A post mill is a form of windmill consisting of a massive central oak post, which is held upright and supported by four “quarter bars” . The whole of the mill body (the buck), which houses the mill stones, all of the process machinery and the bins for temporary storage of grain and milled flour, is free to rotate about the post so that the mill can be turned to face the wind at any time.
The main-post is held in the vertical position and appears to rest on the trestle, a horizontal timber cross beneath it, but this is an illusion. The vertical load is fully supported by the four oak quarter bars and translated into tension in the trestle members, known as cross-trees, which tie together opposite pairs of quarter bars. Each quarter bar is tenoned into the main-post at the top and into a cross-tree at the bottom, but not pinned or doweled at either end. The cross-trees rest on top of four supporting brick piers.
At approximately the vertical centre of the body of the mill is a large oak cross beam (crowntree) with a cast iron bearing in the underside to support the entire weight of the buck resting on top of the central post. The buck is maintained upright by a loose fitting collar, which forms a radial bearing, located in the bottom floor of the buck and bearing against the central post at about its overall mid height. To provide additional stability there is also a vertical reaction on the buck from the rear steps which are hinged to the lower floor at their top end and rest on the ground at the bottom. The steps are downwind of the mill and provide a form of strut to the buck.
The mill has two sets of stones, each set made up of an upper runner stone, positioned over a fixed bed stone. At the front or head of the mill are Derbyshire Peak stones, Used for producing wholemeal flours,each are made from a single piece of rock quarried in the Peak District. In the back or tail of the mill are French Burr stones used for making white flour.
Wind power was converted to stone power with this huge windshaft fitted with a front brake wheel, directing drive to the rear tail wheel. Drive shafts would have then powered the stone sets.
At the top of the windmill is the Bin floor.
At this level the miller would raise the sacks of grain and fill grain bins to provide a constant supply to the stones below.
Powered by large sweeps or sails the windmill was designed to face the wind, as this provided constant drive to the mill’s machinery.
When the wind changed direction the miller would manually rotate the entire top section, using the tail pole as a giant lever. This would have taken some effort as the weight of the upper building is more than 10 tons.