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黄河科技学院毕业设计（文献翻译） 第17 页加工的几何尺寸和公差的提取/转换功能摘要在操作过程中，重要的是保持基于特征系统功能的完整性，尤其是当功能发生相互作用的时候。本文提出了一种用于后续应用领域的转换成设计模型的功能转换方式。这个过程包括成型特征（几何信息）和非几何特征转换。大多数研究人员都集中在几何信息的提取和转换上而忽略非几何特征的重要问题。本文着重探讨对后续加工应用中的几何尺寸和公差（GDT）的提取和转换功能。在本文中将讨论基于CAD/ CAPP/ CAM系统集成特征的主要障碍功能的互动，从而改变它们的几何和非几何的设计特点信息。如何识别和验证这些功能尺寸和公差是特征交互转换的关键问题之一。本文报告工作的主要推力是在使用过程中为规划申请保留功能的完整性而形成强有力的方法。关键词：CAD/ CAPP / CAM集成，功能转换，功能互动，几何尺寸和公差（GDT），加工特征1简介现代设计和先进的加工过程中的一个主要目标是通过各种设计和工艺规划活动之间的无缝集成来实现产品交货时间的显著缩短。使用组件特性的概念来集成设计系统和制造系统已经成为近年来的主要研究方向1-5。在一般情况下，功能是用于赋予组件属性的意义，帮助解剖可识别的、有意义的区域以及改善设计和制造之间联系。特征识别和基于特征的设计是两个主要使用的途径。特征识别驱动器的功能信息来自被识别的几何数据库，通过计算机算法进行提取和分类。特征识别问题与设计意图的损失，不完备的设计和制造信息以及特征识别的复杂性和计算负荷方案3，6，7相关。基于特征的设计涉及到通过使用“积木”定义一个组件，它代表着或设计功能或加工功能。由于功能是取决于过程的，不同的应用需要不同的应用功能。设计阶段使用的功能（称为设计特点）是功能特点，代表着设计意图，如肋，开槽，并加强的功能。在规划阶段使用的功能（称为加工功能）与加工业务有关，如孔、盲孔、槽和袖珍的功能。此外，功能因应用程序的不同而有不同呈现。这些事实是根据需要以正确反映加工特点并实现CAD、CAPP和CAM系统集成在一个基于特征的环境的主要障碍。设计人员使用设计特征构建组件特征模型是最合理的，然后通过加工特征在工艺规划系统中将模型转换。这个过程被称为功能转换或功能映射。许多研究人员对功能转换的方法和算法进行了研究和应用8-10。然而，当代大多数基于特征的设计仍然不能完全处理发展基于特征系统中的一个最重要的，或许是最困难的方面，就是功能的定义和代表性的相互作用。以前使用的功能的特征交互改变了几何形状和属性，但它不会改变它们相应的数据库，这使得后续应用不能从设计数据库获得真正的组件模型。因此，在设计和加工过程中的阶段维护功能的完整性是实现基于特征的CAD / CAPP / CAM一体化问题的关键之一11-13。许多研究已进行到基于提取加工特征设计系统5，7，9，10，14-16的技术。然而，这些研究主要专注于几何信息的识别或转换。在机床特征的非几何信息方面做的还很少，如几何尺寸和公差（GDT）。Shah提出17的方法来创建一个在设计和工艺规划应用中使用的尺寸和公差模型。这种GDT模型被用来捕捉设计师基于特征设计上的GDT方案模型，验证其完整性，然后用特征识别的方法转移GDT到加工特征的自动提取。该方法侧重于在设计和制造阶段如何使用一部分的尺寸时为有效模式。因此，构建出强劲的适合CAD和CAPP系统的GDT模型，是一项复杂而艰巨的任务。对于一个以商业功能为基础的系统（如Pro / Engineer），一旦一个组件的造型过程已经完成，则一个GDT模型即被构建。为了创建所使用的CAD模型，其目标就是去捕获和转换一个过程规划体系所需要的几何和非几何信息并实现CAD / CAPP无缝集成的过程。为了实现一个过程规划模型的完整代表， GDT所需要一个组件需保持完整，并根据加工特点的要求被完整和正确的表示。本文中提出的工作是处理从基于保持一个基于特征加工的完整性和一致性过程的GDT模型的提取/转换。在第2节中通过功能转换模块和STEP文件界面介绍了建议的基于特征CAD / CAPP集成系统的框架。在第3节中，我们讨论了关于功能相互作用所导致的功能信息的改建以及不改变它们数据库等难以解决的问题。加工功能可以通过从设计模型的功能信息中提取和转换来生成。在第4节中，我们介绍为功能GDT的提取和转换所开发的算法。在第5节中，使用一个详细的组件的例子来展示几何尺寸转换的结果。在最后一节，给予本文的结论。2基于CAD/ CAPP集成系统的结构特征组件模型是在基于一系列如突起，槽，孔等的设计特点的环境下被构建的。诸如对CAD模型的过程规划有着不同看法的后续应用，它采用了符合加工操作的一系列加工功能。为实现CAD / CAPP / CAM系统的完整性，代表加工特征的功能几何的提取和转换以及非几何信息加工功能被提出。通过功能转换方法得到的集成系统的框架如图2.1所示。在这个方法中，用基于特征的CAD / CAM软件即Pro / Engineer，来创建一个组件特征模型。通过该功能转换模块和STEP文件接口，可以创建相应的过程规划模型组件，以测试并验证转换的加工特征，这将在18中予以讨论。在功能转换模块中，为加工转换特征属性形成功能映射以开发转换CAD数据库中的几何信息到用于创建一个完整的和正确的代表着有意义“形状”的加工过程和GDT的映射。图2.1 综合功能为基础的CAD / CAM系统框架3特征交互作用在一般情况下，如果特征交互导致原来的功能参与要素类或要素属性的任何改变，那么特征交互就被认为已经发生了。基于特征的设计系统，组件模型被一系列的由一个功能修改另一个的方式的设计特点建立起来。图3.1显示了一个涉及到盲槽1和盲槽2特征的功能交互组件的建模过程，。当相互作用发生时，之前使用的盲槽1和盲槽2功能随着特征尺寸和几何元素的可能改建转变成插槽3和盲槽4功能。图3.1 两个盲槽之间互动功能的例子，随着不同的属性或几何形状生产一个插槽和盲槽特征交互组件模型有着广泛的影响，因为它们不仅改变了预定义的功能几何形状也改变它们的属性。问题是这些产品在组件数据库中是不可替代的，从而导致后续工艺规划的困难。在图3.2中，由于台阶功能的相互作用，以前使用的袖珍功能的几何形状和尺寸发生了变化。图3.2b中，袖珍功能的深度尺寸被改为h1，代替原来的维度h，并将在图3.2c中的袖珍功能改变为盲槽功能，新成立的边缘层面功能也由以前维度a变为a1。然而，组件数据库只记录原始功能，并不包含模拟结果。按照图3.2中的例子，我们可以从数据库中获得图3.2b和图3.2c中两个模型的原始袖珍功能尺寸h和a。图3.2 由袖珍功能和台阶功能的相互作用引起的特征类型和尺寸的不一致为了维护功能模型的完整性，这里有必要由加工特征基础处理规划系统开发一个强大的特征提取/转换算法来检测不一致的功能描述，并将其转换成所需要的正确描述。关于几何方面的设计特点检测和转换算法的加工特征将在17中予以介绍，在这里为了正确地表示转换后的加工特征，我们将重点放在GDT的提取/转换上。4提取几何尺寸和公差的特征4.1几何尺寸特征的提取在基于CAD特征的系统里，数据库中组件和特性的几何尺寸的描述包含了下列项目：1. 尺寸值和公差。2. 如线性尺寸，角度尺寸和直径尺寸的维度类型。3. 维度的边界元素：一个面、轴和一个边缘。例如，维度d4（如图4.1所示）的边界元素是面f1和边缘e1，同时维度d5是面f2和边缘e2的边界元素。