A ball bearing works because it is round. Not approximately round. An ABMA Grade 3 ball deviates from a perfect sphere by no more than 0.08 micrometres, roughly one seventh of the wavelength of green light. Most rolling elements begin as AISI 52100 through-hardening chrome steel: one percent carbon, one and a half percent chromium, vacuum degassed until dissolved oxygen falls below 10 parts per million. Oxide inclusions are the enemy. A single alumina particle 20 micrometres across becomes a stress raiser under repeated Hertzian loading, and the fatigue crack initiates there rather than at the surface. Cleaner steel lifted bearing fatigue life by roughly an order of magnitude between the 1960s and today, with no change to the geometry at all. The forming sequence: - Wire is sheared into slugs and cold headed between hemispherical dies, producing a rough ball girdled by a flash ring - Flashing removes that ring between a rotating grooved cast iron rill plate and a stationary one - So...