External versus internal gearsExternal gears are gears with teeth formed on the outer surface of a cylinder or cone. In contrast, an internal gear is a gear with teeth formed on the inner surface of a cylinder or cone. As with bevel gears, internal gears are gears with pitch angles greater than 90 d

In the past 50 years before the 1970s, the world has developed various types and specifications of mechanical generating instruments for measuring basic parameters such as tooth profile, helix and pitch. These instruments use some precision mechanisms to form a designated standard motion, and then compare with the measured to obtain the size of the measured error.