An involute is the curve that might be traced by a

point on a cord that is being unwound from a line, a

triangle, a square, or other geometric figure. Figure

14-29 shows the pin-and-string method of drawing the

is an instrument used to draw smooth irregular

involute of a pentagon.

curves.

Look at figure 1430. To draw the involute of a line,

A spiral or involute is a constantly changing curve

extend line AB (view A). Using length AB as a radius

that winds, coils, or circles around a center point. For

and A as a center, draw a semicircle (view B). Then,

a practical example, the main spring in your watch is a

using BC as the radius and B as the center, draw a second

spiral. This section gives methods of construction for

semicircle, continuing the curve as shown in view C.

common spirals.

Then, with CD as the radius and C as the center, draw

the next arc as shown in view D. Proceed until the curve

is the desired size.

As you draw the involute of a triangle, refer to

figure 14-3 1. Extend the sides of the triangle (view A).

14-17