"Tumbling" is the process of using a vibratory tumbler to polish metal pinball parts. You can buy a tumbler from a gun shop. you need tumbling media - untreated corn cob - and also a cleaning solution - Turtle Wax that you can buy at bunnings.

You wash the parts, dry them, then place them in the tumbler with the media and cleaner. come back in 4 hours, and the parts will appear completely polished.

I have polish screws etc - they are OK to polish in a tumbler, but I like them to look better then new.