A grain mill grinds between two stone discs: a fixed bedstone below and a rotating runner above, 30-48 inches across for a community mill, with the faces cut into furrows and flat lands. The runner of a 4-foot pair weighs roughly 500-800 kg, so every step of choosing, shaping and lifting the stones has to be planned for that weight.
Part of Community Grain Mill
The pair of stones at the heart of a grain mill: what each stone does, the sizes used, and how stone choice, dressing and balance fit together.
Two Stones, One Fixed And One Turning, Make Flour By Shearing
The bedstone sits level and does not move. The runner turns above it on a spindle, driven through an iron cross-piece called the rynd, and a small gap between the faces is set by raising or lowering the spindle. Grain enters through the central opening (the eye) and is cut, torn and crushed as it moves outward across the faces until it leaves at the rim as meal. The community grain mill note describes the faces as very slightly shaped so that the gap narrows from the eye to the edge, grinding progressively finer.
The work is done by the lands, the flat surfaces between the furrows; the furrows are channels that carry grain and flour outward and give sharp edges that shear against the opposite stone’s furrows. The details of the pattern are in Dressing and Furrowing Patterns.
Sizes Used In A Community Mill
The carrier gives 30-48 in across (0.76-1.22 m) and 6-12 in thick when new. A granite stone weighs its volume times its density. At 2,650 kg/m3 (a typical granite density, recalled) a 36 in (0.914 m) stone has an area of pi x 0.457^2 = 0.656 m2, so at 6 in (0.152 m) thick it is 0.656 x 0.152 x 2,650 = 265 kg (585 lb) and at 12 in thick 530 kg (1,170 lb). The 300-500 lb the carrier gives for a 36 in granite stone corresponds to a stone only about 3-5 in thick, so treat the thickness and weight figures as two different stones, not one. A 1.2 m runner about 25 cm thick comes to about 730 kg, worked out in Runner and Bedstone Mechanics; the bearing page in the corpus puts a loaded millstone at 500-800 kg (Bearing Replacement).
What You Need
- Stone of the right kind - hard, even, slightly porous (Stone Selection and Quarrying)
- A way to move several hundred kilograms - levers, rollers, a crane or a tripod and hoist
- Steel tools - a mill bill and thrift for dressing, hardened and tempered (Tool Steel)
- A frame (the hurst) and a spindle to carry and drive the pair (Grain Mill Design)
Method: From Block To Working Pair
Step 1: Choose the stone. Test for hardness, even texture and a clear ring when struck.
Step 2: Rough out and flatten. Shape the block to a disc, then work the face flat against a straightedge.
Step 3: Cut the eye and set the drive. The runner needs a central opening and a seat for the rynd.
Step 4: Dress the faces. Lay out the harps, cut the furrows, crack the lands.
Step 5: Balance the runner. An unbalanced runner shakes the spindle and hammers the stones together.
Step 6: Set the gap and try a small batch. Adjust by feel and sound until the flour is the right grade.
Common Mistakes
- Using the biggest stone the power cannot turn. A bigger stone needs more watts; see Bushels per Hour by Power Source.
- Skipping the second stone. A mill needs a matching bedstone dressed to mate with the runner, not just a runner.
- Lifting without a plan. Most injuries are a runner dropped during dressing.
Weight and dust
A runner of several hundred kilograms crushes a hand or foot instantly; lift it only on a rated crane or tackle and never stand under it. Dressing throws sharp chips and silica dust: eye protection and a damp cloth over mouth and nose are the minimum, and dressing is done outdoors or with the doors open. Do not dress stones in an enclosed space.
What’s Next
- Learn next: Stone Selection and Quarrying - getting a block that works
- Combine with: Millstone Selection - the longer materials comparison