Diagnose a mill that grinds badly by checking the cheapest causes first: the feed rate, the stone gap and the grain’s moisture, then whether the stones need dressing, then balance and bearings, and last the power supply. A mill that slows when the gap is closed is usually fed faster than its power can grind: 775 W at the stone grinds fine flour at no more than 775 / 60 = 12.9 kg an hour.
Part of Community Grain Mill
A symptom-by-symptom guide to faults in a stone grain mill: coarse, hot, gritty or slow output, vibration and noise, what usually causes each and what to do.
Check Feed, Gap And Moisture Before You Touch The Stones
Most faults come from three settings the miller controls every run: how fast grain is fed, how wide the stones are set, and how dry the grain is. They cost nothing to check. Only when those are right do dressing, balance, bearings and power come into play. The wiki’s mill pages give the gap for coarse meal as 1-2 mm and for fine flour as 0.1-0.5 mm, and a rumble as stones too close and a hiss as too open (Watermill Design).
The Symptom Table
| Symptom | Likely causes, cheapest first | Fix |
|---|---|---|
| Flour coarse | Gap too wide; feed too fast; furrows worn or lands glazed; bearing worn so the runner sits off-level | Close the gap a little; slow the feed; dress (Stone Dressing Schedule); measure the footstep |
| Flour hot, scorched smell | Gap too close; hopper empty and stones running bare; glazed lands; stone speed too high (the wiki’s mill page says speed above 120 rpm heats and burns flour) | Open the gap; refill the hopper; dress; check wheel speed |
| Meal damp, stones clog | Grain above 14% moisture | Dry the grain (Grain Moisture and Milling Readiness); brush the stones |
| Mill slows or stalls when the gap is closed | Feed faster than the power can grind; rising bearing friction; binding gears; low flow or wind | Slow the feed; grease and feel the bearings; check the supply |
| Output down more than 20% at the same gap | Furrows and lands worn | Dress |
| Grit in the flour | Soft or weathering stone shedding; chips from dressing; stones touching | Discard the first batch after dressing; sift through fine cloth; if it persists the stone is unsuitable |
| Thumping, vibration, noise from the stones | Runner out of balance; loose rynd; a damaged gear tooth | Rebalance (Runner and Bedstone Mechanics); tighten the rynd; replace the tooth |
| Feed uneven | Shoe angle wrong or damsel worn | Adjust the shoe; replace the damsel (Feed Mechanism) |
| Wheel slow at the same flow | Choked tailrace; leaks; debris; worn bearings | Clear the tailrace and race; seal the leaks; grease |
| Flour smells rancid or musty | Old flour in the bins; damp grain | Clean the bins; dry the grain; mill less at once |
Worked Example: A Stall At The Fine Setting
A small stream gives 775 W at the stone (the 0.05 m3/s, 3 m head case in Bushels per Hour by Power Source). The miller closes the gap for fine flour, which needs 60 Wh/kg, and sets the shoe to feed 30 kg an hour. The stones need 30 x 60 = 1,800 W, far more than 775 W, so the runner slows and stalls. The fix is not to open the gap but to cut the feed to no more than 775 / 60 = 12.9 kg an hour, or to grind coarse (775 / 15 = 51.7 kg an hour at the low-energy end) and sift. If the stones still stall at a feed of 12.9 kg an hour, the fault is in the drive: grease the bearings, check the gears, and measure the flow.
Method: Finding The Fault
Step 1: Stop and lock out the drive, then look: is the hopper full and the shoe moving?
Step 2: Check the feed and moisture. Is the grain dry and clean, and is the feed within the power?
Step 3: Check the gap by feel and sound with the drive running and empty.
Step 4: Feel the bearings and listen to the gears after a short run.
Step 5: Check the face. Is the flour warmer and coarser at the same gap than last week? Check the hours since the last dressing.
Step 6: Check the power: the wheel’s flow and tailrace, or the wind, or the animals’ condition.
Common Mistakes
- Opening the gap to cure a stall. The stall is a power shortage; cut the feed.
- Dressing a mill whose real fault is a hot bearing. Check all the cheap things first.
- Blowing dust off the stones. Blowing or beating dust raises a cloud that can explode.
If a fire starts in flour dust
Flour dust ignites from sparks, bare running stones and hot bearings. Close the gate or disengage the drive first, get everyone out, and do not blow, beat or hose the dust, because that raises a cloud that explodes. Smother a small fire with damp sacks or sand from a safe distance. The stop is the pinned gate and the disengaged stone nut, and the stones take seconds to tens of seconds to stop after the drive is cut (Runner and Bedstone Mechanics).
What’s Next
- Learn next: Grain Mill Maintenance - the routine that prevents most of these
- Combine with: Stone Dressing Schedule - the fix for the commonest one