Chapter 7 Computer-Aided Drafting and Design 167
drawn about and is tangent to a circle. After
specifying the orientation of the polygon, the
radius of the circle is entered.
Editing and Modifying
Objects
One of the most important advantages of
a CAD program is the ability to easily modify
objects once they are drawn. Objects can be
moved, copied, rotated, and scaled using the
appropriate commands. These commands
may be identified by the software as editing
commands or modifying commands (or both),
depending on the CAD system you are using.
In addition to these basic commands, there are
a number of other editing methods that can be
used to construct objects. This provides great
flexibility. Some of the most common editing fl
and modifying commands in CAD programs
are discussed in the following sections.
Moving Objects
It is often necessary to relocate objects
after they are drawn. This can be done with
the Move command. After selecting one or
more objects to move, you must specify a
base point for the selection set. This may be
the corner point of a rectangle or the center
point of a circle. You are then asked for a
displacement point. The objects making up
the selection set are then moved automati-
cally to the new location using the distance
specified. Distance values can be entered at fi
the keyboard or picked on screen.
Copying Objects
Copying objects is similar to using the
Move command, except the original objects
are not altered by the operation. Objects can
be copied using the Copy command. After
selecting the objects to copy and the base
point, a displacement point is specified. The fi
selected objects are then copied to the new
location.
Depending on the software you are using,
the Multiple option may be active when you
access the Copy command. This option allows
you to copy the original object or selection
set of objects to multiple locations in a single
command sequence.
Rotating Objects
Rotating an object changes the angular
position of the object with respect to the current
orientation. Objects can be rotated using the
Rotate command. When using this command,
the selected objects are rotated about the base
point specifi ed. The objects may be rotated
clockwise or counterclockwise.
If the objects to rotate are already rotated
to a given angle when you enter the Rotate
command, you can enter the angle as a refer-
ence angle. For example, if the objects are
drawn at 45°, enter 45° as the reference angle.
Then, enter the desired angle of rotation.
Scaling Objects
An object can be reduced or enlarged to
a different size by a given scale factor. This
is accomplished with the Scale command.
After selecting the object to be scaled, the
base point and scale factor are specified. A fi
scale factor of 0.5, for example, would be
used to reduce the size of an object to one-
half its original size.
Undoing a Command
CAD programs typically provide a com-
mand that allows you to “undo” a previous
operation. If you enter an incorrect value for
a scale or move operation, for example, you
can reverse the action by using the Undo
command. This command typically allows
you to undo several preceding commands,
one by one. However, the commands must be
undone in sequence.