From Longman Dictionary of Contemporary Englishgrindstonegrind‧stone /ˈɡraɪndstəʊn $ -stoʊn/ noun [countable] TTZa large round stone that turns like a wheel, and is used for making tools and knives sharp when they are rubbed against it → keep your nose to the grindstone at nose1(11)
Examples from the Corpus
grindstone• At the distillery, workers mash agave plants with a huge grindstone drawn around a deep circular pit by an ancient tractor.• He feels as much sympathy for these victims as he does for slaves who died centuries earlier turning some tyrant's grindstone.• The primary bevels were ground back and feathered away on the grindstone, taking care not to overheat and destroy the temper.• I believed the drill instructor was hard core, nose to the grindstone, always screaming and shouting.• Jim had decided he was going to keep his nose to the grindstone and avoid confrontations at work.• Now is the time for the golfer from Welwyn Garden City to put his nose to the grindstone once more.• Remind me not to be caught with my nose to the grindstone for too long.• This grindstone bubble fountain makes an attractive incidental feature in this narrow border and is not difficult to make. 