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 terms of measuring sensors, although high-precision circular gratings are generally used for angle measurement, length measurement varies with different objects. For gears with high accuracy requirements or workpieces with long axial dimensions, dual frequency (or single frequency) laser interferometer is generally used as the length reference (such as measuring involute or spiral template); In other cases, high-precision long gratings are used.