Mill Construction

Why This Matters

A working mill multiplies human labor by a factor of 10 to 50. One person turning a hand quern grinds roughly 2 kg of flour per hour. A water-powered grain mill processes 50-100 kg per hour with no human effort beyond feeding grain into the hopper. Mills are the backbone of any community that has moved beyond subsistence farming.

Site Selection

The single most important decision is where to build. You need three things at the same site: a reliable water source with sufficient head (vertical drop), solid ground for the mill building, and road access for bringing grain in and flour out.

Evaluating Water Supply

Measure the flow rate of your stream during the driest month, not the wettest. You need a minimum of 30 liters per second for a small grain mill. Measure by timing how long it takes to fill a known container at the narrowest point.

Calculate available power:

Head (meters)	Flow (L/s)	Power (watts)	Suitable For
1.5	50	500	Small grain mill
3	50	1,000	Full grain mill
3	100	2,000	Grain + sawmill
6	100	4,000	Multiple operations

Formula: Power (watts) = Head (m) x Flow (L/s) x 9.81 x efficiency (typically 0.65-0.80)

Site Shortcut

Look for existing waterfalls or rapids. Even a 2-meter natural drop saves weeks of construction. Old mill sites are often identifiable by unnaturally straight channel sections or stone foundations near streams.

Ground Requirements

The mill building must sit on firm ground that won’t shift when loaded with heavy stones. Bedrock or dense clay is ideal. The floor of the mill should be at least 1 meter above the highest flood level you can determine from debris marks on trees and banks.

The Mill Race

The mill race is the channel that carries water from the stream to the waterwheel, and the tailrace carries it away after.

Digging the Race

1. Survey the route from the stream diversion point to the mill site
2. Maintain a consistent slope of 1:200 (1 cm drop per 2 meters of length) for the headrace
3. Dig the channel at least 60 cm wide and 45 cm deep
4. Line with clay if the soil is sandy or porous — compact in 10 cm layers
5. For permanent installations, line with flat stones set in clay mortar

Building the Headgate

The headgate (sluice gate) controls water flow to the wheel. Build it at the point where the race leaves the main stream.

Construction:

• Set two vertical posts (15 cm x 15 cm hardwood) in the race walls, with vertical grooves cut into their inner faces
• Slide a thick plank (5 cm oak or similar) into the grooves
• Attach a vertical rack (notched bar) to the gate and a simple pinion handle to raise and lower it
• Add a bypass channel around the gate for overflow during heavy rains

Flood Protection

Always build a bypass spillway upstream of the headgate. Without it, a sudden flood will overwhelm the race and may undermine the mill foundation. The spillway should direct excess water back to the main stream channel.

Millstone Selection and Dressing

The stones are the heart of a grain mill. Poor stones produce coarse, gritty flour that wears teeth and causes intestinal damage.

Choosing Stone

Best to worst options:

Stone Type	Hardness	Grain Quality	Availability	Notes
French burr	Excellent	Superior	Rare	Freshwater quartz, gold standard
Granite	Good	Good	Common	Must be low-mica variety
Basalt	Good	Good	Regional	Excellent if available
Sandstone	Fair	Acceptable	Common	Wears fast, needs frequent dressing
Limestone	Poor	Poor	Common	Too soft, contaminates flour

Select stones at least 75 cm in diameter and 15-20 cm thick. Both stones (runner and bedstone) should be the same material. Look for stone with a consistent grain and no cracks — tap with a hammer and listen for a clear ring, not a dull thud.

Shaping the Stones

1. Rough shaping: Use a pointed mason’s hammer to chip the stone into a cylinder. Work slowly around the circumference.
2. Face flattening: Place the stone face-down on a flat reference surface (large flat rock or leveled timber platform). Grind with sand and water until the face is flat within 1 mm across its diameter.
3. Center hole: For the bedstone, drill or chisel a center hole 8-10 cm in diameter for the spindle to pass through. The runner stone needs a larger rectangular slot (the “eye”) for grain to enter, about 15 cm x 10 cm.

Cutting Furrows

Furrows are the shallow grooves cut into the grinding face. They serve two functions: they cut the grain like scissors as the stones rotate, and they channel flour outward to the edge.

