Post mill
A whole building balanced on one oak post so it can be turned to face the wind — the first machine that let a settlement mill grain where there was no falling water.
Build
Tier 3 · Medieval to early modern
Energy
Hard — the largest timber structure in the early tiers
A season for a crew that has built frames before
Crew 6–10
Home: Mechanical power, Level 3 · Mill, level 1 of 2 in its track. Serves 3 steps across 2 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A watermill needs falling water, and most settled ground has none. Wind is everywhere, and it is free of the one thing that makes water power expensive: the weir, the leat and the flood that destroys them both.
What it costs instead is that the wind changes direction and strength constantly, and the machine has to cope with both. The post mill's answer is the oldest and the most direct: mount the entire building on a single vertical post and turn the whole thing, sails, gearing, stones and miller, until the sails face the wind. It is crude compared with a tower mill, where only the cap turns, and it needs no iron track, no curb and no fantail — which is exactly why it comes first.
The power available is proportional to the swept area and to the cube of the wind speed. Doubling the wind gives eight times the power, and that is the whole difficulty of the machine: the same mill must work in a breeze and survive a gale. Everything below — the twist in the sails, the brake, the furling, the tail pole — exists to manage that range.
Section — trestle, post and buck
Fig 1
Earth / subsoilMasonry / brickTimberGravel / aggregate
The sail, and why it is twisted
Fig 2
Timber
Keyed parts
| 1 | Brick or stone piers | Four piers carrying the cross-trees clear of the ground so the timber stays dry. They are not foundations for the post; they carry the trestle. |
| 2 | Cross-trees | Two heavy beams laid crosswise on the piers. Everything above stands on this cross. |
| 3 | Main post | One oak baulk, the whole mill's bearing. Its head is rounded to take the crown tree; its foot is mortised into the cross-trees. |
| 4 | Quarter bars | Four diagonal braces from the ends of the cross-trees to the post. They stop the post racking; without them the whole trestle folds. |
| 5 | Crown tree | The single beam bearing on the post head. The entire buck pivots on it — one enormous bearing. |
| 6 | Buck (the body) | The building itself: frame, weatherboarding and roof, carrying stones, gearing and miller. Twenty to thirty tonnes turning on the crown tree. |
| 7 | Windshaft | The inclined shaft the sails drive. Tilted so the sails clear the buck and so their weight helps hold the shaft in its bearings. |
| 8 | Brake wheel and brake | A great wooden-cogged wheel on the windshaft, with a wooden brake band round its rim. It transmits the power and it is the only thing that stops the mill. |
| 9 | Stone nut and millstones | A small gear meshing with the brake wheel, driving the runner stone above a fixed bed stone. The step-up sets the stone speed. |
| 10 | Sails, tail pole and ladder | Four sails on two crossed stocks; the tail pole and ladder are levered round to bring the mill into wind and then staked down. |
Specifications
| Sail span, tip to tip | 18–22 m | Four sails on two crossed stocks. Span sets the swept area, and the swept area sets the power |
| Swept area | 250–380 m² | Power is proportional to this and to the cube of wind speed |
| Sail twist | 20–25° at the heel to 5–10° at the tip | Built into the whips. The tip travels far faster than the heel and must meet the wind flatter |
| Windshaft inclination | 5–15° from horizontal | Lifts the sails clear of the buck; the sails' weight settles the shaft into its bearings |
| Sail speed | 10–20 rev/min | Tip speed 5–8 times wind speed. Above that the sails stall and hammer |
| Gear ratio, brake wheel to stone nut | 6:1 to 8:1 | One step up. A post mill drives one pair of stones |
| Runner stone speed | 100–140 rev/min | Faster burns the meal; slower crushes rather than cuts it |
| Millstone diameter | 1.2–1.5 m | Bed stone fixed, runner stone above, both dressed with furrows |
| Working wind | 5 m/s to start, 7–12 m/s working | Below 5 it will not turn under load; the range it actually earns in is narrow |
| Furling wind | About 15 m/s | Beaufort 7. Take cloth off before this, not during it |
| Main post | Oak, 550–750 mm square, 4–5 m | One baulk. The largest single timber in the settlement |
| Buck weight | 20–30 tonnes | All of it bearing on the crown tree and the post head |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Oak for post, cross-trees and quarter bars | ≈ 6 m³ | Heartwood, slow grown, well seasoned. The post is the one timber that cannot be replaced without dismantling the mill. |
