Stone Assembly

Part of Mill Construction

Installing millstones correctly — setting the bedstone level, mounting the runner on the spindle, and adjusting the draft — determines grinding quality and stone longevity.

Why This Matters

A millstone that is installed incorrectly will not grind well regardless of its stone quality or furrow pattern. The bedstone must be perfectly level; the runner stone must run true; the gap between them must be precisely and uniformly controlled. These are not aspirational ideals — they are the specific technical requirements that separate flour from grit-contaminated meal.

Poor installation causes accelerating problems. A runner stone that wobbles puts uneven pressure on the stone surface, wearing furrows unevenly and causing the stone to need re-dressing more frequently. A bedstone that is tilted causes the flour to exit from one side only, losing fine flour to the other. A poorly adjusted gap allows grain to pass unground on one side while scoring the stones on the other.

The assembly process described here represents the accumulated knowledge of generations of millers who discovered through trial and painful error exactly how these tolerances must be maintained. Getting it right the first time saves weeks of corrective work later.

Components of the Stone Assembly

The bedstone (lower stone): stationary, resting on a timber or stone support called the “hurst frame.” The bedstone never rotates. Its upper face (the working face) must be set perfectly horizontal — any tilt causes uneven grinding.

The runner stone (upper stone): rotates on the vertical spindle. The runner is driven through a bar of iron (the “mace” or “rind”) set across the eye of the stone and connected to the spindle. The runner floats on a thin film of grain between the two stones — it is not rigidly attached to any fixed vertical support.

The spindle: the vertical shaft that transmits rotation from the wallower to the runner stone. The spindle’s lower end rests in a footstep bearing (the “bridgetree” bearing), which can be raised or lowered to adjust the runner stone height — and thus the grinding gap.

The stone furniture: the wooden housing that surrounds the stones, keeping flour in and debris out. Includes the vat (the main enclosure), the hopper (the funnel that feeds grain into the stone eye), and the shoe (the vibrating trough that meters grain flow).

Installing the Bedstone

The bedstone is the foundation of the assembly. It must be set once, correctly, and then not moved.

1. Prepare the hurst frame: the timber frame that supports the bedstone. It must be rigid (no flex under the weight of the runner stone and vibration of milling) and level in both axes.

2. Lift the bedstone into position: use a system of levers and rollers — a 1.2m diameter granite stone weighs 500–700 kg and cannot be lifted by hand. Two people with long levers and wooden rollers can move such a stone safely.

3. Level the bedstone: Place a long straightedge across the working face in multiple directions. Use a spirit level (or a water level) on the straightedge. Adjust by shimming under the hurst frame. The stone face must be level to within 1mm across its full diameter — about 1 part in 1,200 for a 1.2m stone.

4. Secure the bedstone: Once level, wedge and pin the bedstone firmly to the hurst frame so it cannot rock. Traditionally, the stone is bedded in a layer of plaster of Paris on the hurst frame, which sets rigid. Do not use ordinary lime mortar — it is too weak to resist the vibration.

5. Install the rynd: the iron bar that crosses the eye of the bedstone and supports the bottom of the spindle bearing. The rynd must be level and centered.

Installing the Spindle and Runner

The spindle sits in its footstep bearing below the hurst frame. It must be perfectly vertical — use a plumb bob to check in two perpendicular directions. The spindle top should be exactly centered in the eye of the bedstone. If it is off-center, the runner will orbit eccentrically and vibrate.

Checking spindle vertically:

• Hang a plumb bob from a fixed point directly above the spindle center
• Measure the distance from the plumb line to the spindle in multiple directions
• Adjust the footstep bearing position until all measurements are equal

Installing the mace (rind): the iron bar that drives the runner. It fits into slots cut in the stone across the eye. The mace must be:

• Level (check with a spirit level)
• Centered (equidistant from both sides)
• Firmly bedded in the stone (traditionally lead-poured into the slots)

Lifting the runner: Use levers and a rope hoist. Guide the eye of the runner over the spindle top carefully — if the edge of the eye contacts the spindle during lifting, it can chip the stone. Lower the runner gently onto the spindle and check that it hangs freely on the mace.

Initial spin test: Turn the spindle slowly by hand. The runner should rotate smoothly without wobbling. If it wobbles, the mace is not level — reset it.

Setting the Draft (Grinding Gap)

The gap between the working faces of the two stones is called the “draft.” It is the most important operational adjustment on the mill.

For grinding wheat into white flour: draft of 0.2–0.4mm (barely perceptible when sliding a piece of paper between stationary stones) For whole wheat flour: 0.4–0.8mm For cracked grain or animal feed: 1–2mm

The draft is adjusted by raising or lowering the footstep bearing. Traditional bearings use a wedge mechanism (the “bridgetree”) — a long wooden lever under the footstep bearing that can be raised or lowered by a wooden or iron screw. One full turn of the screw typically changes the gap by 0.5–2mm depending on the screw pitch.

Setting the draft correctly:

1. Start with the runner resting on the bedstone (zero gap) and mark this position on the bridge screw
2. Raise the bridge screw until a thin paper feeler just barely slips between the stones at the perimeter
3. Run the mill briefly with no grain — the stones should not touch (you’ll hear a harsh grinding and see sparks if they do)
4. Feed a handful of grain — the flour should be fine but the stones should not noticeably warm in the first minute of running

Balancing the Runner Stone

An unbalanced runner stone vibrates and wears unevenly. After installation, test balance by spinning the stone slowly by hand with no grain and observing whether it comes to rest always in the same position (the heavy side down). Small imbalances are corrected by drilling holes in the back of the runner stone and filling them with molten lead. Larger imbalances indicate the stone is not uniform — it may need to be dressed to remove excess material on the heavy side.

The Stone Furniture

Once the stones are set and running correctly, install the wooden furniture:

The vat: a cylindrical wooden enclosure surrounding the stones, catching flour and directing it to the flour chute. Fit it as closely to the stone diameter as possible without touching the runner.

The hopper and shoe: the hopper is a four-sided funnel above the stone eye; the shoe is a trough that vibrates (driven by a small cam or the eccentric motion of the spindle) to meter grain. The grain flow rate is adjusted by tilting the shoe or changing the vibration amplitude.

Run the mill through a full batch of grain before declaring installation complete. Inspect the first output for grit (indicating stones touching), uneven particle size (indicating uneven draft), and unusual heating. Make small adjustments to the draft and confirm the results before bringing the mill to full load.