The harps pattern (most common):

1. Divide the stone face into 6 or 8 equal sectors
2. In each sector, cut 4-6 furrows radiating from the center toward the edge
3. Furrows should be 2 cm wide, 1 cm deep, with a sharp leading edge and a sloped trailing edge
4. The flat areas between furrows (called “lands”) do the fine grinding
5. Angle the furrows slightly — about 15 degrees from the radius line — so they create a scissoring action against the opposing stone’s furrows

Use a mill bill (a double-pointed chisel) and a thrift (a short-handled hammer). Wear eye protection — stone chips are razor-sharp.

Testing Your Dress

Sprinkle dry sand between the stones and rotate the runner by hand. The sand should move steadily outward and emerge evenly around the entire circumference. If it piles up in one area, the furrows in that sector need deepening.

Grain Mill Assembly

The Bedstone

The bedstone sits fixed and level on a sturdy timber frame (the “hurst frame”). Level it precisely — use a straight edge and water level. The bedstone never moves during operation.

The Runner Stone

The runner stone sits on top of the bedstone, separated by a gap of 0.5-2 mm. It rotates on the spindle via the rynd — an iron cross piece set into the underside of the runner stone that fits over the top of the spindle.

Spindle assembly:

1. The main vertical spindle passes up through the center of the bedstone
2. At the top, it supports the rynd and runner stone
3. At the bottom, it rests on a bearing (the “toe bearing” — a hardened steel cup on an adjustable bridge called the “tentering beam”)
4. Raising or lowering the tentering beam changes the gap between stones, controlling flour fineness

The Hopper and Shoe

The hopper is a large funnel-shaped wooden box mounted above the runner stone’s eye. Grain pours from the hopper into the shoe — a sloped trough that vibrates to feed grain at a controlled rate.

The shoe vibrates because it rests against the damsel — a square or octagonal rod attached to the spindle that wobbles as it turns, shaking the shoe. The classic “clacking” sound of a mill comes from this mechanism.

Feed rate control: Adjust the angle of the shoe. Steeper means faster feed. A string or cord from the shoe to a peg allows the miller to adjust from the floor below.

Power Transmission

From Waterwheel to Millstone

The waterwheel turns a horizontal shaft. The millstone needs to turn on a vertical shaft. This 90-degree power conversion is the central engineering challenge.

Standard gear train:

1. Pit wheel: A large gear (120-180 cm diameter) mounted on the waterwheel shaft inside the mill
2. Wallower: A smaller gear (40-60 cm diameter) mounted on the top of the vertical main shaft, meshing with the pit wheel
3. Main shaft: Vertical timber shaft running from wallower up through the bedstone to the runner stone
4. Great spur wheel: (optional, for multiple sets of stones) A large horizontal gear on the main shaft that can drive multiple smaller gears (“stone nuts”), each turning a separate pair of millstones

Gear Ratio	Waterwheel RPM	Millstone RPM	Application
3:1	8	24	Very slow — coarse meal
4:1	10	40	Standard wheat flour
5:1	10	50	Fine flour, fast output
6:1	8	48	Fast, risk of overheating

Overheating

If millstones turn too fast or the grain feed runs dry, friction heats the stones and can scorch flour or even crack the runner stone. A competent miller never leaves the mill unattended while stones are turning. Always listen for the change in pitch that indicates the hopper is running low.

Sawmill Design

A sawmill converts the same waterwheel power into linear cutting motion.

Vertical Sash Saw

The simplest and most historically common design:

1. Saw blade: A straight blade 1.5-2 m long, mounted vertically in a wooden frame (the “sash”)
2. Crank mechanism: The waterwheel shaft drives a crank that converts rotary motion to reciprocating (up-and-down) motion
3. Sash guides: The sash slides in vertical grooves in a heavy timber frame
4. Log carriage: The log sits on a movable carriage that advances toward the blade by a small, consistent amount on each down-stroke

Advance mechanism: A ratchet and pawl system on the carriage, triggered by the sash movement, advances the log 3-5 mm per stroke. This produces boards of consistent thickness.

