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extraction of machining features for cad

By Beverly Peters,2014-10-18 02:25
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extraction of machining features for cad

    Extraction of machining features for CAD/CAM integration

    J.Gao?D.T.Zheng?N.Gindy

    Abstract Feature-based CAD/CAM integration is a technology used to realize automatic transmission and conversion of component information among CAD, CAPP and CAM applications. A feature is the medium of information transmission in the integration. Since a process planning downstream application has a different viewpoint from the component designer, feature conversion or feature extraction methodology is used to create a machining feature model based on the design feature model. One of the major difficulties in generating machining features is the preservation of feature integrity because of feature interactivity. The most current research, therefore, focuses on the planar-type form feature conversion. This paper discusses the problem of feature interactivity and proposes a feature-based approach to generating hole-series machining features from a design feature model. Hole-series features are important machining features for gearbox components used in machine tools. These kinds of features cannot be obtained directly from a design feature atabase. A constraint-based method is developed in this paper to define a hole-series feature model based on the geometric and topological information extracted from the design database. In addition, a STEP file is generated to interface the converted machining feature model with the downstream CAPP application. The implemented feature conversion system is verified by considering its application to some component examples and the developed prototype CAPP system.

    Keywords CAD/CAM integration ? Constraint ? Feature

    conversion ? Feature interaction ? Fundamental feature ? STEP interface

1 Introduction

    Feature-based design and manufacturing systems have been widely used in mechanical component CAD/CAM systems. Feature-based CAD/CAM integration is a technology used to realise automatic transmission and conversion of component information for CAD, CAPP and CAM [14]. A feature is the medium

    of information transmission in the integration. It carries a mass of engineering information, geometrical and non-geometrical, and organically links each system of CAD, CAPP and CAM. However, features definitions vary from different viewpoints. Design features are form features related to a component’s function, design intent, or model construction methodology. Machining features are form features associated with distinctive machining operations. Some of the current computer-aided design (CAD) systems such as Pro/Engineer, are feature-based. Feature-based design means that components are designed by starting with a simple extrusion or revolution (such as a cylinder or a block), and then modified these by attaching features such as holes, slots, etc., to the various faces of the existing design. The designer specifies the position, size and orientation of these features [5]. Depending on the downstream machining application, it might be necessary to convert design features into machining features through feature conversion or feature extraction. Feature extraction defines machining features from a geometry-based design model, whereas feature conversion converts machining features from a feature-based design model [610].A

    set of extracted machining features is often called a machining feature model. Usually, a component can be represented by more than one machining feature model. These feature sets correspond to different manufacturing methodologies to machine the component [6, 8, 11, 12]. A cell-based

    decomposition approach was proposed to generate machining models in [11, 12]. The volume to be removed (delta volume) is decomposed into cells by extending and intersecting all of its surfaces or half space. A subset of these cells is then combined into a machining feature. In this way, cell composition is repeated until all the cells of the delta volume are consumed. As a result of cell composition, the delta volume is completely decomposed into a set of machining features. Alternative machining features can be generated by changing the composition sequence of the cells. Han [8] proposed a procedure for generating a machining feature model on request from a process planner. However, this approach focuses on features with planar cells or no-cylindrical volumes and cannot be used in complex components, such as gearbox components where coaxial hole features are important machining features [9].

     1

    Figure 1 shows two typical components used in machine tool manufacturing in the Guangzhou Machine Tool Manufacturing Factory (GMTMF). Feature conversion or feature extraction on hole-series type components is new and, so far, it hasn’t been reported. This paper focuses on gearbox components and develops a feature mapping approach to generate the coaxial hole-series machining features based on the design feature model. Section 2 introduces the general CAD/CAM integration strategy through feature conversion and the difficulty caused by feature interaction. Section 3

    gives the definition of a fundamental feature used for feature conversion, and Sect. 4 introduces the constraint relationship and hole-series machining feature description. In Sect. 5 we give the conversion algorithm for the hole-series machining features and design a STEP file interface to integrate the CAD and CAPP system. Finally, conclusions close the paper.

2 Feature conversion

    In this paper, a feature-based conversion approach is proposed to generate machining features from a design feature model. The approach defines the machining feature model in the machining viewpoint and the design feature information is extracted and maintained during feature-based modelling and machining feature conversion.

     2

    As illustrated in Fig. 2, the proposed CAD/CAM integration system through feature conversion includes a form feature conversion module, dimension feature conversion and feature attribute conversion.

3 Fundamental feature definition

    To implement the constraint-based hole-series feature conversion, an appropriate definition of the fundamental features is introduced. A fundamental feature is defined as an elementary geometrical shape with some characteristics which includes geometrical and topological information. A fundamental feature can be expressed by a mathematical set Fb = {T,G,C}

4 Constraint-based hole-series feature description

5 Implementation of the hole-series feature conversion

6 Conclusion

    In this paper, a feature-based CAD/CAM integration system is proposed and the difficulties for the integration are discussed, which lie in the different feature definition for different application and feature interaction and cause not only geometrical information changes, but also non-geometrical attribute changes.Therefore, feature conversion contains every aspect feature about the component, not only geometrical features, but also nongeometrical features, such as dimension features and material features. We have focused in this paper on geometrical features, that is the hole-series feature conversion for gearbox components and developed an approach to convert the design feature model into the machining feature model, in order to realise the integration of feature-based CAD and CAPP activities. This paper presented a feature conversion algorithm for coaxial hole-series machining features based on the design feature model. The converted machining feature model can be transferred to a CAPP downstream application through the developed STEP file interface. Combined with our planar-type form feature conversion and dimension and attributes feature conversion algorithms, the complete

    machining feature model can be generated from the design feature model and can be transferred directly to a CAPP system through the STEP file. This work was implemented in the Pro/Engineer environment on a Sun Sparc 20 workstation, using the Pro/Develop tool.

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