A side comment regarding terminology: B&S and others originally called these heads spiral milling heads and they could indeed cut a spiral as in a scroll, which is a spiral in proper mathematical terminology. Helical gears and helical cutters are not spirals as their edges curve around a cylinder. A spiral is a curve that lies in a plane. You can also have a helical spiral (?) that is a combination that has a curve on the surface of a cone. Spiral bevel gears are a machinery application. Someone...