图4.1 特征交互和尺寸边界元素除了上面列出的项目1和2，工艺规划系统根据CAD模型有不同的尺寸边界表示，该模型需要用加工操作的一个有意义的几何元素尺寸来描述，如一个面或一个轴元素，但不用边界元素。由于是由两个面元素形成的一个边缘，它是加工流程建模使用的某种模糊的方式。例如，在图4.1中的维度“d”是描述的长度|d|以及e1，e2的两个边元素。加工过程建模的要求，应该是用面f5和面f8的边界元素描述的。由于设计特点和加工特点之间不同的维度描述，有必要区分和改造过程建模的CAD模型相对应的面元素中的维度边界元素的边缘。该算法是以GDT映射模块中的一个模块作为以下步骤实现的。第1步：搜索组件上的每个维度dim(i)，让i=1，2，N_GD，N_GD组件几何尺寸的总数。第2步：让i = 1，提取维度dim(i)的值，类型和边界元素标识id1，id2。第3步：检查维度dim(i)的边界元素。如果一个边界元素不是一个真正的元素，那么在功能操作后维度的边界就不再存在。这意味着维度的边界被改变。在这种情况下，一个处理互动功能的额外算法是需要的，这将在下一节予以讨论。如果这两个边界元素都是真实的，那么维度的边界类型就能直接确定。第4步：如果一个维度的边界元素是边e0，另一个面是f0，那么面f1和f2的其中一个形成的边e0必须得以确定（转到第5步），并使之作为加工特征的三维边界元素。如果维度的两个边界元素是边e1和e2，就转到第6步把两个边都转换成两个面，作为作为维度的两个边界元素。如果维度dim(i)的两个边界元素是两个面或两根轴，亦或一个面和一根轴，那么它们的标识是在一个数组记录，再转到第7步;第5步：比较长度d01和d02的真实值，它们分别代表了面f0和f1、面f0和f2之间的距离。如果距离d01等于（维度dim(i)）的真实值，那么代替边界识别e0的id1也伴随着f1的鉴定，这意味着边元素的表示已改为面元素的表示。如果距离d02等于（维度dim(i)）的真实值，那么代替边界识别面f2中的一边e0的id1，并转到第7步。第6步：比较长度d11，d12，d21，d22（dij代表面f1i和面f2j之间的距离，其中i，j=1,2）尺寸的真实值，必须存在一个有维度dim(i)真实值的距离。如果d21等于(维度dim(i)的真实值，那么面f12和f21就替代尺寸d21的两个边界标识，并转到第7步。步骤7：如果i基准数N，就结束；否则，设置i= i+1，返回到第2步。-通过上述算法，共平面阵列存储相应的基准元素以对应真正的功能元素。算法流程图如图4.5所示。结合两个阶段的转换算法，位置几何公差可以完全转换。图4.5 组件位置几何公差基准转换的流程图5一个组件的例子在本节中介绍一个棱柱组件几何尺寸转换的例子。组件模型的使用如图5.1所示。通过功能转换模块形式开发的介绍19，从组件工艺规划应用的角度来看，是槽，盲孔，孔和盲槽功能等基本特征的复合功用。功能属性（非几何特征），尤其是模型的几何尺寸，需要转换，以正确描述这些加工特点。在转换算法应用到组件模型之前，几何尺寸被提取并在表5.1中列出。研究发现，一些边界元素的尺寸是边缘元素并且这些都需要进行转换。转换后，所有与边缘边界元素相关的维度被转换为面边界元素的尺寸（如表5.2所示）。图5.1 组件几何尺寸转换的例子表5.1 功能转换前的尺寸边界表5.2 功能转换后的尺寸边界6结论特征技术已成为应用于CAD/ ACPP/ CAM一体化方面重要的研究课题。由于特征的呈现因应用程序的不同而不同，现在主要未解决的问题是基于特征组件模型功能设计的完全解释，特别是关于功能相互作用方面的问题。根据建模过程中设计功能的调查和分析，发现最终的组件模型和数据库提供的信息之间的不一致是后续应用要解决的关键问题。为了用大量的加工特征正确地表示组件模型，本文着重对非几何特征的提取和转换，也就是，几何尺寸和公差（GDT）给予了论述，从而在过程规划建模过程中起到了关键的作用。在本文中，详细介绍了GDT的功能转换算法的开发过程。由于设计特点和加工特点之间的不同表述，一种算法的开发是为了维度转换的边界元素去识别过程建模元素。在特征交互的情况下，由于在数据库中的功能信息不发生相应的变化，该算法实施检测特征尺寸是否符合特征模型并将那些虚拟尺寸转换为能正确地描述加工功能的实际尺寸。对于形位公差转换，主要的任务是要找到每个组件功能相应的公差，并将设计模型中的虚拟数据元素转换成加工应用中能识别的元素。结合17中几何信息转换的描述，加工特征可以生成符合工艺规划系统要求的服务条款。通过STEP文件的接口，基于CAD / CAPP集成特征的功能可以实现18。基于Pro / Engineer系统特征可以用于建立设计模型，以及通过Pro/Develop工具在Sun SPARC20工作站写入C + +实现功能转换算法。参考文献1 Shah JJ,Mantyla M.基于CAD/CAM的参数和特征：概念，技术和应用.纽约：威利出版社，1995.2 Liu X.CFACA：基于设计和处理规划特征的框架组件.计算机辅助设计32期,2000：397-408.3 Han J, Requicha A.CAD模型的特征识别.IEEE Comput Graphs Appl 18，1998，(2):80-94.4 Gindy N, Yue Y, Zhu CF.基于组件数据模型的创建/编辑功能的自动功能验证.研究制品36期，1998，（9）：2479-2495.5 Wang H-F, Zhang Y-L，协同开发环境下的CAD / CAM集成系统.Robot Comput Integr Manuf 18,2002:135-145.6 Han J.关于特征识别3D几何推理算法的论文.洛杉矶:美国南加州大学出版社，1996.7 Yang MH.通过局部约束满足识别加工的功能.在IEEE国际会议上关于系统、人与控制论的议程，1997：12-15.8 Bidarra R, Kraker KJ, Bronsvoort WF.单元模型中功能信息的表征和管理.计算机辅助设计30期，1998，（4）：301-313.9 Narayan GA, Nalluri SRPR, Gurumoorthy B.基于几何推理功能的工艺规划.Sadhana 22,1999,(2):217240.10Lee JY, Kim K.加工功能产生的的另一种解读.Int J Adv Manuf Technol 15,1999:38-48.11Gao J, Zheng DT, Sun J.域在设计和制造之间数学模型的功能转换研究.Chin J Mech Eng 8,1998:411.12Zheng DT.对组件从设计域到制造域特征映射的研究.中国清华大学论文,1997.13Chen YM,Wen CC,Ho CT.几何特征提取的可制造性评估.Robot Comput Integr Manuf 19,2003,(4):371385.14Yan X,Yamazaki K,Liu J.NC程序加工特征识别和拓扑功能的识别.计算机辅助设计32期，2000:605-616.15Tseng Y,Joshi SB.相互作用的加工特征的多重解读.计算机辅助设计26期，1994，(9)：667-688.16Suh YS.一致工程环境下CAD系统的转换功能.纽约州特洛伊市伦斯勒理工学院论文，1995.17Shah JJ,Yan Y,Zhang BC.基于尺寸及公差的建模并转换的设计和制造功能.J Intell Manuf 9,1998:475488.18Gao J,Zheng DT,Sun J.