Millstone - Grinding With Millstones

Grinding With Millstones

Grain is fed by gravity from the hopper into the feed-shoe. The shoe is agitated by a shoe handle running against an agitator (damsel) on the stone spindle, the shaft powering the runner stone. This mechanism regulates the feed of grain to the millstones by making the feed dependent on the speed of the runner stone. From the feed shoe the grain falls through the eye, the central hole, of the runner stone and is taken between the runner and the bed stone to be ground. The flour exits from between the stones from the side. The stone casing prevents the flour from falling on the floor, instead it is taken to the meal spout from where it can be bagged or processed further.

The runner stone is supported by the rind, a cross-shaped metal piece, on the spindle. The spindle is carried by the tentering gear, a set of beams forming a lever system with which the runner stone can be lifted or lowered slightly and the gap between the stones adjusted. The weight of the runner stone is significant (up to 1500 kg) and it is this weight combined with the cutting action from the porous stone and the patterning that causes the milling process.

Especially in the case of wind powered mills the turning speed can be irregular. Higher speed means more grain is fed to the stones by the feed-shoe and grain exits the stones quicker because of their faster turning speed. The miller has to reduce the gap between the stones so more weight of the runner presses down on the grain and the grinding action is increased to prevent the grain being ground too coarsely. It has the added benefit of increasing the load on the mill and so slowing it down. In the reverse case the miller may have to raise the runner stone if the grain is milled too thoroughly making it unsuitable for baking. In any case the stones should never touch during milling as this would cause them to wear down rapidly. The process of lowering and raising the runner stone is called tentering and lightering. In many windmills it is automated by adding a centrifugal governor to the tentering gear. Depending on the type of grain to be milled and the power available the miller may adjust the feed of grain to stones beforehand by changing the amount of agitation of the feed-shoe or adjusting the size of the hopper outlet. Milling by millstones is a one-step process in contradiction with roller mills in modern mass production where milling takes place in many steps. It produces wholemeal flour which can be turned into white flour by sifting to remove the bran.