Maintenance and Capacity

Part of Mill Construction

Keeping a mill running at full capacity requires systematic maintenance of all mechanical components — water wheel, gearing, millstones, and structural elements.

Why This Matters

A mill that breaks down at harvest time is worse than no mill at all. Grain piling up in the fields while the millstones sit idle means spoiled food, missed planting windows, and a community that loses faith in the technology that was supposed to help them. Mill maintenance is not an afterthought — it is the core skill that separates a mill that serves for generations from one that fails after a single season.

Understanding capacity — how much grain a mill can process in a day, a week, a season — is equally important. A community needs to know whether to build one mill or two, how large to build the storage bins, how to schedule the grinding queue, and when to plan maintenance shutdowns around the agricultural calendar. A miller who cannot answer these questions is not fully doing their job.

The principles here apply across mill types: grain mills, sawmills, fulling mills, and edge mills all share the same fundamental maintenance challenges of wear, alignment, and structural integrity.

Calculating Mill Capacity

Capacity depends on three variables: the speed of the millstones, the gap between them, and the hardness of the grain being ground.

A pair of millstones 1.2 meters in diameter turning at 90 RPM (a common working speed) will grind approximately 60 to 100 kg of wheat per hour under good conditions. Harder grains like rye grind more slowly; softer grains like millet somewhat faster. Wet grain grinds poorly and can clog the furrows — grain must be dry before milling.

To calculate daily capacity, assume the mill operates 10 to 12 hours in daylight (traditional mills rarely ran at night due to fire risk from unattended machinery). At 80 kg/hour for 10 hours, you get 800 kg per day. Allow 20% downtime for re-dressing stones, clearing blockages, and minor repairs: effective daily capacity is about 640 kg.

Weekly and seasonal capacity then follows: 640 kg/day × 6 working days = 3,840 kg/week. A community consuming 500g of flour per person per day (a moderate ration) needs about 350 kg/day for 700 people — well within one mill’s capacity for most of the year, with room for local trade.

Mill Diameter	Speed (RPM)	Output (kg/hr)	Daily Capacity
0.9 m	100	40–60	400–480 kg
1.2 m	90	60–100	480–800 kg
1.5 m	80	100–150	800–1,200 kg
1.8 m	70	150–220	1,200–1,760 kg

Wear Patterns and Inspection Schedule

Every component of a mill wears, but different components wear at different rates. Knowing what to inspect and when prevents catastrophic failures.

Daily inspection (before starting):

• Check that the millstone gap (the “draft”) is correctly set — use a feeler gauge or known-thickness wood sliver
• Listen for unusual vibration or grinding noise as the mill comes up to speed
• Check that grain feeds smoothly from the hopper into the eye of the stone
• Inspect the sluice gate and mill race for debris that could restrict water flow

Weekly inspection:

• Check the main shaft bearings for wear — look for vertical play (the shaft should not have measurable up-and-down movement)
• Inspect all wooden gear teeth for cracks, splits, or worn faces
• Check the drive belts or wooden cogs for loose or missing pins
• Inspect the millstone skirt (the wooden housing around the stones) for damage that could allow grain to escape

Monthly inspection:

• Measure the vertical wear on the main bearing — mark a reference line when new and check against it monthly
• Inspect the water wheel paddles or buckets for rot, splitting, or loose attachment
• Check the sluice gate mechanism — the gate must close fully to stop the mill
• Inspect the mill building structure: look for roof leaks that could wet the stones or soak the wooden gearing

Bearing Maintenance

The main shaft bearing is the single highest-wear item in a grain mill. In traditional construction, the main shaft rests in a “footstep bearing” — a metal cup (iron or bronze) set into a wooden block at the base of the shaft. The shaft end is rounded or pointed to minimize the contact area.

The footstep bearing requires lubrication. Traditional lubricants include:

• Tallow (rendered animal fat) — most common, applied every 2–3 days
• Lard — softer than tallow, good for high-speed bearings
• Olive oil or linseed oil — acceptable but burns off faster than grease
• Beeswax mixed with fat — longer-lasting than pure tallow

The bearing cup itself will eventually wear a hollow. When the hollow reaches about 3mm depth, the bearing needs replacing. Keep a supply of cast or forged iron bearing cups — they are small enough that even a basic forge can make them.

Side bearings (where the shaft passes through a wall or frame) should be checked for the shaft running true. If the shaft wobbles visibly at speed, a bearing is worn or misaligned.

Signs of imminent bearing failure

A bearing that squeals, produces smoke, or runs noticeably hot is on the verge of failure. Shut down immediately, allow to cool, re-lubricate, and inspect for damage. Running through a failed bearing can destroy the shaft.

Wooden Gear Maintenance

Traditional mill gearing used wooden teeth (called “cogs”) mortised into iron or hardwood wheels. The cogs are intentionally sacrificial — they are designed to fail before the iron wheel, protecting the more expensive component.

Cogs are typically made from apple, pear, hornbeam, or other dense fruitwoods. They are cut so the grain runs across the face of the tooth (end-grain cogs), which wears more slowly and evenly than side-grain.

Replacing cogs is routine work. Keep a supply of pre-cut blanks shaped to the correct profile. To replace a worn cog:

1. Stop the mill and lock the wheel against rotation
2. Drive out the old cog with a mallet and punch — it is held by wedging and friction
3. Clean the mortise (the slot in the wheel rim) with a chisel
4. Drive in the new cog blank, initially slightly oversized
5. Mark the contact point by rubbing the mating gear with chalk or charcoal
6. Pare the new cog to fit using a chisel until the contact is even across the full tooth face
7. Run the mill slowly and check for binding

A skilled miller can replace a cog in 20–30 minutes. Having a stock of pre-shaped blanks is the difference between a 20-minute repair and a two-day shutdown.

Stone Re-dressing Schedule

Millstones dull with use and must be periodically re-dressed (the furrows re-cut and the land surfaces re-leveled). The frequency depends on the hardness of grain being ground and the hardness of the stone.

A typical French burr stone grinding wheat needs re-dressing every 4–8 weeks of continuous operation. Harder stones (granite) may need re-dressing more frequently because the grain glazes the flat surfaces. Signs that re-dressing is needed:

• The flour becomes noticeably coarser even with the same stone gap
• The stones run hotter than usual
• Output drops for the same water flow

Re-dressing requires shutting down the mill, lifting the runner stone, and working the stone surface with a mill bill (a small pick). This is skilled work — see the Stone Dressing article for full technique.

Seasonal Maintenance Calendar

Season	Priority Tasks
Late summer	Full stone re-dress before harvest rush; inspect and replace worn cogs
Autumn	Run at full capacity; daily inspection only; stockpile spare cogs
Winter	Major overhaul — replace bearings, repair water wheel, re-dress stones, repair mill race
Spring	Test run before sowing season; re-lubricate all bearings

The winter shutdown is critical. Most mills froze their races and reduced water flow in winter anyway, making it the natural time for major work. A well-maintained mill entering harvest season should have fresh bearings, a re-dressed stone, new cogs where needed, and a clean, clear mill race.