Selection of machining datum and allocation of tolerance through tolerance charting technique

Tolerance charting is an effective tool to determine the optimal allocation of working dimensions and working tolerances such that the blueprint dimensions and tolerances can be achieved to accomplish the cost objectives.The selection of machining datum and allocation of tolerances are critical in any machining process planning as they directly affect any setup methods/machine tools selection and machining time.This paper mainly focuses on the selection of optimum machining datums and machining tolerances simultaneously in process planning.A dynamic tolerance charting constraint scheme is developed and implemented in the optimization procedure.An optimization model is formulated for selecting machining datum and tolerances and implemented with an algorithm namely Elitist Non-Dominated Sorting Genetic Algorithm(NSGA-II).The computational results indicate that the proposed methodology is capable and robust in finding the optimal machining datum set and tolerances.     

A Graph Representation Scheme for Process Planning of Machined Parts

A new generic graph representation scheme for process planning is presented. It has been specifically developed to facilitate the planning and control of geometric dimensions and tolerances. Relationships between cut, location, and datum surfaces and machining operations are represented by a graph consisting of connected boxes. A box is a multi-terminal node that corresponds to a machine set-up. Machining operations are depicted as arrows inside the box. These connect input nodes, representing datum and location surfaces, to output nodes, representing machined surfaces. External connectors join boxes together to form a directed graph. The main purpose of these connectors is to indicate the surfaces (from prior set-ups) that are used as location and datum surfaces for later set-ups. The representation covers cutting operations and non-cutting operations, such as surface finishes and coatings. The paper concludes with an example that illustrates the application and power of the scheme.     

面向公差技术的几何要素自由度表示与操作及其应用

描述了几何要素的本征方向和本征自由度概念、几何要素自由度的表示方法和约束自由度的计算规则。根据基本几何元素的几何特征以及与基准要素的位置关系确定了几何要素的本征方向,根据几何量测量原理,提出了基准要素约束自由度的计算方法,建立了基准体系约束自由度的计算规则。根据公差类型与基准要素和目标要素的关系,建立了公差设计正确性、完整性验证的具体规则,提出了基于自由度分析的公差标注正确性验证方法。     

基于同步约束解除的零件爆炸图自动生成方法

面向结构教学及维修人员培训,提出了一种基于零件几何约束关系同步解除的爆炸图自动生成方法。在定义零件拆卸轴向的基础上,建立了零件邻接拆卸约束关系矩阵及约束类型矩阵,按照可同步解除几何约束的顺序对零件进行分层,并利用判断规则识别子装配体。结合应用OBB和FDH两种包围盒,提出了一种“由外向内”的等速率分层牵引零件爆炸分离方法,实现了装配体组成零件爆炸图的自动生成。     

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.     

基于三维几何特征的产品报价方法

针对当前主流报价系统对产品实例库的多样性和时效性要求高且报价响应效率低的问题,在研究自动特征识别技术和CAD二次开发方法的基础上提出了基于三维几何特征的产品报价方法。首先,对常用设计特征进行分类并建立设计特征标识号、设计特征和加工特征间的映像关系,利用特征标识号和设计特征间的一一映像关系完成设计特征的识别,结合设计特征与加工特征的映像关系实现加工特征的识别;其次,通过零件的材料费用和特征加工费用进行零件的报价研究,综合产品、部件及零件的报价层次关系建立产品报价模型;再次,为快速响应用户个性化需求,建立了从用户需求到产品配置再到快速响应报价的服务模型;最后,开发了基于Creo的三维几何特征的报价系统,并将该报价系统应用于某企业大功率逆变器报价中,提高了产品报价效率,验证了基于三维几何特征的产品报价方法的可行性。     

An approach to incremental feature model conversion

A new approach to converting a design feature model (DFM) to a machining feature model (MFM) step-by-step during a design process has been proposed. In the approach, the MFM is incrementally set up and updated through performing a local machining feature recognition on certain areas of a design part’s B-rep when its design features are updated. The areas to be recognized as machining features are determined through geometric reasoning according to the statuses of the DFM and the dynamically generated MFM of the part. To facilitate the above geometric reasoning, a dynamic link list (DLL) has been developed to record the changed topological elements during the design process and the relationships between the DFM and the MFM. Based on the automatic and effective conversion process from design models to machining feature models, the proposed approach can support the establishment of a concurrent product development environment to link product design and manufacturing processes seamlessly.     

