grindstone

详细解释:n. 磨石, 砂轮, 石磨
【机】 磨石

英语解释：

名词 grindstone:

a revolving stone shaped like a disk; used to grind or sharpen or polish edge tools

例句：

The teacher believed in hard work and always keep our nose to the grindstone.老师提倡勤奋，而且也总是要我们埋头读书。