How it's made - Ball bearings
The surface of the cast grinding steel ball is not allowed to have the casting defects such as the crack, and the obvious gas porosity, inclusion, shrinking cavity, cold shut, elephant skin, etc.. It is not allowed to have the shrinkage cavity, shrinking porosity, gas porosity, inclusion, and other cavity defects on the section including plane pasting through the pouring gate and the ball centre.After this operation the balls may be heat-treated. This hardens the balls, but it also changes their size. The size of bearing balls has to be perfect, sometimes within millionths of an inch, so a few more operations are needed after heat-treating. The balls next go through a grinding operation.