Page 285 - Aeronautical Engineer Data Book

Basic mechanical design 231
contains all the changes required to adapt
vendor software for custom use.
12.7.2 Types of modelling
CAD software packages are divided into those
that portray two-dimensional or three-dimen-
sional objects. 3D packages all contain the
concept of an underlying model. There are
three basic types as shown in Figure 12.12

Wireframe models
Although visually correct these do not contain
a full description of the object. They contain no
information about the surfaces and cannot
differentiate between the inside and outside.
They cannot be used to link to a CAM system.

Surface models
Surface models are created (conceptually) by
stretching a two-dimensional ‘skin’ over the

Wireframe model
It is possible to get
No differentiation meaningless
between inside and ‘nonsense’ models
outside like this

Surface model
All surfaces and their
boundaries are defined Although the model
and recognized by the appears solid, there
model is no recognition of
what lies inside the
surfaces
Solid model
Various techniques of solid
modelling include:
The model is • BR (Boundary
recognized as a Representation)
solid object • CSG (Constructive Solid
Geometry)
• FM (Faceted Modelling)
Fig. 12.12 Types of modelling