Spur Gears, 20 Teeth: Cut One Space at a Time on a Dividing Head

Without a hobbing machine, a spur gear is cut one tooth space at a time: a formed involute cutter on a milling machine, the blank turned by a dividing head between cuts. On a 40:1 head, a 20-tooth gear is two full turns of the crank per tooth. Eight involute cutters cover every gear from 12 teeth to a rack. The picture: cutting a spur gear with the spiral (dividing) head, from "A Practical Treatise on Milling and Milling Machines".