Chapter 6 Basic Geometric Constructions 199
Figure 6-50. Constructing a circle or an arc of a given radius and tangent to two given circles. Note that if the
circles are to be inside of the arc that the radii of the two given circles are subtracted from the given radius.
A B
R
A
A
C
R + ra R + rb
D
B
B
R
R – rb R – ra
Application
D
R
C
Referring to Figure 6-50D, in order to
locate the center of an arc tangent to two
enclosed circles (circles that are “inside” the
arc), subtract the circle radii from the arc
radius when drawing the construction arcs.
For example, use a radius of R – ra for the
fi rst arc (where ra is the radius of Circle A).
Note
Using the Tangent, Tangent,
Radius Option (CAD Procedure)
The
Tangent, Tangent, Radius
option of the
Circle
command allows you to draw a circle
with a specified radius that is tangent to two fi
objects you select. The circle can be tangent to
lines, circles, or arcs. After entering the option
and selecting the two objects, the radius of the
circle is specified and the circle is generated fi
automatically. This is a useful option that
saves considerable drafting time. The follow-
ing procedure is used to draw a circle that is
tangent to two given circles.
1. Circle A, Circle B, and Radius R are
given. Refer to Figure 6-50A.
2. Enter the
Circle
command and enter
the
Tangent, Tangent, Radius
option. To
specify the fi rst tangent point, move the fi
screen cursor toward Circle A and pick
when the object snap icon appears. To
specify the second tangent point, move
the screen cursor toward Circle B and
select it in the same manner.
3. Enter the given radius. The required tan-
gent circle is drawn. Refer to Figure 6-50C.
Note
If you need to construct tangent arcs
rather than tangent circles in relation to exist-
ing geometry, you can use the
Trim
command
to trim away unwanted portions of a circle
after using the
Circle
command. There are
cases where you will find it easier to use the fi
Circle
and
Trim
commands rather than the
Arc
command to draw arc segments. For example,
if you draw a tangent circle for the construc-
tion in Figure 6-50, you can quickly trim
away the portion of the circle you do not need
to complete the tangent arc. Enter the
Trim
command, select Circle A and Circle B as the
cutting edges, and then select the portion of
the circle that lies above the two intersections.
The circle is trimmed to the two intersections
to form the tangent arc.