The Millstones in a mill are where the grain is turned into flour. A set of stones consists of two discs of flat course stone that sit horizontally above one another. The Stones are set into a wooden canister called the Stone Case. The top stone or runner stone is the only one that moves, and it rotates above the bed stone. In between the stones is a small gap that is set and controlled by the miller. Grain is fed into this gap and is crushed between the stones releasing the flour. The flour then moves down channels that are cut into the surface of the stones and falls to the bottom of the stone case and through a chute in the floor.
Mills can have multiple sets of stones and Wheatley mill has two sets. The stones themselves are made from up of pieces of French Burr stone. They are not a single piece of stone but rather chunks of Burr stone cemented together, this is said to improve the quality of the flour. The Wheatley stones were in a poor condition but recently have been removed and reconditioned.
The picture above shows the stones before they were reconditioned, and the picture below shows them after the work was completed.
The Millstones in Wheatley mill are under-driven. This means that the power is applied from below the Millstones. This is accomplished by the Main Shaft turning the Spur Wheel, which then turns the individual Stone Nut for the set of stones. Protruding from the top of the Stone Nut is an iron Spindle (the Stone Spindle) that passes through the centre of the Bed Stone and it is to this Runner stone is connected and rests upon.
The Stone Spindle can be seen along with the Stone Nut in the picture of the restored stones below.
To keep the stones working to maximum efficiency regular maintenance is required to keep the milling surface flat, to learn more about this click on the following link to see about Dressing the Millstones.