Wiktionary
ball mill
n. A device that grinds material very finely by means of a horizontally rotating drum containing ceramic or metal balls
Wikipedia
Ball mill
For the type of end mill, see Ball nose cutter.
A ball mill is a type of grinder used to grind and blend materials for use in mineral dressing processes, paints, pyrotechnics, ceramics and selective laser sintering.
Usage examples of "ball mill".
If you can imagine a huge cement mixer filled with cannon balls and limestone, you understand a ball mill.