| Oak and elm framing for the buck | ≈ 8 m³ | Elm tolerates the wetting and drying at the sills. |
| Weatherboarding and roof shingles | ≈ 120 m² | Consumable. Design so boards can be replaced from a ladder. |
| Sail cloth | 40–60 m² | Heavy canvas. Carry at least one full spare set; cloth is the first thing a gale takes. |
| Iron gudgeons, straps, bands and the neck bearing | 80–150 kg | Forged. The windshaft neck is the hardest-worked bearing in the mill — see the forge build. |
| Millstones, bed and runner | 2 | Hard, open-grained stone that will hold a dressed furrow. Quartz conglomerate is best; soft stone sheds grit into the flour. |
| Hornbeam, apple or crab for gear cogs | ≈ 0.4 m³ | Cut so the grain runs the length of the cog. Cogs are meant to break before the wheel does — keep a box of spares cut and seasoned. |
| Brake band timber and iron | 1 set | Wood on wood. The band wears; the wheel rim must not. |
| Tallow and beeswax | Continuous | Bearings and the brake path. A dry bearing here starts a fire. |
Tools: Pit saw and adze for converting the post and cross-trees · Augers, chisels and a framing square for the mortices · A gin pole or shear legs to raise the post and set the crown tree · Forge and anvil for gudgeons, straps and bands · Mill bill and thrift for dressing the stones · Plumb bob and long straightedge
Construction
Choose ground for wind, not for convenience
A mill wants clear, exposed ground — a ridge, a rise, the windward edge of the settlement. Obstacles matter far more than people expect: a building or a belt of trees disturbs the air for roughly twenty times its own height downwind, and the turbulence that arrives is worth less than the wind speed it steals, because it hammers the sails instead of pushing them. Watch a candidate site for a season if you can, and note the quarter the working winds come from.
✓ Check: Nothing taller than the piers within about twenty times its height upwind of the chosen spot, in the prevailing quarter.
Lay the piers and cross-trees
Four masonry piers on firm subsoil, levelled to each other within 10 mm across the whole trestle. The cross-trees are notched over each other at the centre and bedded on the piers, carrying the mill clear of wet ground. Dig a drain so water never stands under the trestle.
✓ Check: Cross-trees level in both directions, bearing evenly on all four piers with no rock.
Raise the post and fit the quarter bars
The post foot is mortised into the crossing of the cross-trees. Raise it with shear legs and stay it immediately. The four quarter bars then run from the ends of the cross-trees up to the post — these are what stop the whole trestle folding sideways, and they must be tight and equally tensioned, not merely present. The post is finished with a rounded head that the crown tree bears on.
✓ Check: Post plumb within 1:200 in both planes, and all four quarter bars driven home so none can be moved by hand.
Set the crown tree and frame the buck on it
The crown tree is a single beam seated on the post head; the buck is framed around and on it. The whole building will pivot here, so this bearing surface and its collar decide how easily twenty-odd tonnes can be turned by two people with a lever. Grease it, and build so it can be greased again without dismantling.
✓ Check: With the buck framed but unboarded, two people on the tail pole can swing it through a full circle.
Hang the windshaft, brake wheel and stone nut
The windshaft goes in inclined 5–15°, its neck running in a greased bearing at the breast and its tail carried at the back. The brake wheel is keyed to it inside the buck, and the stone nut meshes below. Set the gearing for a 6:1 to 8:1 step-up, so a sail turning 15 rev/min drives the runner stone near 120 rev/min. Cogs are shaped and eased individually until the mesh rolls without knocking.
✓ Check: Turned by hand with no stones engaged, the gear train runs a full revolution with no tight spot and no cog knocking.
Make and hang the sails
Two stocks pass through the shaft head, carrying four whips. Each whip is framed with sail bars and a hemlath, and twisted along its length — 20–25° of weather at the heel easing to 5–10° at the tip. Hang opposite sails within a few kilograms of each other: an unbalanced pair shakes the whole mill on the post. Cloth is spread on the frames and reefed by the miller from the ladder, sail by sail, with the mill stopped.
✓ Check: Sails balance — with the brake off in still air, the cross settles in no preferred position.
