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166 Reverse Engineering – Recent Advances and Applications such as (Thompson, 1999). One promising development in recent years was the geometric feature recognition (GFR), which automatically recognizes solid features embedded in the Brep models. However, none of the method is able to fully automate the construction process and generate fully parametric solid models. Some level of manual work is expected. 3.3.1 Boundary representation Based on segmented regions (with fitted surfaces), a region adjacent graph is built. This graph reflects the complete topology and serves as the basis for building the final B-rep model, also called stitched models, where the individual bounded surfaces are glued together along their common edges to form an air-tight surface model. In general, there are three steps involved in constructing B-rep models, flattening, edges and vertices calculations, and stitching (Várady et al., 1998). In flattening step, regions are extended outwards until all triangles have been classified. Note that this step is necessary to remove all gaps between regions. Sharp edges can be calculated using surface-surface intersection routines, and vertices where three surfaces meet are also determined. During the process, a complete B-rep topology tree is also constructed. A B-rep model can then be created by stitching together the faces, edges, and vertices. This operation is commonly supported by most solid modeling kernels. 3.3.2 Solid feature recognition B-rep models are not feature-based. In order to convert a B-rep model into a feature-based solid model, the embedded solid features must be recognized, and a feature tree that describes the sequence of feature creation must be created. One of the most successful algorithms for geometric feature recognition has been proposed by (Venkataraman et al., 2001). The algorithm uses a simple four step process, (1) simplify imported faces, (2) analyze faces for specific feature geometry, (3) remove recognized feature and update model; and (4) return to Step 2 until all features are recognized. The process is illustrated in Fig. 6. Once all possible features are recognized, they are mapped to a new solid model of the part (Fig. 6d) that is parametric with a feature tree. This feature tree defines the feature regeneration (or model rebuild) sequence. Fig. 6. Illustration of GFR algorithm, (a) imported surface model with hole surface selected, (b) hole recognized and removed, extruded face of cylinder selected, (c) cylindrical extrusions recognized, base block extrusion face selected, and (d) all features recognized and mapped to solid model Venkataraman’s method was recently commercialized by Geometric Software Solutions, Ltd. (GSSL), and implemented in a number of CAD packages, including SolidWorks and
A Review on Shape Engineering and Design Parameterization in Reverse Engineering CATIA, capable of recognizing basic features, such as extrude, revolve, and more recently, sweep. This capability has been applied primarily for support of solid model interchanges between CAD packages with some success, in which not only geometric entities (as has been done by IGES�Initial Graphics Exchange Standards) but also parametric features are translated. One of the major issues revealed in commercial GFR software is design intent recovery. For example, the flange of an airplane tubing would be recognized as a single revolve feature, where a profile sketch is revolved about an axis (Fig. 7a). However, current GFR implementations are not flexible. As shown in Fig. 7b, without adequate user interaction, the single sketch flange may be recognized as four or more separate features. While the final solid parts are physically the same, their defining parameters are not. Such a batch mode implementation may not be desirable in recovering meaningful design intents. Fig. 7. Feature recognition for airplane tubing flange, (a) single revolved feature, and (b) four features: revolve, extrude, cut, and fillet 3.3.3 Design parameterization A feature-based parametric solid model consists of two key elements: a feature tree, and fully parameterized sketches employed for protruding solid features. A fully parameterized sketch implies that the sketch profile is fully constrained and dimensioned, so that a change in dimension value yields a rebuilt in accordance with design intents as anticipated. To the author’s knowledge, there is no such method proposed or offered that fully automates the process. Some capabilities are offered by commercial tools, such as Rapidform, that support designers to interactively create fully parameterized sketches, which accurately conform to the data points and greatly facilitates the solid modeling effort. 3.4 Solid model export Since most of the promising shape engineering capabilities are not offered in CAD packages (more details in the next section), the solid models constructed in reverse engineering software will have to be exported to mainstream CAD packages in order to support common engineering assignments. The conventional solid model exchanges via standards, such IGES or STEP AP (application protocols), are inadequate since parametric information, including solid features, feature tree, sketch constraints and dimensions, are completely lost through the exchanges. Although feature recognition capability offers some relief in recognizing geometric features embedded in B-rep models, it is still an additional step that is often labor intensive. Direct solid model export has been offered in some software, such as liveTransfer module of Rapidform XOR3 as well as third party software, such as TransMagic. More will be discussed for liveTransfer. 167
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REVERSE ENGINEERING - RECENT ADVANC
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Contents Preface IX Part 1 Software
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Preface Introduction In the recent
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Preface XI and con’s related to t
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Preface XIII the step-by-step appli
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Part 1 Software Reverse Engineering
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4 Reverse Engineering - Recent Adva
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10 Reverse Engineering - Recent Adv
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58 2. Model driven architecture: An
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62 Fig. 1. Model, metamodels and tr
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100 Reverse Engineering - Recent Ad
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108 Table 1. Number of smoothers an
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238 5. Summary and discussions Reve
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268 Fig. 6. A closed polygon mesh r
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272 3. Results Reverse Engineering
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