Abstract: In this paper, the gearing principle of the spherical hob is introduced. A new definition of lead has been given. It explains the method of the formation and evolution of the spherical helicoidal surface. The helical surface equations of the spherical hob are set up in spherical coordinates. The formulas of the side clearance angle and the calculation are given.

Key words: 滚刀, 螺旋线, 内齿轮