功能转换与产品工艺信息模型代表性的研究.香港制造业自动化国际会议，1997:28-30.19Gao J,Zheng DT,Gindy N.CAD/CAM一体化加工特征的提取.Int J Adv Manuf Technol,DOI:10.1007/s00170-003-1882-9.DOI 10.1007/s00170-004-2195-3ORIGINAL ARTICLEInt J Adv Manuf Technol (2005) 26: 405414Jian Gao De Tao Zheng Nabil Gindy Doug ClarkExtraction/conversion of geometric dimensions and tolerances for machiningfeaturesReceived: 28 January 2004 / Accepted: 31 March 2004 / Published online: 28 July 2004 Springer-Verlag London Limited 2004Abstract It is important for a feature-based system to preservefeature integrity during feature operation, especially when fea-ture interaction occurs. The paper presents a feature conversionapproach to convert design features used in a design model intomachining features for the downstream applications. This pro-cess includes both form features (geometric information) andnon-geometric features conversion. Most researchers have con-centrated on geometric information extraction and conversionwithout tackling the important problem of non-geometric featureinformation. This paper focuses on the extraction and conver-sion of feature geometric dimensions and tolerances (GD&T) fordownstream machining application.The main barrier to the integration of a feature-based CAD/CAPP/CAM system feature interaction is discussed in thispaper, which alters design features in their geometries and non-geometric information. How toidentify andvalidate these featuredimensions andtolerances isoneofthekeyissuesinfeatureinter-action conversion. The development of robust methodologies forpreserving feature integrity for use in process planning applica-tionis the main thrust ofthe work reported inthis paper.Keywords CAD/CAPP/CAM integration Featureconversion Feature interaction Geometric dimensionsand tolerances (GD&T) Machining featuresJ. Gao (u) N. GindySchool of Mechanical, Materials, Manufacturing Engineeringand Management,The University of Nottingham,University Park, Nottingham NG7 2RD, UKE-mail: Jian.GaoNottingham.ac.ukTel.: +44-115-951-4116Fax: +44-115-951-3800J. Gao D.T. ZhengSchool of Mechanical and Electronic Engineering,Guangdong University of Technology,729 East Dongfeng Road, 510090 Guangzhou, P.R. ChinaD. ClarkSchool of Mathematical and Computer Sciences,Heriot-Watt University,Riccarton, Edinburgh EH14 4AS, UK1 IntroductionA main objective in modern design and advanced machining pro-cess is to achieve a significant reduction in product lead timethrough seamless integration between the various design andprocess planning activities. The concept of using component fea-tures to integrate a design system and a manufacturing systemhas been a major research direction in recent years 15.In general, features are used to give meaning to compon-ent attributes, helping in dissecting component geometry intorecognisable, meaningful regions, and improving communica-tion between design and manufacture. Feature recognition andfeature-based design are two major approaches to using thefeatures. Feature recognition drives form feature informationfrom a geometric database where form features are recognized,extracted and categorized using computer algorithms. Featurerecognition has problems associated with