Chapter 7 Computer-Aided Drafting and Design
157
Drawing Circles and Arcs
Curves making up CAD-generated circles
and arcs are defi ned mathematically by the
program based on the coordinates entered.
Circles are typically drawn by specifying
the center point and a radius or diameter,
Figure 7-14A. A circle may also be drawn by
specifying points along the perimeter of the
circle or by entering a radius and selecting
two lines or two circles to which the circle
should be tangent. The Circle command is
most commonly used to draw circles. The
center point location and radius value may be
entered at the keyboard or picked on screen.
Arcs can be drawn with the Arc command.
A number of methods are usually available.
Arcs typically require a center point, radius,
and endpoint. See Figure  7-14B. Arcs may
also be drawn by specifying three points, or a
starting point, a center point, and a third entry,
such as a chord length.
Drawing Ellipses
Ellipses are drawn with the Ellipse
command. An ellipse has a center point, a minor
to layers as needed. This provides a means to
organize a wide variety of content.
Many of the drawing aids previously
discussed can be saved in a drawing template.
A template is a fi le with standard user settings
used to start a new drawing fi le. Different
templates can be created for different drafting
disciplines. A typical template includes settings
for the unit format, sheet size, and drawing
scale. It also includes predefi ned text styles,
dimension styles, layers, and block defi nitions.
The use of templates saves drawing time and
allows drafters to focus on the drawing project
at hand.
Creating Objects
There are a variety of ways to create
objects using CAD. In most cases, the creation
of an object begins with a command. While
a command is active, objects may be created
by specifying coordinates, such as absolute
coordinates, or parameters, such as linear
measurements and radius or diameter values.
Coordinates and parameters may be entered
at the keyboard or specifi ed dynamically on
screen with the cursor.
Most of the basic geometric shapes discussed
in Chapters 3 and 6 can be drawn quickly with
drawing commands. The following sections
discuss the common methods used to create
basic geometric shapes in CAD.
Drawing Lines
Lines are most commonly drawn with the
Line command. A line may be drawn horizontal,
vertical, or inclined by specifying coordinates
at the keyboard or by using the cursor to
pick points on screen. A line requires two
coordinates, Figure 7-13. Additional coordinates
may be entered within a single command
sequence to create as many line segments as
needed. As previously discussed, lines may be
displayed using different line conventions by
applying the proper linetype. Before drawing
the line, the layer with the appropriate linetype
setting must be set as the current layer.
(11,10)
(6,6)
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Figure 7-13. A line is created by drawing a segment
between two coordinates.
Radius
A B
(3,0)
(0,3)
(0,0)
(0,-3)
(3,0)
(0,0)
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Goodheart-Willcox Publisher
Figure 7-14. Drawing circles and arcs. A—Circles are
defined with a center point and a radius or diameter.
B—Arcs are commonly defined with a center point, a
starting point, and an endpoint, or with points along
the arc.
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