Output: A water-powered sash saw cuts roughly 100-200 board-feet per day, compared to 30-50 board-feet for two men using a pit saw. The cut quality is also more consistent.

Circular Saw

More advanced but far more efficient:

1. Requires a forged steel disc 60-90 cm in diameter with teeth cut around the edge
2. The disc mounts directly on a horizontal shaft driven by the waterwheel
3. The log passes through on a carriage with rail guides
4. Cuts on both directions of travel (no wasted return stroke)
5. Output is 3-5 times higher than a sash saw

Starting Simple

Build a sash saw first. It requires no precision metalwork — just a straight saw blade and basic carpentry. Upgrade to a circular saw only when you have the metalworking capability to forge, harden, and sharpen a large steel disc.

Specialty Mills

Fulling Mill

Fulling mills process woven cloth by pounding it with wooden hammers to compress and thicken the fibers. The waterwheel turns a shaft with cams (raised bumps) that lift heavy wooden hammers (stocks) which then fall by gravity into a trough containing the wet cloth. Run cloth through warm water with fuller’s earth (a type of clay) or stale urine for degreasing.

Bark Mill

For leather tanning, you need large quantities of finely ground tree bark (oak bark is preferred). A bark mill uses a heavy edge-runner stone (a vertical stone rolling in a circular trough) driven by the waterwheel. Load chopped bark into the trough and let the stone crush it over several passes.

Threshing Machine

A rotary threshing drum separates grain from stalks. Mount a wooden cylinder with iron-tipped beaters on a horizontal shaft. Sheaves of grain are fed between the spinning drum and a curved iron grate (the concave). The beaters strip the grain from the heads, and the grain falls through the grate while straw passes over it.

Maintenance Schedule

Task	Frequency	Time Required	Tools
Check stone gap	Daily	5 min	Feeler gauge or paper strip
Grease bearings	Weekly	15 min	Tallow or rendered fat
Inspect furrows	Monthly	30 min	Straight edge
Redress furrows	Every 100-200 hours	4-8 hours	Mill bill, thrift
Replace toe bearing	Every 1-2 years	2 hours	Forge, lathe
Replace rynd	Every 3-5 years	Half day	Blacksmith
Race cleaning	Seasonally	1 day	Shovels

Stone Redressing

When flour quality drops (becomes coarser or gritty), the furrows need redressing. Lift the runner stone using a crane or lever hoist. Flip it upside down on timber supports. Using the mill bill and thrift, re-cut each furrow to its original depth and sharpen the leading edges. The lands between furrows should be slightly concave (dished) — about 0.5 mm lower in the center than at the edges.

Output Capacity

Plan your mill size based on community needs:

Population	Daily Flour Need	Millstone Size	Hours/Day
50	25 kg	75 cm	1-2
200	100 kg	90 cm	3-4
500	250 kg	120 cm	6-8
1,000	500 kg	150 cm (or 2 sets)	8-10

Single Point of Failure

A community that depends on one mill is vulnerable. If the stones crack, the waterwheel breaks, or drought reduces water flow, everyone goes hungry. Build at least one backup hand-powered mill (a large quern) and keep spare rynd iron and bearing stock on hand.

What’s Next

With a working mill, you can move into:

• Food Processing — Flour is just the beginning: bolting (sifting), malting for brewing, and oil pressing all use similar power take-off systems
• Woodworking — A sawmill transforms construction capability, enabling dimensional lumber production at scale
• Gear Making — Understanding the gear train in your mill is the foundation for all mechanical power transmission

Mill Construction — At a Glance

Minimum water: 30 L/s with 1.5 m head for small grain mill Millstone size: 75-150 cm diameter, 15-20 cm thick Best stone: French burr or granite; avoid limestone Key furrow pattern: Harps — 6-8 sectors, 4-6 furrows each, angled 15 degrees Gear ratio: 4:1 gives ~40 RPM for standard wheat flour Sawmill output: 100-200 board-feet/day (sash saw), 300-1,000 (circular) Critical maintenance: Redress furrows every 100-200 operating hours Safety rule: Never run stones without grain — friction will scorch or crack them Capacity: 75 cm stones process ~50 kg flour/hour for a community of 50-100