the loss of design in-tent, incompleteness of design and manufacturing informationand the complexity and computational load of feature recogni-tion programs 3,6,7. Feature-based design involves defininga component through the use of “building blocks,” which repre-sent either design features or machining features. Since featuresare process dependent, different applications require differentsets of features. The features used in the design stage (calleddesign features) are considered to be functional features, repre-senting design intents, such as rib, slot and step features. Thefeatures used in the process planning stage (called machiningfeatures) are associated with machining operations, such as hole,blind-hole, slot, and pocket features. In addition, feature repre-sentations vary from application to application. These facts arethe main barriers to correctly representing the machining fea-tures as required, and to realise CAD, CAPP and CAM systemintegration in a feature-based environment.It is most reasonable for the designer to use design featuresto construct a component feature model and then convert themodel in terms of machining features for use in a process plan-ning system. This process is called feature conversion or featuremapping. Methods and algorithms for feature conversion have406been studied and applied by many researchers 810. Never-theless, the majority of contemporary feature-based design isstill incapable of fully handling one of the most important andperhaps the most difficult aspects in the development of feature-based systems, that is,the definition and representation of featureinteractions. Feature interaction changes the geometries and at-tributes of the features previously used, but it does not changethem accordingly in the database, which makes the downstreamapplication incapable of obtaining the real component modelfrom the design database. Therefore, preserving feature integrityduring the design and machining process phases is one of the keyissues in realising the feature-based CAD/CAPP/CAM integra-tion 1113. Much research has been conducted into techniquesfor extracting machining features from a design feature-basedsystem 5,7,9,10,1416. However, such research has mainlyfocused on the geometric information recognition or conversion.Little has been done in the aspect of non-geometric informationof machine features, such as geometric dimensions and toler-ances (GD&T). Shah proposed an approach in 17 to createa dimension and tolerance model for use both in design andprocess planning applications. This GD&T model was used tocapture the designers GD&T scheme on a feature-based designmodel, validate its completeness, and then transfer the GD&T tomachining features extracted automatically by feature recogni-tion methods. The method focused on how to model effectivelythe dimensions of a part both for use in the design and manu-facturing stages. It is therefore a complex and difficult task toconstruct robustly such a GD&T model suitable for CAD andCAPP systems. For a commercial feature-based system (such asPro/Engineer), a GD&T model was constructed once a compon-ent modelling process was finished. To make use of