Keep Your Nose to the Grindstone means keep your eye on the milling
or else you may clog up the millworks. I better keep my nose to the
grindstone to finish this blog today.
Put through the mill means to be put between two stones and ground
up. When you go to college you are put through the mill.
Run of the mill means what is normally ground up at the mill. The
Aldie Mill's run of the mill was wheat and corn.
Have a millstone around one's neck means to have a heavy burden
to solve. According to Al Gore the global warming problem is a
millstone around our world's neck.