Lay out large angles accurately. Useful for Fabrication/Boilermaking.

2. The chord of a circle is a straight line joining
any two points on its circumference.

3. 1000
45°
765.4
A B
C
The Chord length of 45° is 0.7654

4. 1000
45°
765.4
A B
C
Draw lineA-B at 1000 units.
This is our scale factor.
Values are derived from
the Unit Circle.

5. 1000
45°
765.4
A B
C
Using the measurementA – B (as radius) and
pivot pointA produce an arc.

6. 1000
45°
765.4
A B
C
From point B produce an arc equal to 1000 times
the value obtained from aTable of Chords relative
to the required angle.
0.7654 multiplied by 1000 = 765.4

7. 1000
45°
765.4
A B
C
Mark the intersection of the two arcs ©.
Draw a line from the pivot A through point C.
The angle BAC is the required angle.

8. 1000
45°
765.4
A B
C
If aTable of Chords is unavailable, obtain the
chordal value from a scientific calculator using
the following method.
If the required angle is 45° then :
sin
45deg
2






2 0.7654
T

9. 1 UNIT
A
B
C
22.5°
0.3826
0.7654
The Unit Circle
The sine of an angle is the ratio of the
length of the opposite side divided by the
length of the hypotenuse, which in the
example below is one unit.
The sine of an angle is equal to half the
chord of twice the angle.
sin ( )
opposite
hypotenuse

Sin (22.5 deg) = 0.38268
0.38268 x 2 = 0.7654