Chapter 7 Computer-Aided Drafting and Design 157
Figure 7-8 A third axis (the Z axis) is needed to
generate three-dimensional drawings like the one
shown. The ability to visualize objects in 3D space is
fundamental to creating 3D drawings.
(ROBO Systems)
Objects drawn with XY coordinates are
sufficient for 2D drawings. A third coordi- fi
nate axis, the Z axis, is required for 3D draw-
ings. This axis is used for coordinate entry
above or below the XY plane. A point with
a positive Z coordinate, such as (0,0,1), is
located “above” the XY drawing plane. When
looking at a drawing on screen, this loca-
tion can be thought of as a point projecting
out of the monitor toward you. A point with
a negative Z coordinate, such as (0,0,–1), is
located “below” the XY drawing plane. When
looking at a drawing on screen, this location
can be thought of as a point projecting into
the monitor away from you. In order to draw
objects in 3D space with a CAD system, it is
important to be able to visualize them in three
dimensions with coordinates along each axis.
See Figure 7-8.
A basic way to locate points when drawing
objects is to pick points on screen using the
mouse. In addition, there are three common
forms of coor dinate entry used to specify
point locations. Coordinates can be entered
as absolute coordinates, relative coordi-
nates, or polar coordinates. See Figure 7-9.
When using absolute coordinates, objects are
drawn using points in relation to the coordi-
nate system origin (0,0). The absolute coordi-
nate (2,2) indicates that the point is located
two units from the origin along the positive
X axis and two units from the origin along
the positive Y axis. When using relative coor-
dinates, objects are drawn using coordinates
in relation to the last coordinate specified (or
the origin, if a previous point has not been
specifi For example, entering the relative fied).
coordinate (@5,4) after entering the absolute
coordinate (2,2) places the next point fi ve units
along the positive X axis and four units along
the positive Y axis “relative” to the absolute
coordinate (2,2) or at the absolute coordinate
(7,6). Refer to Figure 7-9B.
When using polar coordinates, coor-
dinates are located at a given distance and
angle. Polar coordinates are entered using a
format such as (distanceangle). This format
specifi a linear distance and angle in the fies
XY plane relative to the previous point or the
origin. Angles are most commonly measured
counterclockwise, with the positive X axis
representing 0°. Entering the @ symbol before
the polar coordinate locates the point relative
to the previous point. If the @ symbol is not
entered before the polar coordinate, the point
is located relative to the origin. Referring
to Figure 7-9C, the polar coordinate entry
(545) locates the point fi ve units away from
the origin at an angle of 45° in the XY plane.
The polar coordinate entry (8120) locates the
point eight units away from the origin at an
angle of 120° in the XY plane.
Each of the different types of coordinate
entry can be used when creating a drawing.
The entry used typically depends on the
object drawn. For example, it may be useful
to use absolute coordinates when a given
point is known, or when drawing straight
lines. Relative coordinates may be used when
locating points from a common feature. Polar
coordinates are useful for drawing inclined
lines and round objects.