2004

296 | Chapter 16 Draw Geometric Objects
AutoCAD supports three types of 3D modeling: wireframe, surface, and solid.
Each type has its own creation and editing techniques.
A wireframe model is a skeletal description of a 3D object. There are no sur-
faces in a wireframe model; it consists only of points, lines, and curves that
describe the edges of the object. With AutoCAD you can create wireframe
models by positioning 2D (planar) objects anywhere in 3D space. AutoCAD
also provides some 3D wireframe objects, such as 3D polylines (that can only
have a
CONTINUOUS linetype) and splines. Because each object that makes up
a wireframe model must be independently drawn and positioned, this type
of modeling can be the most time-consuming.
Surface modeling is more sophisticated than wireframe modeling in that it
defines not only the edges of a 3D object, but also its surfaces. The AutoCAD
surface modeler defines faceted surfaces using a polygonal mesh. Because the
faces of the mesh are planar, the mesh can only approximate curved surfaces.
With Mechanical Desktop
®
, you can create true curved surfaces. To differen-
tiate these two types of surfaces, AutoCAD calls faceted surfaces, meshes.
Solid modeling is the easiest type of 3D modeling to use. With the AutoCAD
solid modeler, you can make 3D objects by creating basic 3D shapes: boxes,
cones, cylinders, spheres, wedges, and tori (donuts). You can then combine
these shapes to create more complex solids by joining or subtracting them or
finding their intersecting (overlapping) volume. You can also create solids by
sweeping a 2D object along a path or revolving it about an axis. With
Mechanical Desktop, you can also define solids parametrically and maintain
associativity between 3D models and the 2D views that you generate from
them.
3D wireframe
mesh
solid