Computer Aided Engineering Design

272 COMPUTER AIDED ENGINEERING DESIGN

intersection between the bounding surfaces of two primitives. Related are the issues of determining
points and curves on the bounding surface. Segmentation and trimming operations may also be seen
as intersection problems between the segmenting or trimming surface and the solid. Segmentation
involves splitting a solid into two parts using a plane. Note that post segmentation, each sub-solid
must have its own topology and geometry. Trimming entails intersecting the solid with the trimming
surfaces followed by the removal of solid portions outside these surfaces. Determining inersection
between freeform curves, surfaces or solids is computationally involved and algorithmically complex.
The following chapter discusses some ways to find intersections within the realm of computational
geometry.
An important manipulating mode is the editing of solid primitives. Most new designs are alterations
of existing ones, and editing a solid involves changing its existing topological (rarely) and geometric
(mostly) information. Generating solid models for complex engineering parts and assemblies can
be arduous, and for a few changes, one may have to regenerate the entire set in absence of the
editing capability. It thus seems imperative that solids are represented in their parametric form
wherein design dimensions and relations between features (constraints) are also stored in the data
structure. Consider, for instance, the CAD model of a bolt in Figure 8.25. For different applications,
one may require the bolt to have different nominal diameters, thread and bolt lengths, and head
sizes. These seem notable features in bolt design and can be treated as its parameters. Among this
set, some parameters may depend on others via some design rules, for instance, the head size
(diameter of the circumcircle of the hexagon and head thickness) may be governed by the nominal
diameter. One may then treat the nominal diameter, and thread and bolt lengths as independent
parameterswhich may be altered as required. The user would expect that if the nominal diameter
is altered, appropriate changes would be carried through the bolt. That is, the shank and helix
diameter (for threads) would change and so would the head size. Note, however, that the relative

Figure 8.27 Sweep solids

Extruded crosssection generated using splinesRevolved crosssection

(a) Linear sweep, translational (left) and rotational (right)

(b) Nonlinear sweep (wireframe in left showing crosssection and path)