the CADmodel created, the objective is to capture and convert what a pro-cess planning system needs in geometry and non-geometric in-formation and to realise CAD/CAPP seamless integration.To achieve a complete representation for a process-planningmodel, a component GD&T needs to be complete and correctlyrepresented according to the requirements of the machining fea-tures. The work presented in this paper deals with the extrac-tion/conversion of the GD&T from a feature-based model toFig.1. Framework of the integrated feature-based CAD/CAMsystempreserve the integrity and consistency of a machining feature-based process model.The framework of the proposed feature-based CAD/CAPPintegration system through a feature conversion module anda STEP file interface is introduced in Sect. 2. In Sect. 3 we dis-cuss the difficult issues concerning feature interactions that causefeature information alterations and do not change them in thedatabase. Machining features can be generated through the ex-traction and conversion of the feature information from a designmodel. In Sect. 4 we introduce the algorithms developed for theextraction and conversion of feature GD&T. A component ex-ample is discussed in detail in Sect. 5 to show the results of thegeometric dimension conversion. In the final section, the conclu-sions of the paper are given.2 Feature-based CAD/CAPP integration systemstructureIn a feature-based design environment, a component model isconstructed from a series of design features, such as protru-sions, slots, holes, etc. Adownstream application such as processplanning has a different view of a CAD model, it uses a se-ries of machining features that correspond to machining oper-ations. To implement the integration of the CAD/CAPP/CAMsystem, extraction and conversion of the feature geometric andnon-geometric information for the representation of machiningfeatures is proposed. The framework of the integrated systemthrough a feature conversion approach is shown in Fig. 1. In thisapproach, feature-based CAD/CAMsoftware Pro/Engineer isused to create a component feature model. Through the featureconversion module and a STEP file interface, the correspond-ing component process-planning model can be created to testand verify the converted machining features, which is discussedin 18.In the feature conversion module, the form feature mappingis developed to convert the geometric information in the CADdatabase into the meaningful “shapes” to the machining process,and the GD&T mapping is used to create a complete and correctrepresentation for the converted machining feature attributes.4073 Feature interactionsIn general, feature interaction is said to have occurred if it causesany change in feature class or feature attributes of the originalfeatures involved. In a feature-based design system, a compon-ent model is built up by a series of design features by theway of one feature modifying another. Figure 2 shows a com-ponent modelling process that involves the feature interactionof a blind-slot1 and a blind-slot2 features. When the interac-tion occurs, the previously used blind-slot1 and blind-slot2 fea-tures are changed to through-slot3 and blind-slot4 