ComputerAided_Design_Engineering_amp_Manufactur.pdf

stage. Feature-based design essentially involves the feedback to the user which can be on the validity of
the designs, design for manufacturability (DFM), and other related factors. Iterative loops between FBDS
and the user until the user reaches a satisfactory design are crucial here. It can be considered as a predesign
method.
Many researchers frequently use the term “feature-based design” to describe post-design modeling
methods. (In this chapter, feature-based design denotes the conceptual designing of the part in terms of
features). In fact, the characteristic of the feature-based design systems that distinguishes it from other
systems is the origin of the model.
Other approaches (FRES and FBMS) start with an existing design (a drawing or CAD data) to build
the model, whereas this approach solely depends on the imagination and ability of the designer to use
the features for designing the part. Hence, FBDS must provide the designer with the tools necessary to
carry out the design analysis. These can also be developed using dedicated systems or general purpose
CAD systems.
This offers the total solution because the part modeling and the design representation match with
each other (both are in terms of features) and there will be no special need to carry out part modeling.
The design representation can serve the purpose of supplying the required data to the CAPP function.
Representation of Technological Details
The information pertaining to tolerances and surface finish can be referred to as technological data.
Technological data have considerable influence on the determination of machining sequence, manufacturing
procedures, machine selection, and chucking positions (Halevi and Weill, 1985; Weill, 1988). To
enable the CAPP system to make realistic decisions, it is necessary to include the technological details in
the part model.
Although representation of technological details is as important as geometrical details, it does not
seem that this aspect has enjoyed much interest in the initial stages of CAPP system development. The
reason for this may be attributed to the adhoc modeling methods followed when CAD systems are not
widely used.
In the later stages, CAD systems are used for part modeling. Because of the limitations of CAD systems,
FBS has become popular in recent years. In parallel to these developments, issues concerning the representation
of technological details in CAD and FBS are addressed by many researchers. Subsequent
discussion gives a brief chronological presentation of the work done in this area.
In the early stages, a data base management system (DBMS) for storing technological and material
information along with information obtained from the geometric modeler was proposed by Iwata and
Arai (1983).
Representing dimensioning and tolerancing in solid models was studied by Requicha (1983). He
proposes a tolerancing theory based on the “variational class” concept. This theory treats tolerances as
properties or attributes of an object’s features (surfaces and edges), the variational information is represented
by a graph, called Vgraph. Requicha and Chan (1986) also designed a variational graph (Vgraph)
to implement this theory in a CSG-based system. As an independent graph, Vgraph stores all the
variational information about an object and is attached to a CSG tree through a set of nominal faces
(which are indexed through a suitable method) of the object.
A number of other schemes have also been developed by other researchers such as Kimura et al. (1986),
Suzuki et al. (1988), Ranyak and Fridshal (1988), and Gossard et al. (1988). Faux (1988) classified surface
features (of ANSI standard) into resolved primitives (size and form can be separately defined from
position and orientation) and unresolved primitives. These primitives are then used for the attachment
of geometric tolerances to explicit data, called datum reference frame (DRF).
In a recent work, Jasthi et al. (1994) developed a part modeling scheme for rotational parts in which
the technological details are divided into two categories: (a) intra-feature data and (b) inter-feature data.
In their scheme, feature-specific details are termed intra-feature data (e.g., diametric tolerance, surface
finish, circularity, cylindicity, etc.) while inter-feature data denote those tolerances controling a feature