In this animation of an overshot wheel water pours over water wheel spinning the horizontal shaft and the crown gear which in turn spins the lantern pinion which in turn spins the runner stone against the bed stone. Grain poured into the hopper through the center or eye of the runner stone is ground into flour or meal which is pushed out from between the stones and into the vat for collection in the meal chest.
The initial preparation of the stone depends very much on its source. Blanks would be cut from a mill stone grit quarry as a single piece, but a set of French burr stones has to be hand assembled from a number of distinct pieces. However, mill stone grit is not as resilient as French Burr and small pieces of grit would go into the flour making it unsuitable for domestic use. But mill stone grit can be and is used in the milling of animal food.
To turn blocks of burr stone into a usable millstone, the millwright will first prepare the pieces of his jigsaw puzzle such that they fit together to give him the overall shape and size he requires. A typical millstone will be about 4ft in diameter. Once the pieces have been put together, a set of iron tyres are then fitted around them. This is done in exactly the same way as a wheelwright would prepare a wooden wagon wheel. When cold, the tyre would be just a little bit too small to fit around the stones, but when heated it would expand until it was just large enough to drop into place. As the metal cools, it will attempt to shrink back to its original size, thus gripping the pieces of the stone very tightly indeed.
Dressing the millstones is one of the most important aspects of routine maintenance carried out in a mill. A poorly maintained set of millstones is of no use to anyone. After the basic stone has been prepared, the surfaces are smoothed and a pattern of grooves is cut into them and it is these furrows that do the real work. As the stones are used, the furrows gradually become worn and must be refreshed or dressed. This work was often done by the miller himself, but in many areas you would find itinerant stone dressers, who would travel from mill to mill doing a few days work here and a few days work there.
As an aside, the growing of wheat and the milling of wheat are intertwined so it is of no surprise that the grooves that are cut into the wheel are called furrows in the same way that a plough cuts a furrow in a field.
From the animation on the right you can see here how the rotation of the stones causes the furrows to cross one another giving the effect of hundreds of pairs of scissors. You can also see how the angle at which the furrows cross will cause the ground meal to work its way out towards the edge of the stones. In real life, the runner stone (coloured red) would be rotating at approximately 100 to 150 revolutions per minute. The bed stone (coloured black) remains static.