How Ball Bearings Are Made

Taken from Rolling Bearings and their Contribution to the Progress of Technology, by
FAG Kugelfischer George Schaefer KGaA, Schweinfurt, West Germany (c) 1986. ISBN 0 7124 1500 9.

1.9 How Do You Get Round Balls?

Three centuries ago, an astonishingly simple manufacturing method had been developed for balls made of stone, e. g. marble (Figure 33, left). Lumps of stone were hammered into squares with rounded edges and put on stone plates with concentric grooves; then a wooden plate was put on top and run as a water wheel, until, after one, two or three days, the squares had become balls.

The deviation in diameter of these balls from the ideal circular shape, the out-of-roundness, is less than 0.1 millimetre (Figure 34, left).
Who knows why steel balls were not machined, from the very beginning, to a principle similar to that of stone ball production. This production method guarantees for a large number of balls equal diameters when machined between two plates, since only the ball contacting the upper and lower rotating surface simultaneously with a certain load can be ground. Due to the numerous contact points, a set of balls of equal size is produced almost automatically. For mass production of steel balls in the 19th century another method was chosen first. After all, not relatively soft stones, but steel had to be treated. On lathes, balls of a remarkable accuracy were cut off from a rod of steel and the ends machined to form a sphere. At the turn of the century, balls manufactured by this method in England were within tolerances of 0.025 to 0.050 millimetre [1]. The Schweinfurt mechanic Friedrich Fischer, son of Philipp Moritz Fischer who built the first pedal bicycle, did not want to accept this accuracy, mainly because the imported steel balls were too expensive. In his workshop for velocipedes, bicycles and tricycles he therefore worked intensively on the production of high-quality balls. The marble ball mills described above were probably not the model for Friedrich Fischer's ball milling machines. He developed, however, a similar production method (Figure 35, below).

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