features withthe possible alterations in feature dimensions and geometricelements.Feature interactions have a wide range of effects to a com-ponent model because they not only change the predefined fea-ture geometries but also alter their attributes. The problem isthat these alternatives are not made available in the componentdatabase, which causes difficulties for the downstream processplanning application. In Fig. 3, both geometries and dimensionsFig.2. An example of feature interaction between two blind-slot features, producing a through-slot feature and a blind-slotfeature with different attributes or geometryFig.3. Inconsistency of feature type anddimensions caused by the interaction ofa pocket feature and a step featureof the previously used pocket feature were changed due to the in-teraction of the step feature. The depth dimension of the pocketfeature in Fig. 3b was changed to h1, instead of the original di-mension h, and the pocket feature in Fig. 3c was changed toblind-slot feature, and the edge dimension of the newly formedfeature has been altered to a1instead of the previous dimen-sion a. However, the component database only records the orig-inally used features, and does not contain the modelling results.For the example in Fig. 3 the original pocket feature with dimen-sions h and a is what we can obtain from the database for the twomodels in Fig. 3b,c.In order to preserve the feature model integrity it is necessarytodevelop a robust feature extraction/conversion algorithm tode-tect inconsistent feature representations and convert them intothe correct representations required by the machining featuresbased processing planning system. The detection and conversionalgorithms of design features to machining features in the ge-ometric aspect were introduced in 17, here we focus on theextraction/conversion of feature GD&T in order to correctly rep-resent the converted machining features.4084 Extractionof feature geometric dimensions andtolerances4.1 Extraction of feature geometric dimensionsIn a feature-based CAD system, the geometric dimensions rep-resentation of a component and a feature in the database containthe following items:1. Dimension value and tolerance.2. Type of dimension, such as linear dimension, angle dimen-sion and diameter dimension.3. Boundary elements of the dimension: a face, an axis and anedge. For instance, the boundary elements of the dimensiond4(shown in Fig. 4) are the face f1and the edge e1, and theboundary elements of the dimension d5are the face f2andthe edge e2.Besides items 1 and 2 listed above, a process planning systemhas a different dimension boundary representation from a CADmodel, which needs to describe the dimensions by a meaningfulgeometric element to machining operations, such as a face or anaxis element, not an edge element. Since an edge is formed bytwo face elements, it is somehow ambiguous for the use of a ma-chining process modelling. For example, the dimension “d” inFig. 