Throughout the course of the development of CAD, CAPP, and CAM systems, unambiguous representation of a design’s geometry and topology remain an essential part of the task. Since the mid-1990’s, the technology has matured enough to enable such a representation. While geometry and topology provides a basic description of a design part, direct use of it for creation of the part and other applications, can be cumbersome. Take creation of a simple plane with four straight edges as an example. For a B-rep model to fully define the plane, four points are to be created first to be used as four vertices. They are used to define four edges, which are connected one after another to form a closed loop. Finally, a flat surface is fitted onto the loop to form the plane. When a cube is to be designed, the above process needs to be repeated five more times for the other five faces though some of the vertices and edges may be re-used. In addition, the directions of the solid have to be defined through each face. Clearly, this is not a trivial task. Users would find it helpful if the creation of geometry and topology is hidden behind them and only some meaningful parameters of the solid are provided. In the case of a simple cube, length, width, and depth would be the parameters. Hence, the concept of features (i.e. cube or block in the above example) emerged, as did the associated technologies. The same applies for other domains, such as manufacturing and engineering analysis. This chapter aims to give a succinct introduction to various feature technologies such as feature defintions, feature taxonomy, feature representation schemes, and feature-based methodologies. Several important issues are highlighted. These include the application-dependent nature of features, and surface features versus volumetric features.
TopFeature Definition
Features can be thought of as ‘engineering primitives’ suited for some engineering tasks. They originate in the reasoning processes used in various design, analysis and manufacturing activities, and are therefore often strongly associated with particular application domains. This explains why there are many different definitions for features. According to Shah and Mäntylä (1995), a feature should have,
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a physical constituent of a part;
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a generic shape that can be mapped to;
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engineering significance; and
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predictable properties.
In the context of CAD/CAPP/CAM, several more specific definitions have been suggested. One of such examples is,
A feature is referred to as a distinctive or characteristic part of a workpiece, defining a geometrical shape, which is either specific for a machining process or can be used for fixturing and/or measuring purposes (Erve & Kals, 1986).
Of particular relevance in this chapter are the machining features, which are normally related to machining methods or machining operations. Thus, a machining feature can be defined as
A portion of the workpiece generated by a certain mode of metal cutting. (Choi, Barash & Anderson, 1984), or
Feature information can be considered to be about volumes of material to be removed (Anderson, 1990).
A component with four machining features is shown in Figure 1, where the slot may require an end mill, the holes may require drilling operations and the pocket may require a slotting cutter or a slotting cutter and an end mill.
Figure 1. A component with four machining features. ©2001, Engineers Australia used with permission.
When dealing with features, there is a further complication: viewpoint dependence. That is, depending on the application domain, one could have different views towards the same, or combination of, feature(s) on a part. When a part is designed by features, the resulting model is not usually in a form convenient for other applications such as manufacturing process planning. Indeed, design features are stereotypical shapes related to a part’s function, its design intent, or the model construction methodology, whereas manufacturing features are stereotypical shapes that can be made by typical manufacturing operations (Shen & Shah, 1994). To this context, design features may also be called function features. Figure 2 illustrates a part whose different geometrical entities may be of interest for different applications.
Shah and Mäntylä (1995) distinguish between various types of features by using a sub-classification of features such as form features, tolerance features, assembly features, functional features and material features. Form features, tolerance features and assembly features are all closely related to the geometry of a part, and are therefore called geometric features.