ComputerAided_Design_Engineering_amp_Manufactur.pdf

A feature is a geometric shape specified by a parameter set, which has special meaning to design or
manufacturing engineers. Features represent a collection of entities in an intelligent form (like hole, slot,
thread, groove, etc.) and hence provide information at a higher conceptual level. The use of such groups
of geometry, coupled with the necessary information needed for the process planning function, is seen
as a practical means of linking the design and manufacturing (Drake and Sela, 1989; Klein, 1988; Clark
and South, 1987; Butterfield et al., 1986).
The attractive prospect in the adoption of form-features is the elimination of exhaustive postprocessing
of the part data. Because of the compatibility between the feature-data and the application-data, the
development of CAPP systems and other related applications (such as checking the part model for DFM,
geometrical validity, etc.) also become relatively easy.
Unfortunately, attempts to define the precise nature of features are fraught with difficulty because of
the wide interpretation placed upon the term by different researchers. Features originate in the reasoning
processes used in various design, analysis, and manufacturing activities and are strongly associated with
application domains (Cunningham and Dixon, 1988; Case and Gao, 1993; Kang and Nnaji, 1993).
Researchers are realizing that the feature definition is relative, depending on such factors as application,
context, state of the product, structure, and configuration. When representing product knowledge in
terms of features, application domains do not stand alone.
Several informal definitions of feature can be found (Wilson and Pratt, 1988; Dixon et al., 1987).
However, none of these definitions is entirely satisfactory because the feature definition depends on
the application for relevance; therefore it can be surmised that there can be as many definitions as
applications.
Though there is no universal definition for a feature, the following views are of interest to CAPP (Shah,
1991):
• From the manufacturing point of view, features represent shapes and technological attributes
associated with manufacturing operations and tools.
• From the geometric modeling point of view, features are groupings of geometrical or topological
entities that need to be referenced together.
• From the design point of view, features are elements used in generating, analyzing, or evaluating
designs.
Three important definitions of features adopted in the modeling systems of CAPP systems are shown
in Figure 5.7 and are briefly described below. It can be seen that the amount of information attributed
to the feature is increasing with each definition.
Surface Generated after Machining
In this method, the feature represents the final specification of the machined region. For example, consider
a through hole (Figure 5.7a). Here, the hole surface and its attributes together define the feature. These
are called surface features.
Surface features, however, do not relate explicitly to the extent of the material to be removed. In spite
of that, the reason for selecting the surface features is the simplicity they offer during the part modeling
(design) stage. The designer can define the feature as given in the part specification and gives little
consideration to the machining aspects (e.g., operations, material removal volumes, etc.) while defining
a feature, since these are supposed to be the tasks of CAPP. This method is widely followed in CAPP
systems dealing with rotational parts. Examples include TECHTURN (Hinduja and Barrow, 1986),
CIMROT (Domazet and Lu, 1992), and GIFTS (Jasthi, 1993).
Volume Removed during Machining
In this method, the feature represents the material to be removed to generate the final specification
(Figure 5.7b). These can be considered as volumetric features that correspond to machining operations
(Henderson and Anderson, 1984; Dong and Wozny, 1990; Kusiak, 1989; Kusiak, 1990). This definition
is widely used in CAPP systems dealing with prismatic parts. An Example is MACHINIST (Hayes, 1990).