High speed, cold heading machines take a wire or rod feed, chop it to length and cold forge a ball to fairly close limits of size and sphericity. Basically this is done by taking a cut slug of the right volume and "bonking" it between two
hemispherical dies.
Heat treatment follows..in continuous conveyor fed furnaces.
Then the balls are fed in a continuous stream to a series of grinding and lapping machines. These look like the old flour grinding mills..two discs contra-rotating (or rotating at differential speeds) the balls feed in the center and sphericity is 'Generated" just as you would form a ball of play-do by
rolling it between your hands.
Gauging is automatic..usually by rolling the balls along two, slightly divergent hardened knife edges..amazing accuracy!
Balls are graded for size and sent off to be assembled with similarly graded inner and outer rings on a "Selective Assembly" basis. This may be done in the building next door..or one half way around the world!! Bearings are a
Multinational concern!
The assembly plants run, 24/7/365 fully automated, on an almost "lights out" basis. Noisy places they are..Busy but no one about unless something needs fixing. A modern ball bearing can go through every stage of manufacture, assembly, packaging, shipping and assembly (For instance into an automobile) without being touched by human hand or seen by human eye.