104 Section 1 Introduction to Drafting
The types of coordinate entry shown in
Figure 4-1 are sufficient for making 2D draw- fi
ings. In 2D drafting, the Cartesian coordinate
system provides a basic XY drawing plane (a
surface upon which objects are drawn using XY
coordinates). However, in 3D drafting, a third
coordinate axis, the Z axis, is used to locate
points. See Figure 4-2. When using 3D coordi-
nates (XYZ coordinates), points are represented
as (X,Y,Z). Notice how the third coordinate
provides a “vertical” measurement and allows
the object to be represented in three dimensions.
Also, notice how the view is rotated to show the
object in 3D. This view also shows the orientation
of the XY drawing plane. The plane is parallel to
the X and Y axes and is oriented at 90° to the Z
axis.
The coordinate systems shown in Figure 4-1
and Figure 4-2 both have points located in rela-
tion to the 0,0,0 origin. In both cases, the origin
has a fi xed location and points located from fi
the origin are absolute coordinates. This is the
default coordinate system for most CAD draw-
ings and is commonly referred to as the world
coordinate system. This system is sufficient for fi
2D drawing and can also be used for 3D draw-
ing. In 3D drawing, however, it is often useful
to establish a different coordinate system so
that object features in 3D space can be drawn
more easily. A user-defi coordinate system fined
is known as a user coordinate system (UCS). A
UCS can be established at any origin and orien-
tation so that points can be located in relation to
a point in space, such as the corner of an object
surface. User coordinate systems can typically be
defined with the fi
UCS
command. This command
and other 3D drawing functions are discussed
later in this chapter.
Drawing Commands
Drawing commands form the foundation
of any CAD program. These commands allow
you to actually create objects on the computer
screen. The most basic drawing command is the
Line
command. After all, any object is made up
of at least one line. In addition, many CAD pro-
grams have commands to automate the creation
of certain objects, such as circles, rectangles, and
polygons.
Line
The
Line
command is the most frequently
used command in a CAD program because lines
are the basic elements of most drawings. Each
straight line requires information as to the place-
ment of the fi rst point (one end) and the second fi
point (other end). Generally, you can enter
specifi coordinates for the endpoints or pick the fic
endpoints on screen, Figure 4-3.
AutoCAD Example:
Command: line↵
Specify fi rst point: 3,5↵
(or pick a point on screen)
Specify next point or [Undo]: 6,4↵
(or pick a point on
screen)
Specify next point or [Undo]: ↵
Command:
Double Line
Some CAD packages provide a
Double Line
command, although it may not have this name.
This command is useful in creating grooves on
parts and in similar applications where parallel
Figure 4-2. A third coordinate axis (the Z axis) is used
in 3D drawing. The point locations making up the 3D
object shown have XYZ coordinate values. Note the
relationship of the coordinates to the origin and the XY
drawing plane.
Z axis
(3,3,1)
Y axis
Origin (0,0,0)
X axis
XY drawing
plane
(3,2,0)
. A line consists of two endpoints and a
segment.