books.google.comhttp://books.google.com/books/about/Gear_Hobbing_Shaping_and_Shaving.html?id=_cH57g5T7acC&utm_source=gb-gplus-shareGear Hobbing, Shaping, and Shaving
Page 5 - The form also does not need to be of any special curvature. The only limiting factor is that the indentations or projections must be of sufficient width in proportion to height to roll the hob in and out without Interference.