Chapter 7 Computer-Aided Drafting and Design
153
make it easy to specify distances and locate
coordinates when drawing objects. The typical
drawing aids in a CAD program include grid
and snap, object snap, orthogonal mode, and
direct distance entry.
In CAD, a grid is a network of uniformly
spaced points used to determine distances.
Displaying a grid is similar to using graph
paper in manual drafting. The grid spacing
may be set to any value that simplifi es the
process of locating points at specifi c increments.
When used, the grid display is for reference only.
It does not print when the drawing is plotted.
Snap is a function that allows the user to
“snap” (align) the cursor to specifi c increments,
or snap points, in an invisible grid. Snap can be
used with or without the grid display turned
on. However, it is common to set the snap
spacing in conjunction with the grid spacing.
For example, if the grid spacing is set to .50″,
snap may be set to .25″ so that the cursor can
be “snapped” to the grid points as well as the
midpoints between grid lines.
When creating drawings in 3D, however, it
is often useful to change the world coordinate
system to a different coordinate system to
establish a different drawing plane. A user
coordinate system is a user-defi ned coordinate
system with an orientation different from that of
the world coordinate system. As with the world
coordinate system, coordinates are located in
relation to a fi xed origin. The origin used may
be a specifi c point on an object, such as a corner
or center point. See Figure 7-10.
User coordinate systems greatly simplify
the 3D drawing process. There are also viewing
tools and drawing commands that are useful
for 3D drawing. Viewing tools and 3D-based
modeling methods are discussed later in this
chapter.
Drawing Aids
In addition to coordinate systems, CAD
programs offer a number of drawing aids that
simplify the drawing process. These features
X
Z
Y
Goodheart-Willcox Publisher
Figure 7-10. A user coordinate system is used to establish a drawing plane in 3D space so that objects can be
drawn on the surface of the plane. The XYZ coordinate axes identify the orientation of the drawing plane. In this
3D model, the XY drawing plane is oriented so that objects can be added to the top surface of the part. Note the
direction of the axes.
Copyright Goodheart-Willcox Co., Inc.