Chapter 6 Basic Geometric Constructions 209
of this projection is increased when a
greater number of divisions is used.
3. Lay off the same number of chord lengths
along the arc. The length of Arc EG is the
approximate length of Line EF.
This type of construction is used in deter-
mining the true length of a formed metal part,
Figure 6-61C.
Using the Divide and Circle
Commands (CAD Procedure)
The following procedure uses the
Divide
command to divide a line of a given length
that is tangent to the arc. You can also draw
an arc segment if you know the chord length
by using the
Center, Start, Length
option of the
Arc
command. The chord length is the length
of a line that connects the endpoints of the
arc. Use the following procedure when the
rectified length of the arc is known. fi
1. Arc AD and the length of the tangent
line (Line AB) are given. Refer to
Figure 6-61A.
2. Enter the
Divide
command. Divide
Line AB into an equal number of parts.
3. Enter the
Circle
command. Draw a circle
that intersects the arc at Point D. Specify
Point C as the center point and specify
the radius by selecting Point B. Use object
snaps as needed.
4. Arc AD is equal in length to Line AB.
Figure 6-61. Laying off a given length on an arc. A—Using the construction method. This method is accurate to
.6% for angles of less than 90°. B—The equal chord method can also be used for this construction, but it is not
as accurate. C—Drafters may use this construction in many different applications.
A C B
D G
Application
True length
A
E F
B C
Using the Construction Method
(Manual Procedure)
Note that to use this method, the specifiedfi
length must be given as a line tangent to the
given arc.
1. Arc AD and the length of the tangent
line (Line AB) are given, Figure 6-61A.
2. Divide Line AB into four equal parts.
3. Using the first division point (Point C) fi
as the center, strike an arc with a radius
equal to CB. This arc will intersect the
arc at Point D.
4. Arc AD is equal in length to Line AB.
This method is accurate to six parts in a
thousand, or .6%, for angles less than 90°.
Using the Equal Chord Method
(Manual Procedure)
Note that to use this method, the specified fi
length must be given as a line tangent to the
given arc.
1. Arc EG and the length of the tangent line
(Line EF) are given, Figure 6-61B.
2. Using dividers, divide Line EF into an
equal number of parts. The accuracy
Lay Off a
Given Length
on an Arc
CAD