逆向工程中基于几何约束的汽车零部件模型重建

结合汽车变速籀、发动机零部件的精密测量和数字建模，考虑零部件实物模型中的隐含约束关系，提出了基于几何约束的模型重建和优化求解方法。该方法把约束按照优先等级顺序添加到约束图中，应用自由度分析法建立协调的约束系统，把复杂约束问题分解为约束子问题进行求解。将该方法应用于变速箱零部件模型重建中，所得到的几何模型较好地反映了原始设计意图，满足产品装配精度要求。     

Automatic feature recognition from engineering drawings

Recognising features from 2D engineering drawings is one of the most important issues facing advanced integrated design and manufacturing. Although the problem of recognising features from a solid model has been extensively studied, most existing product models are expressed as engineering drawings. Moreover, the solid model can only provide complete 3D topological and geometrical data. Some essential machining information cannot be retrieved. In this study, an automatic transfer system is proposed with the capability of recognising linear swept features from engineering drawings. The swept profiles are first extracted from drawing views, then features can be identified from the swept profiles using a set of recognition rules. In addition to the feature types and their geometry, some annotations of drawings, e.g. dimensions and geometric tolerances, can also be recognised and attached to the recognised features by this approach.     

CAD系统中并行公差的建模方法

提出一种基于产品设计与制造、支持公差工程语义和并行公差的几何公差计算机辅助建模表示新方法。在CAD系统中，进行装配公差和零件制造公差建模，利用特征的几何公差结构块，求出各种公差带的空间表示函数，计算有关公差带在三维空间中相应自由度的允许变动量，为公差分析和综合提供实用的模型。用工程实例验证了所提出的方法。     

圆柱齿轮滚切多刃断续切削空间成形模型及应用

滚切加工作为当前圆柱齿轮制造应用最为广泛的工艺具有高效但成形过程复杂的特点。基于展成原理,由规则分布在滚刀基本蜗杆螺旋面上的一系列切削刃相继从圆柱齿坯上切除材料,最终包络形成渐开线齿面。研究其成形原理可以为提升该工艺性能提供有效指导。针对齿轮滚切工艺过程多刃切削成形特征,建立圆柱齿轮滚切的空间成形模型。基于滚刀几何结构参数的理论约束关系,建立滚刀切削刃序列的参数方程;根据滚刀和工件的几何参数以及切削参数确定滚切过程中滚刀与工件的空间位置和运动关系,并基于空间运动学方法推导求得滚刀刀刃序列形成的空间成形曲面参数模型;采用离散数值方法获得滚切过程中的切屑几何形态及切屑厚度,并根据切屑几何计算各刀刃去除材料时的瞬时主切削力。该模型可以为滚切工艺参数优化、滚刀几何设计等研究提供支撑。     

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.     

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.     

Neutral representation of tolerance information for process planning using STEP AP 224

The focus of CIM is on information as it is the crucial element for integrating all the manufacturing activities. CAPP, as one of the key elements in CIM, needs to extract the manufacturing information such as machining features and precision specifications like surface roughness and tolerances from a geometric model in order to link CAD and CAM. However, these data are not real attributes of the geometric model in most of the current CAD systems. Therefore, human interpretation is inevitable for further processing of CAD model for downstream application like process planning or inspection. This paper proposes a scheme to represent the manufacturing information in a neutral format using STEP technology in order to enable downstream users such as process planner and inspection planner to make correct decisions on process selection, processing conditions, etc. It is shown that by using STEP AP224 manufacturing information encompassing machining features, surface roughness, dimensional and geometric tolerances can be completely represented together with part geometry, which certainly contributes to successful implementation of CIM.     

