The crown wheel and pinion in a rear axle takes every newton metre the engine makes, turns it through ninety degrees, and multiplies it three or four times. It is the most heavily loaded gear pair in the drivetrain, and cutting one is closer to sculpture than to ordinary machining. Most road sets are hypoid rather than plain spiral bevel. The pinion axis sits below the crown wheel centreline, typically by 20 to 45 mm. That offset drops the propshaft, lowers the transmission tunnel, and puts more teeth in mesh at once. It also introduces lengthwise sliding along the tooth face, which is why hypoid axles demand GL-5 oil loaded with sulphur phosphorus additives. Ordinary gear oil would be wiped out in minutes. The route from bar to axle: - Blanks are forged from case hardening steel such as 8620, 4320 or 20MnCr5, then turned in the soft state - Teeth are cut with a face mill or face hob cutter head, either single indexing with tapered tooth depth or continuous indexing with uniform dep...