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

Gear measuring instrument is a relatively rich concept. It not only includes instruments for detecting various gears, but also includes instruments for detecting worm gears, worms, gear cutters and transmission chains. There are many kinds of gears and complicated geometric shapes, so there are many parameters to represent their errors. Therefore, there are many kinds of gear measuring instruments. The research on gear measuring technology and its instruments has a history of nearly a hundred years.