4 isdescribed by the length |d|, and the twoedge elements ofe1, e2. As required by a machining process modelling, it shouldbe described by the boundary elements of face f5and face f8.Because of the different dimension representations betweenthe design features and machining features, it is necessary to dis-tinguish and transform a dimension boundary element edge ina CAD model to its corresponding face element for process mod-elling. The algorithm is implemented by the following steps asone of the modules in the GD&T mapping module.Step 1: Searching each dimension dim(i) on a component, leti = 1,2,.,N_GD, where N_GD is the total number of geomet-ric dimensions of a component.Step 2: Let i = 1, extracting the value, type, and boundaryelement identifications id1, id2of the dimension dim(i). Step 3:Checking the boundary elements of the dimension dim(i). If oneof the boundary elements isnt a real element, then the bound-ary of the dimension doesnt exist after feature operation. ThisFig.4. Feature interaction and dimensionboundary elementsmeans the dimension boundary was changed. In this case, an ad-ditional algorithm is needed to deal with the interactive features,which will be discussed in the next section. If both boundaryelements are real, then the types of the dimension boundary canbe determined directly.Step 4: If one of the dimension boundary elements is an edgee0, and the other is a face f0, then one of the two faces f1and f2,whichformtheedgee0mustbedetermined(gotostep5)andusedas the dimension boundary element of the machining feature. Ifbothboundaryelementsofthedimensionareedgese1ande2,thengo to step 6 to convert both edges into faces as the dimensionstwo boundary elements. If two boundary elements of the dimen-sion dim(i) are two faces or two axes, or one face and one axis,thentheiridentificationsarerecordedinanarray,thengotostep7;Step 5: Comparing the real value of a dimension with the dis-tance d01and d02, which represents the distance between facef0and f1and face f0and f2, respectively. If the distance d01is equal to the Value(dim(i), then substitute the boundary iden-tification id1of e0with the identification of f1, which means therepresentation of the edge element has been changed to the rep-resentation of the face element. If the distance d02is equal toValue(dim(i), then substitute identification id1of edge e0withthe one of face f2and go to step 7.Step 6: Comparing the real value of a dimension with the dis-tances d11, d12, d21, d22(dijrepresents the distance between facef1iand face f2j, and i, j = 1,2), there must exist a distance thathas the value of dim(i). If d21is equal to the Value(dim(i), thensubstituting dimension d21s two boundary identifications withthose of face f12and f21, and go to step 7;Step 7: If i datum number N, then end; otherwise, seti =i+1,go back to step 2By means of the above algorithm, the co-planar array storesthe datum elements corresponding to the real feature elements.The algorithm flowchart is shown in Fig. 8. Combining with thetwo-stage conversion algorithms, the positional geometric toler-ances can be fully converted.413Table2. Dimension boundary after feature conversionFeature idDimDimension typeDimension boundaryDimension boundarysignelement1element1#id_feat#dLINANGDIA/RADSRFAXISEDOSRFAXISEDO#1dOYes#14#18basedlYes#16#12featured2Yes# 7#2#20d3Yes#37#33slotd4Yes#16#35asYes#33#18asYes#16#68#59d9Yes#71#73blind-holedlOYes# 