1956_数控工作台三维造型设计及关键零部件工艺设计
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1956_数控工作台三维造型设计及关键零部件工艺设计,1956,数控,工作台,三维,造型,设计,关键,零部件,工艺
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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 feature conver-sion algorithm was implemented through the Pro/Develop toolwritten in C+ on a Sun Sparc 20 workstation.References1. Shah JJ, Mantyla M (1995) Parametric and feature-based CAD/CAM:concepts, techniques, and applications. Wiley, New York2. Liu X (2000) CFACA: component framework for feature-based designand processing planning. Comput-Aided Des 32:3974084143. Han J, Requicha A (1998) Feature recognition from CAD models. IEEEComput Graphs Appl 18(2):80944. Gindy N, Yue Y, Zhu CF (1998) Automated feature validation for cre-ating/ editing feature-based component data models. Int J Prod Res36(9):247924955. Wang H-F, Zhang Y-L (2002) CAD
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