Abstract: A plane that contains a central axis of a worm shaft and that is orthogonal to a central axis of a worm wheel is arranged at a central position of the worm wheel in a tooth width direction thereof when no load is applied to a worm speed reducer. The worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position toward an offset direction corresponding to a first direction along the tooth width direction. A pair of tooth flanks has different pressure angles at the central portion in the tooth width direction.

Abstract: A forming device (100) for bevelling includes a base (102), a rail (104), a workbench (106) and a bar (108). A pair of support arms (114) extends upwardly from the base. An axle (120) is received in pivot holes (118) defined in the support arms. One end of the rail is fixedly connected with the axle. The workbench rests on the rail, and is slidably moveable along the rail. A beveller (132) is fixedly installed in a top surface of the workbench. The bar is fixedly connected to an opposite end of the rail. The bar includes a large bar (134) abutting the support plate, and a coaxial small bar (136). The rail can be adjusted to allow the forming device to bevel at an angle of less than one degree, by placing one or a combination of standard gauges between the small bar and the base.