7#76dl 1Yes#18#76#81dl JYes#93#95holed!4Yes# 7#98dlSYes#33#98d29Yes#207#203#190d30Yes#16#205blind-slotd31Yes#207# 2d34Yes#198#335 A component exampleAn example of geometric dimension conversion to a prismaticcomponent is presented in this section. The component modelused is shown in Fig. 9. Through the form feature conversionmodule developed and introduced in 19, the component in theviewpoint of process planning application is a compound of thebase feature, slot, blind-hole, hole and blind-slot features. Fea-ture attributes (non-geometric feature), especially the geometricdimensions of the model, need to be converted so as to de-scribe correctly these machining features. Before the conversionalgorithm is applied to the component model, the geometric di-mensions are extracted and listed in Table 1. It is found that someboundary elements of the dimensions are edge elements andthese need to be converted. After the conversion, all dimensionswith edge boundary elements are converted into the dimensionswith face boundary elements (shown in Table 2).6 ConclusionsFeature technology has been an important research topic used forCAD/ACPP/CAM integration. Due to the variety of feature rep-resentations from application to application, the main unsolvedproblem now is to completely explain the design-feature-basedcomponent model, in particular regarding feature interactions.Based on the investigation and analysis of the design featuremodelling process, it was found that the inconsistency betweenthe final component model and the information available indatabase is the key issue to be solved for downstream applica-tions. In order to correctly represent the component model bya number of machining features, this paper focuses on the ex-traction and conversion of the non-geometric features, that is, thegeometric dimensions and tolerances (GD&T), which play a keyrole in the process planning modelling process.In this paper, GD&T feature conversion algorithms were de-veloped and described in detail. Due to the different represen-tations between the design features and the machining featuresan algorithm was developed to convert the dimension bound-ary elements to recognisable elements for process modelling. Inthe situation of feature interactions, since the feature informa-tion in the database does not change accordingly, the algorithmwas implemented to detect whether the feature dimensions areconsistent with the feature model and to convert those virtual di-mensions into the real dimensions that correctly describe the ma-chining features. For geometric tolerance conversion, the maintask is to locate each tolerance of the component to the corres-ponding feature, and convert the virtual datum elements in thedesign model into recognisable elements for the machining ap-plication.Combined with the geometric information conversion de-scribed in 17, the machining features can be generated in termsof the process-planning system requirements. Through the inter-face of the STEP file, the feature-based CAD/CAPP integrationcan be implemented 18. The feature-based Pro/Engineer sys-tem was used to build the design model, and the featur