How are ball bearings made?
The ball bearing balls are made from wires of steel with diameter equal to the design diameter of the balls. The wire is cut in to small pieces slightly longer than ball diameter. Each of these piece is pressed between two dies, each having semi-spherical cavities. This results in the cylindrical pieces of wire taking on spherical shape of the dies. However the balls produced in this way have some projection called flash, which looks like ring of Saturn, because some gap remains between the two halves of the dies even when they are pressed together to the maximum.
The flash is ground off by placing the balls in a rough groove between cast iron disks. One of the disk is stationery while the other rotates. This causes the balls to rub and rotate against the rough surface of the groove, which grinds off the flash.
The ball after removal of the flash are heat treated to give them required hardness. The hardened ball are then ground and lapped to the exact required diameter and degree of smoothness.