Feature-Based Design
ME 459 Advanced Topics in Design and Manufacturing
(Extract from online paper by Otto Willem Salomons )
Features can be viewed upon as information sets that refer to aspects of form or other attributes of a part, in such a way that these sets can be used in reasoning about design, performance and manufacture of the part or the assemblies they constitute. Figure 3.1 shows some examples of features occurring on some different parts.
Feature technology, is expected to be able to provide for an adequate basis for the integration of design and the subsequent applications such as engineering analysis, process planning, machining and inspection. Over the last few years, a vast number of papers and other publications on feature technology has come forward. The most important aspects dealt with in these papers are summarized in the remainder of this chapter and in Appendix A. The focus is on feature based design, feature based manufacturing and CAD/CAPP integration. Other overviews of feature technology are provided in [Case 93], [Catania 91], [Roller 89], [Ruf 91], [Salomons 93a], [Shah 91c], and [Wierda 91]. Currently, three main views are discerned on how to obtain application features, such as manufacturing features, analysis and inspection features, from a product model (e.g. [Joshi 90], [Shah 90b] and [Houten 91]):
In feature recognition, application features are automatically or interactively recognized from a model of the object under consideration. Product models from both solid modellers and feature based modellers can be subjected to feature recognition (see figure 3.2). Reviews on feature recognition are provided in references [Joshi 90] and [Shah 90b]. Feature recognition is mainly used in CAPP systems; for this reason feature recognition has been allocated to the CAPP environment of figure 3.2. In general, however, feature recognition may also be part of CAD functionality as will become clear shortly. Appendix A.2 further elaborates on feature recognition.
A product model can be built by using (design) features; this is known as design by features or feature based modelling. One can start either with a more or less complete geometric model and define form features on it, or one starts from scratch by combining form features from a standard library. Design with pre-defined form features can reduce the number of input commands substantially. This is especially advantageous in re-design. The parametric representation of features provides a powerful way to change features with respect to their dimensions. Features can serve as functional elements to designers. Design features often differ from "downstream" application features. However downstream application features that are considered in this thesis are manufacturing features as used in process planning. The possible cases of obtaining manufacturing features in a CAPP system are indicated in figure 3.2. The features are either similar to the "downstream" application features, in which case no special actions are required, or the design features differ from the manufacturing features in which case feature mapping or feature recognition are necessary. In figure 3.2 the mapping functionality has been identified as part of the CAD functionality while feature recognition has been identified as part of CAPP functionality. This has historic reasons. Initially (academic) feature based design systems were equipped with mapping functionality for linkage with downstream applications. Process planning systems were equipped with feature recognition modules to be able to transform CAD models into appropriate chunks of manufacturing information. Feature mapping need not necessarily be part of feature based design systems and equally feature recognition and feature identification need not necessarily be part of process planning systems. The discrepancy between the different types of features is detailed further in section 3.5 and in Appendix A.4.5. An overview of feature based design is provided in section 3.3.
In this approach, features are "defined" interactively. Most often, this is carried out by identifying the faces belonging to a certain feature on the product model that is under consideration. The feature that has been identified in this way, often is not generic in the sense that after having been identified, it can automatically be recognized from another product model. Therefore, in this thesis, the term feature identification is used in two different contexts. It can be used for the process of identifying the elements of a feature as a part of a product model without the possibility to reuse this information on other product models. It can also be used as a part of more generic feature definition. Feature definition is defined here as the definition of generic features, which can be used over and over again, either in CAD or in CAPP or even in both CAD and CAPP. Therefore, when speaking of feature identification, the context is to be mentioned. Feature definition and feature identification in the generic sense are dealt with in Appendix A.4.2 and in the following chapters. In figure 3.2 feature identification can be regarded as specific. Otherwise, a feedback link with the feature recognition functionality must be present.
Presently, the first two views - feature recognition and design by features - prevail. In [Husbands 91] the belief is advocated that feature recognition and feature based design alone are not sufficient to fulfil the requirements of CAD/CAPP integration. It is presently believed that future CAD/CAPP systems should support both feature recognition and design by features (e.g. [Houten 91], [CAM-I 90], [Wing#rd 91], [Wang 91a] and [De Martino 94a,b]). M#ntyl# and Laakko presented a hybrid feature recognition/feature based design system [Laakko 93]. This system is based on the belief that not all shapes can be represented by features. Thus, in addition to feature based modelling, conventional CAD techniques are required. This CAD-based geometry should then be subjected to feature recognition in order to arrive at a complete feature based model at the end of the design process. This seems like a contradiction: admitting the fact that not all shapes can be represented by features, but at the same time aiming at a complete feature based model. In [Sreevalsan 91] it is even advocated that all three methods should be integrated in a unified approach.
In comparison with the other two approaches, feature based design has the advantage of offering the possibility of capture and storage of relevant information during design which can be used for applications that take place after the design process has finished. Feature based design also offers the possibility for considering manufacturing and assembly aspects early in the design process. When using only feature recognition or interactive feature definition, this is not possible. Thus, feature based design seems a promising method supporting CAD/CAPP integration in a CE approach. However, feature based design systems not only need to communicate with feature based CAPP systems, but also need to receive feedback information from CAPP systems. These feedback information flows are not shown in figure 3.2, but are addressed later.
