Chapter 7 Computer-Aided Drafting and Design 165
(11,10)
(6,6)
•
•
Figure 7-13 A line is created by drawing a segment
between two coordinates.
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 file. Different fi
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, layer assignments,
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 or linetype
must be set current.
Drawing Circles and Arcs
Curves making up CAD-generated circles
and arcs are defined 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. As with circles,
arcs may be drawn tangent to lines, other arcs,
or circles.
Radius
A B
(3,0)
(0,3)
(0,0)
(0,-3)
(3,0)
(0,0)
• • •
•
•
•
Figure 7-14 Drawing circles and arcs. A—Circles
are defined with a center point and a radius or diameter.
B—Arcs are commonly defi ned with a center point,
a starting point, and an endpoint, or with points along
the arc.