This is how metal balls of steel be manufactured.

MANUFACTURING OF METAL BALLS

Metal balls start as a wire. The wire is sheared to give a pellet with a length approximately the size of the desired ball outer diameter (OD). This pellet is then headed into a rough spherical shape. Next, the balls are then fed into a machine that de-flashes them. The machine does this by feeding the balls between two heavy cast iron or hardened steel plates, called rill plates. One of the plates is held stationary while the other rotates. The top plate has an opening to allow balls to enter and exit the rill plates. These plates have fine circumferential grooves that the balls track in. The balls are run through the machine long enough so that each ball passes through many of these grooves, which ensures each ball is the same size, even if a particular groove is out of specification. The controllable machine variables are the amount of pressure applied, the speed of the plates, and how long the balls are left in the machine.

During the operation coolant is pumped between the rill plates because the high pressure between the plates and friction creates considerable heat. The high pressure applied to the balls also induces cold working, which strengthens the balls.

Sometimes the balls are then run through a soft grinding process afterward to improve precision. This is done in the same type of machine, but the rill plates are replaced with grinding stones.

If the balls are steel they are then heat treated. After heat treatment they are descaled to remove any residue or by-products.

The balls are then hard ground. They are ground in the same type of machine as used before, but either an abrasive is introduced into coolant or the rotating plate is replaced with a very hard fine-grain grinding wheel. This step can get the balls within ±0.0001 in (0.0025 mm). If the balls need more precision then they are lapped, again in the same type of machine. However, this time the rill plates are made of a softer material, usually cast iron, less pressure is applied, the plate is rotated slowly. This step is what gives bearing balls their shiny appearance and can bring the balls between grades 10 and 48.

If even more precision is needed then proprietary chemical and mechanical processes are usually used.

The inspection of bearing balls was one of the case studies in Frederick Winslow Taylor's classic Principles of Scientific Management.