Fit the brake and learn it before you need it
A wooden band round the rim of the brake wheel, tightened by a lever the miller can reach from the stone floor and from outside. This is the only thing that stops the mill, and it is also the commonest cause of a mill burning down: applied too hard in a strong wind, the band and the rim heat until the timber lights. Stop by taking area out of the wind first — reef or furl the sails — and use the brake to hold a mill that is already slowing.
✓ Check: The brake holds the mill stationary in a fresh breeze with the sails bare, and releases without the lever fighting back.
Dress and set the stones
Both stones are dressed with radial furrows that cut and then carry the meal outward. The gap between them is set while running — tentering — to a fraction of a millimetre, the miller judging it by the feel and warmth of the meal between finger and thumb. Stones must never touch under load: they spark and glaze, and a spark in a mill full of flour dust is how mills explode.
✓ Check: A handful of meal from the spout is even and cool, and the stones have not touched at any wind the mill has run in.
Turn her to wind, and learn to watch the sky
The tail pole and its ladder are levered round until the sails face the wind, and then staked. The mill must be turned before the wind comes round, not after: wind arriving from behind gets under the sails and can lift the buck off the post. In a rising wind, reef; in a gale, furl completely and leave the sails bare and set in a cross, with the brake on and the mill turned side-on to the wind.
✓ Check: The crew can turn the mill and furl all four sails inside the time it takes a visible squall to arrive — timed, in practice, before it is needed.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Post plumb | 1:200 | An out-of-plumb post loads the crown tree unevenly and the buck becomes hard to turn. |
| Cross-tree level across the trestle | < 10 mm | Rock in the trestle works the mortices loose over a season. |
| Brake wheel run-out at the rim | < 15 mm | A wheel out of round grabs the brake band in one spot and burns it there. |
| Sail pair balance | Within a few kg | Unbalanced sails shake the buck on the post at every revolution. |
| Stone gap, running | A fraction of a millimetre, never touching | Touching stones glaze the dress, burn the meal and throw sparks into flour dust. |
| Cog mesh | Rolling contact, no knock at any point | A knocking cog breaks, and a broken cog at speed takes several more with it. |
How this fails
Fire from the brake. Applying the brake to stop a mill running hard in a strong wind heats band and rim until they ignite. Take the sails out of the wind first; brake only to hold.
Fire from the stones. Stones allowed to touch under load spark, and flour dust in air is explosive. Tenter carefully and never run empty stones together.
Being tailwinded. The wind comes round while the mill is unattended, gets behind the sails, and lifts the buck off the post. Turn ahead of the weather, and never leave a mill set with the wind forecast to back or veer.
A gale on set sails. Sails left clothed in a rising wind run away, and a runaway mill destroys its own gearing or throws its sails. Furl early; cloth is cheaper than a windshaft.
Quarter bars fitted slack. The trestle racks, the post leans, and the buck becomes impossible to turn — then unsafe.
A sail tip strike. Sail tips sweep to within a metre or so of the ground and are moving far faster than they look. Nobody walks in front of a mill with sails set; this has killed millers, children and livestock for centuries.
Green timber in the post or cross-trees. They shrink, the mortices slacken, and the whole trestle has to be taken apart.
Siting behind an obstacle. A mill in disturbed air works a fraction of the hours it should and is shaken while it does.
How you know it worked
VerificationThe mill has run under load through a full season, ground at its rated output in a working wind, been furled and turned ahead of at least one gale without damage, and the brake has stopped it from working speed without the band scorching.
Scaling and variants
Smaller: a single-pair post mill of 12–14 m span is a two-person build and still grinds for a hamlet, at perhaps a third of the output. Below about 8 m span the trestle and gearing cost nearly as much as a full mill and return very little; use wind for pumping instead.
Larger: more sails, or longer, gain area — but stocks and whips become the limit, and a sail that fails at speed destroys the mill. The real step up is a different machine: a tower or smock mill, where the body is fixed and only the cap turns, so the building can be masonry, carry several pairs of stones, and be turned by a fantail automatically rather than by hand.
Different work: with the stone nut replaced by a crank and rods, the same trestle and sails drive pumps for drainage or irrigation — the machine that drained the Fens and the Dutch polders. For lifting water only, a multi-blade fan wheel on a lattice tower is far simpler, self-regulating and self-furling, and is the right answer where the job is water rather than flour.