Comment by mikewarot

Having made an 4 year excursion from IT into the world of making gears, I can attest that what seems like something fairly simple from the outside really isn't.

Just like a bearing, gears (ideally) have a rolling contact. The smoother and harder the faces the better, until you get to the point where fractures and spalling occur. The best gears are cut, sent out to be hardened via heat treat, then ground (with "superfinishing") to exact size. As with bearings, you have to get the size just right for long life.