Chapter 7 Computer-Aided Drafting and Design 161
for 3D drawing. Viewing tools and 3D-based
modeling methods are discussed later in this
chapter. Commands used in 3D drawing are
discussed in Chapter 13.
Drawing Aids
In addition to coordinate systems and
the numerous commands used to create
basic geometric shapes, CAD programs offer
a number of drawing aids that simplify the
drawing process. These features 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, and orthogonal mode.
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 simplifies the fi
process of locating points at specific incre- fi
ments. 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 align (“snap”) the cursor to specific incre- fi
ments, 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.
Object snap is a function that allows the
cursor to be “snapped” to specific locations fi
on an existing object. It can be set to one or
more modes that determine the type of loca-
tion. This is useful for drawing objects using
points on an existing object, or entity, such as a
line or circle. For example, the Endpoint object
snap mode allows you to snap the cursor to
an endpoint of an object, such as a line. The
Midpoint object snap mode is used to snap the
cursor to the midpoint of an object. The Center
object snap mode is used to snap the cursor to
the center of a circle or an arc. Object snap can
also be used to draw objects that are parallel,
perpendicular, or tangent to other objects.
X
Z
Y
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.