In order to eliminate twist and crown in the final gear grinding operation, a bore-to-face perpendicularity of fewer than 10 microns is critical, as the gear is piloted off of both face and bore.

Glenn Muffly, GEAR-GRINDING AND TOOTH-FORMS, SAE Transactions, Vol. 18, PART I (1923), pp. 568-612 ...

A gearbox is an important component of the vehicle that transfers power from the engine to the wheels.