Tentering

Part of Mill Construction

Tentering is the art and mechanism of controlling the grinding gap between millstones — the most sensitive adjustment in mill operation, affecting flour quality, stone wear, and fire risk.

Why This Matters

The gap between the millstones — called the “draft” or “tenter” — is the single most critical operating parameter of a grain mill. Too wide and grain passes through partially ground, producing coarse flour or cracked grain. Too narrow and the stones touch, creating intense heat that scorches the flour, damages the stone faces, and in extreme cases can ignite the flour dust in the air — causing a devastating mill explosion.

Getting the tenter right is what separates an experienced miller from a novice. The correct gap for fine white flour is less than half a millimeter — thinner than most paper. Adjusting to this precision while the mill is running at full speed, in a dust-filled environment, with no measuring instruments more precise than the miller’s fingertip and nose, requires developed skill and deep understanding of the machine.

The tentering mechanism is the device that makes this adjustment possible. Its design must allow fine adjustments (fractions of a millimeter), hold its position under the continuous vibration of milling, and be adjustable quickly if something goes wrong. Traditional millers spent years developing the feel for their particular mill’s tentering system.

The Bridgetree and Footstep Bearing

The standard tentering system in a traditional grain mill operates through the footstep bearing — the cup that supports the lower end of the millstone spindle.

The footstep bearing sits on a wooden beam called the “bridgetree” (or “bridging box”), which is supported at both ends. The center of the bridgetree can be raised or lowered by a screw mechanism. Raising the center raises the footstep bearing, which lifts the spindle and the runner stone attached to it — opening the gap. Lowering does the reverse.

The adjustment screw is typically a wooden or iron screw threaded through a nut mounted in the mill floor or hurst frame. One full rotation of the screw raises or lowers the bridgetree center by a distance equal to the screw pitch. For a screw with 3mm pitch (3mm advance per revolution), each quarter-turn changes the stone gap by approximately 0.75mm.

The bridgetree design must be rigid enough not to flex under the weight of the runner stone (which exerts a downward load on the spindle through the stone’s own weight plus the grinding pressure), yet the adjustment mechanism must be smooth enough to move without undue force.

Tentering from Below vs. Above

Below tentering (most common traditional design): The bridgetree acts upward against the bottom of the footstep bearing. Raising the bridgetree lifts the stone. The default position (if the screw were removed) would be stones touching — the screw pushes the bridge up to create gap. This arrangement means the stones naturally try to come together, requiring active adjustment to maintain gap. It is simpler to build but slightly less controllable.

Above tentering (governor-controlled mills): A vertical screw above the spindle presses down on the runner stone from above, adjusting gap by pulling the stone away from the bedstone. This allows the use of a centrifugal governor to automatically regulate gap based on spindle speed — a significant advance that maintains consistent flour quality as water flow varies. The governor mechanism is a later development but buildable with modest metalworking skill.

The Centering Mechanism

A simple tentering screw works well if water flow (and thus spindle speed) is consistent. But if flow varies — lower in dry seasons, higher after rain — spindle speed varies, and the optimal gap changes. Running too fast requires a wider gap (or the stones heat up); running too slow requires a closer gap (or grinding is ineffective).

Traditional millers responded to this by constant manual adjustment — tasting the flour, feeling the heat, adjusting the tenter screw continuously throughout the working day. An experienced miller made dozens of small adjustments per hour.

For a more consistent result, the centrifugal governor:

Two balls on hinged arms are connected to the spindle. As spindle speed increases, centrifugal force throws the balls outward and upward, lifting a collar on the spindle. This collar, through a linkage, raises the runner stone slightly — increasing the gap as speed increases, preventing overheating. As speed decreases, the balls fall, the collar drops, and the gap closes — maintaining grinding efficiency.

Building a centrifugal governor requires:

• Two iron balls, 1–2 kg each
• Hinged iron arms connecting them to a rotating collar on the spindle
• A linkage from the collar to the tentering mechanism
• Careful balancing of the arm lengths and ball weights to respond at the correct speed range

Reading the Mill

Before automatic governors, the miller “read” the mill to know when to adjust the tenter:

Temperature: The miller held the back of a hand near the flour chute outlet. Flour should feel warm, not hot. Temperatures above 45–50°C indicated stones running too close (grinding too fine and creating too much friction) or too fast. This is the most important warning sign.

Smell: Overheated flour smells toasty or slightly burnt. Fresh flour has a clean grain smell. Any burnt smell means immediate action — open the tenter and check for stone contact.

Sound: A correctly running mill has a steady grinding sound — even, rhythmic, moderate volume. An unusual high-pitched sound or intermittent scraping often means a stone running too close or grain not feeding properly. A change in the general sound level indicates a change in load.

Flour texture: Periodically, the miller rubbed a pinch of flour between thumb and forefinger. The texture should be consistent — fine and slightly damp-feeling. Coarse texture meant the gap was too wide; excessive warmth meant too close.

Touch at the eye: Experienced millers would briefly touch the outside of the wooden vat just above the eye — the temperature there indicated what was happening at the stone center.

Tenter Log

In a well-run mill, the miller kept a tenter log: a record of the screw position (marked on the shaft) for each type of grain at each season. Over time, this became a reference that allowed the mill to be set up quickly for optimal grinding:

Grain Type	Season	Screw Marks from Datum	Notes
Wheat, fine flour	Summer	+3.5 turns	Dry grain, close gap
Wheat, fine flour	Autumn	+4 turns	Higher moisture harvest grain
Rye	Any	+5 turns	Harder grain, wider gap
Coarse meal	Any	+7 turns	Feed grain, fast output
Barley	Any	+4.5 turns	Variable hardness

These settings were specific to each mill and its particular stones. A new miller had to develop their own record through trial and observation.

Emergency Procedures

If the stones come into contact (indicated by sparks, sudden heat increase, or a harsh metallic grinding sound):

1. Immediately open the tenter screw several turns to create gap
2. Kill water flow to the wheel
3. Allow the stones to stop
4. Inspect for damage — stone contact can chip the working faces
5. Check for fire risk — inspect all flour surfaces for smoldering material before re-starting
6. Re-dress if the stone faces show contact damage

The risk of mill fire from stone contact is real. Traditional wooden mill buildings with flour dust in the air are extremely flammable. Keep water buckets at hand whenever milling, and never leave a running mill unattended.