposed according to typical feature’s assembly mode. By these ways, the geometric consistency among units as well as unit/workpiece is maintained. In the end, design efficiency and quality is expected to be improved more automatically.     

Extraction/conversion of geometric dimensions and tolerances for machining features

It is important for a feature-based system to preserve feature integrity during feature operation, especially when feature interaction occurs. The paper presents a feature conversion approach to convert design features used in a design model into machining features for the downstream applications. This process includes both form features (geometric information) and non-geometric features conversion. Most researchers have concentrated on geometric information extraction and conversion without tackling the important problem of non-geometric feature information. This paper focuses on the extraction and conversion of feature geometric dimensions and tolerances (GD&T) for downstream machining application.The main barrier to the integration of a feature-based CAD/CAPP/CAM system – feature interaction – is discussed in this paper, which alters design features in their geometries and non-geometric information. How to identify and validate these feature dimensions and tolerances is one of the key issues in feature interaction conversion. The development of robust methodologies for preserving feature integrity for use in process planning application is the main thrust of the work reported in this paper.     

基于图的夹具特征识别方法研究

夹具特征识别是实现夹具自动化设计的基础。本文在属性邻接图的基础上,给出了一种通过几何推理实现夹具特征识别的方法。在该方法中定义了制造特征方向,并通过最大凹边凝聚的方法,实现了极限表面的识别,在此基础上通过一系列几何推理算法来识别夹具特征。该方法可以有效地解决CAD与夹具设计CAD之间的集成问题,为自动化夹具设计探索了一种新的途径。     

An improved tolerance charting technique using an analysis of setup capability

The tolerance charting method enables the calculation of working tolerances in machining process planning. The method has been used as a basic tool for analysing process plans for many decades. Process capability in tolerance charting is modelled using the tolerances of the working dimensions. The literature shows that machining process capability can be analysed from the point of view of surface position errors. During setups, it is possible to perform decomposition into two surface position tolerances: a datum surface position tolerance and a machining surface position tolerance. This type of analysis has the advantage of producing simplified tolerance chains. This paper provides an adaptation of the tolerance charting technique that uses a capability model based on datum and machining surface position tolerance. The results show an improvement in the working tolerance stackup that reduces the capability required for productive resources. As a result, reductions in manufacturing costs can be achieved. The proposal is valid for manual or computer-assisted techniques.     

Tolerance analysis of mechanical assemblies based on modal interval and small degrees of freedom (MI-SDOF) concepts

Tolerance analysis is a key analytical tool for estimation of accumulating effects of the individual part tolerances on the design specifications of a mechanical assembly. This paper presents a new feature-based approach to tolerance analysis for mechanical assemblies with geometrical and dimensional tolerances. In this approach, geometrical and dimensional tolerances are expressed by small degrees of freedom (SDOF) of geometric entities (faces, feature axes, edges, and features of size) that are described by tolerance zones. The uncertainty of dimensions and geometrical form of features due to tolerances is mathematically described using modal interval arithmetic. The two concepts of modal interval analysis and SDOF are combined to describe the tolerance specifications. The algorithm is presented which explains the steps and the procedure of tolerance analysis. The proposed method is compatible with the current GD&T standards and can incorporate GD&T concepts such as various material modifiers (maximum material condition, least material condition, and regardless of feature size), envelope requirement, and bonus tolerances. This method can take into account multidimensional effects due to geometrical tolerances in tolerance analysis. The application of the proposed method is illustrated through presenting an example problem and comparing results with tolerance charting method.