Internal gear is a gear with its teeth cut in the internal surface of a cylinder and meshes with spur gears. Internal gears are primarily used for planetary gear drives. In its manufacturing, because of its shape, the usual hobbing machine used in spur gear production cannot be used. Generally, it is made with a gear shaper (or gear shaping machine) equipped with a pinion cutter. Internal gears mesh with spur gears. Spur gears are generally seen as best for applications that require speed reduction and torque multiplication, such as ball mills and crushing equipment. However, gears with teeth parallel to the axis have higher demand.