Many parts of machine tools like lathes need to be as flat as possible, because if the surface of the tool is not flat then that error is transferred to the object you are tooling. To get, say, a lathe bed flat, you buy or make a reference flat surface, paint the lathe bed blue, and then rub the surfaces together. The parts where the blue rubs off are the high parts. You take a scraper -- basically a modified chisel that takes off a thin layer of metal -- and scrape down the high parts. Wipe off the paint, repeat until there are no more high spots. It is very tedious.