FLOUR MILL
A gristmill or grist mill is a building in which grain is ground into flour, or the grinding mechanism itself. In many countries these are referred to as corn mills or flour mills.
Classical British and American mills.
Classical mill designs are usually water powered, though some are windmills, or powered by livestock. A sluice gate is used to open a channel and so start the water flowing and a water wheel turning. In most such mills the water wheel was mounted vertically, i.e., edge-on, in the water, but in some cases horizontally (the tub wheel and so-called Norse wheel). Later designs incorporated horizontal steel or cast iron turbines and these were also sometimes refitted into the old wheel mills.
In most wheel-driven mills, a large gear-wheel called the pit wheel is mounted on the same axle as the water wheel and this drives a smaller gear-wheel, the wallower, on a main driveshaft running vertically from the bottom to the top of the building. This system of gearing ensures that the main shaft turns faster than the water wheel, which typically rotates at 10 rpm, or so.
The millstones themselves turn at around 120 rpm. They are laid one on top of the other. The bottom stone, called the bed, is fixed to the floor, while the top stone, the runner, is mounted on a separate spindle, driven by the main shaft. A wheel called the stone nut connects the runner's spindle to the main shaft, and this can be moved out of the way to disconnect the stone and stop it turning, leaving the main shaft going to drive other machinery. This might include driving a mechanical sieve to refine the flour, or turning a wooden drum to wind up a chain used to hoist sacks of grain to the top of the mill house.
The grain is lifted in sacks onto the sack floor at the top of the mill. The sacks are emptied into bins, where the grain falls down through a hopper to the stones on the stone floor below. The flow of grain is regulated by shaking it along a gently sloping trough (the slipper) from which it falls into a hole in the center of the runner stone. The milled grain (flour) is collected as it emerges through the grooves in the runner stone from the outer rim of the stones and it gets fed down a chute to be collected in sacks on the ground or meal floor. A very similar process is used for grains such as wheat, kamut, etc to make flour as well as for maize to make corn meal.
In order to prevent the vibrations of the mill machinery from shaking the building apart, a gristmill will often have at least two separate foundations.
Historically, gristmills contained rotating stones powered by water or by wind; later mills used steam engines for power, and modern mills typically use electricity or fossil fuels to spin heavy steel rollers. These techniques produce visibly different results, but can be made to produce nutritionally and functionally equivalent output.
Gristmills only grind clean grains, that is, grain from which stalks and chaff have previously been removed, but some mills also housed equipment for threshing, sorting, and cleaning prior to grinding. Gristmills also grind corn into meal.
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