An edge mill, or chilian mill, is a grinding device used to crush or mill ore and grain. The mill utilizes a pair of round stones or weights positioned on their edges that revolve around a central pivot in a shallow, round milling bed. The grain or ore is poured into the milling bed where the edges of the rotating stones crush them. Early examples of the edge mill were powered by animals, wind, or water, with later power sources including steam and internal combustion engines or electric motors. In the early to mid-1900s, the edge mill was also frequently used in the production of explosives such as black powder.
Stones have been used as universal milling agents since the beginning of time. Round stones rolled across grain spread on flat surface proved to be one of the most efficient methods of milling, with stone mills still in everyday use in many global regions. The edge mill was one of the more efficient stone mill types. The mills consist of a driven vertical shaft through which an axle passes at right angles. A pair of large, round stones or weights with holes in their centers are then fitted onto the axle, one slightly closer to the central shaft than the other.
When the driven shaft rotated, the stones rolled around its axis on their edges in a round, flat milling bed. The milling bed was typically fitted with a low rim that formed a shallow basin to stop spillage of milled material. Various types of ore and grain were poured into the milling bed where the rotating stones crushed them between their edges and the bed surface. The fact that the one wheel was placed closer to the central shaft than the other meant that the stone's tracks overlapped, covering a wider area in the process. Varying edge textures and adjustable stone heights gave mill operators a degree of control over how finely the material was ground by the mill.
Various power sources have been used to drive the edge mill over the ages. Early mills were driven by oxen, donkeys, or horses, with later types driven by wind-mill or water-wheel mechanisms. As technology advanced, the edge mill was usually driven by electric motors, and steam, petrol, or diesel engines. In addition to the grinding of ore and grain, these mills were widely used in the production of chemicals and explosive components. Many plants producing high-quality black powder for blasting explosives and munitions production used the edge mill to grind the powder components prior to mixing.