Hob is a milling cutter with cutting edge installed on the worm. Since the cutting mechanism is all rotation movement except feeding, it is excellent in productivity compared with other gear cutters.
Cutting principle of hob is shown in Fig.1. Along with rotation of hob, each cutting tooth on the screw surface appears one by one on to the surface where tooth profile is generated. Then, the rack, which goes straight on this surface, is projected. Gear is generated by rotating the work piece so as to mesh with this rack ideally, and at the same time, by feeding the surface, where tooth profile is generated, in the direction of thread helix of gear. Fig.2 below shows gear generation when is fixed.