In marking out involute curves, first strike a circle as shown by the sketch; divide this into a number of equal parts, as A, B, C, etc., the more the better; from each of the points draw a tangent to the circle DEFGHIJKL. Now, supposing the curve to commence from C, make D equal to the distance from C to B measured on the circumference of the circle, E twice the length of D, E three times the length of D, G four times the length of D, and so on; from C, through the ends of D, E, F, G, etc., draw the curve freehand. This curve may be described mechanically as follows. Take a cylinder of wood, and on this wind a cord with a loop at the end; place a pencil point in the loop, and the cylinder with one end on a piece of paper; press the point on the paper and unwind the cord, keeping it tight, and the pencil will trace a curve similar to that shown on the sketch. Variations of the curve may be made by using an ellipse or any other form for the section of the solid from which the cord is unwound.

Marking Out Involute Curves.