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.     

Computer-Aided Fixture Design Verification. Part 2. Tolerance Analysis

Tolerance analysis is the most important issue in computer-aided fixture design (CAFD), and it is an important part of computer-aided fixture design verification (CAFDV). This study presents a new approach for fixture tolerance analysis that is more generalised and can be used to assign locator tolerances based on machining surface tolerance requirements. The tolerance analysis is also generalised to handle any type of fixture design, workpiece, datum feature, and machining feature tolerance. Locator tolerance assignment distributes tolerances to locators based on a sensitivity analysis.     

Cylindricity modeling and tolerance analysis for cylindrical components

The circular and cylindrical features are fundamental geometric features in machines. Cylindricity error affects the fitting conditions of cylindrical components and impacts the performance of the precision products. In this paper, the cylindricity error was modeled using L-F functions and evaluated by particle swarm optimization algorithm. Then the contact method is developed to determine the position accuracy through the virtual assembling of the bore and shaft using Monte Carlo simulation. The effects of the cylindricity error and the number of lobes on the position error were analyzed in detail. The results indicate that the cylindricity error has more significant influence on the position accuracy between the cylindrical parts than the roundness error. Using the suggested method in the paper, the position accuracy can be rapidly predicted after the design tolerances are allocated or the geometrical errors are measured on manufactured parts.     

夹具定位误差分析自动建模方法

提出一般夹具的定位误差分析模型建立方法。根据工件—夹具系统的误差形成和传递路线分析,将工件—夹具系统分为定位元件、定位基准、工序基准和加工特征等四个要素组成,通过求解四个要素之间的位置变动获得工件—夹具系统整体的定位误差。将四个要素之间的位置及其变动关系用连杆机构模型等价表示,工件—夹具系统转换为接触副等价机构、公差关系等价机构、工序尺寸等价机构等三个等价机构的组合。研究工件和夹具定位元件接触副与等价机构之间的映射关系的建立方法,研究定位基准与工序基准、工序基准与加工特征之间的尺寸与公差关系所对应的等价连杆机构的建立方法,研究采用机构的结构参数和运动参数表示工件—夹具系统的所有工序信息及其内在联系的方法。利用机构学的机构位置计算方法求解定位误差,实现定位误差分析的自动化。     

Optimal tolerance design for mechanical assembly considering thermal impact

In this paper, a cost–tolerance model based on neural network methods is proposed in order to provide product designers and process planners with an accurate basis for estimating the manufacturing cost. Tolerance allocation among the assembly components is carried out to ensure that the functionality and design quality are satisfied considering the effect of dimensional and geometric tolerance of various components of the assembly by developing a parametric computer aided design (CAD) model. In addition, deformations of various components of mechanical assembly due to inertia and temperature effects are determined and the same is integrated with tolerance design. The benefits of integrating the results of finite element simulation in the early stages of tolerance design are discussed. The proposed method is explained with an application example of motor assembly, where variations due to both dimensional and geometric tolerances are studied. The results show that the proposed methods are much effective, cost, and time saving than the ones considered in literature.     

Fuzzy-small degrees of freedom representation of linear and angular variations in mechanical assemblies for tolerance analysis and allocation

Tolerances naturally generate an uncertain environment for design and manufacturing. In this paper, a novel fuzzy based tolerance representation approach for modeling the variations of geometric features due to dimensional tolerances is presented. The two concepts of fuzzy theory and small degrees of freedom are combined to introduce the fuzzy-small degrees of freedom model (F-SDOF). This model is suitable for tolerance analysis of mechanical assemblies with linear and angular tolerances. Based on the fuzzy concept, a new index (called the assemblability index) is introduced which signifies the fitting quality of parts in the assembly. Graphical and numerical representations of tolerance allocation by this method are presented. The goal of tolerance allocation is to adjust the tolerances assigned at the design stage so as to meet a functional requirement at the assembly stage. The presented method is compatible with the current dimensioning and tolerancing standards. The application of the proposed methodology is illustrated through presenting an example problem.