A watch-winding stop (form 2) is a geared stop-work mechanism that limits how many turns the mainspring arbor can make during winding by counting revolutions on a small auxiliary wheel. The key part is the stop-finger, a single radial tooth on the winding arbor that advances a notched count wheel one step per turn until a blocking face stops further rotation. It exists to prevent overwinding past the safe arc of the mainspring and to keep the watch operating in the linear-torque portion of the spring's curve. The result is steadier rate, longer mainspring life, and a defined power reserve.
Watch-winding Stop (Form 2) Interactive Calculator
Vary the count-wheel tooth count, stop-boss travel, and finger window to see the permitted winding turns and lock arc.
Equation Used
The count wheel advances one tooth for each full turn of the winding arbor. Tooth pitch is 360/N. Dividing the stop-boss travel angle beta by that pitch gives the number of permitted winding turns before the boss meets the fixed stop pin. The rim-lock arc is the part of the arbor revolution outside the stop-finger engagement window.
- One stop-finger engagement advances the count wheel exactly one tooth per arbor turn.
- Stop-boss travel beta is measured from the open position to the fixed stop pin.
- Finger engagement occurs once per arbor revolution; the remaining arc is rim lock.
Inside the Watch-winding Stop (form 2)
Form 2 stop work is the geared cousin of the better-known Maltese cross (form 1). Instead of a finger sweeping a star with one blocked slot, form 2 uses a small pinion or single-tooth driver on the winding arbor that meshes once per turn into a count wheel mounted alongside the going barrel. Each full turn of the arbor advances the count wheel by exactly one tooth. After a fixed number of turns — typically 4 to 7 depending on mainspring length — a raised stop face on the count wheel collides with a fixed pin or bridge boss, and the arbor will not rotate further in the winding direction. Unwind the spring through normal running, and the count wheel reverses tooth-by-tooth back to its open position.
The geometry has to be exact. The stop-finger engagement window is narrow — usually 25° to 35° of arbor rotation — and the count wheel must be locked the rest of the turn by a circular rim that rides against a matching concave on the arbor pinion. If that rim-lock clearance opens past about 0.03 mm the count wheel can chatter forward under shock and you lose a turn of reserve. Tighten it below 0.01 mm and the wheel binds when oil thickens in cold weather.
What fails in practice is almost always the stop-finger or the leading face of the blocking tooth. Both see an impact load every time the user winds to the end of travel. If you notice a watch that suddenly winds half a turn further than it used to, the stop face has peened over — the steel has yielded under repeated impact and the effective stop angle has rolled back. The fix is replacement, not dressing, because a re-ground face changes the power reserve.
Key Components
- Stop-finger (single-tooth driver): A single radial tooth or pin pressed onto the winding arbor. It engages the count wheel once per arbor revolution and advances it by one tooth pitch. Tooth thickness is typically 0.25–0.40 mm in pocket-watch sizes; the leading face must be hardened to 700 HV minimum or it will peen within a few hundred winding cycles.
- Count wheel (notched stop wheel): A small wheel with 4 to 7 indexing notches and one raised stop boss. It tracks the cumulative number of arbor turns. Notch pitch tolerance is tight — typically ±0.02 mm on pitch diameter — because cumulative error multiplies across the full count.
- Rim lock (circular locking arc): A concave arc on the arbor pinion that rides against the convex rim of the count wheel between engagement events. It holds the count wheel motionless for roughly 325° of every arbor turn. Clearance must sit between 0.01 and 0.03 mm to prevent both chatter and cold-weather binding.
- Stop boss / blocking face: The raised tooth on the count wheel that meets a fixed bridge pin at end of travel. This is the part that absorbs the user's winding impact. Heat-treated to 60 HRC and usually presented at a 5–8° back-rake so the contact is wedging rather than purely shear.
- Fixed stop pin: A hardened steel pin pressed into the barrel bridge or a dedicated cock. It is the dead-stop reference. The pin diameter sets the stop angle resolution; a 0.50 mm pin gives roughly 1° of angular play in a typical 6 mm count wheel.
Who Uses the Watch-winding Stop (form 2)
Form 2 stop work shows up wherever a wound mainspring drives a precision rate-sensitive train and the designer needs to bound winding turns to the linear region of the torque curve. It is most common in higher-grade pocket watches and marine chronometers, but the same kinematics appear in instrument clocks, fusee timepieces, and a handful of modern micro-mechanical devices.
- Horology — pocket watches: Patek Philippe and Vacheron Constantin used geared form 2 stop work on top-grade hunter-case pocket watches through the late 19th century, typically set to 5 or 6 turns of reserve.
- Marine chronometers: Thomas Mercer and Ulysse Nardin chronometers used form 2 stop work on the fusee winding arbor to keep the spring on its flat torque section for the full 56-hour run.
- Precision instrument clocks: Riefler regulator clocks built in Munich pre-1930 used a geared stop arrangement to limit weekly winding turns on the gravity remontoire spring.
- Modern mechanical watch restoration: Independent restorers in La Chaux-de-Fonds fit reproduction form 2 stop work to vintage Hamilton 992B railway-grade movements when the original Geneva stop is missing.
- Scientific instruments: Wind-up Bourdon-tube barograph drives from Short & Mason used a 4-turn geared stop on the chart-drum mainspring to prevent overrun of the inked trace.
- Music boxes and small automata: Reuge cylinder music boxes use a simplified form 2 stop on the larger mainspring barrels to bound winding to 4 full turns before the click would otherwise be over-stressed.
The Formula Behind the Watch-winding Stop (form 2)
What you actually need from a form 2 design is the number of permitted winding turns as a function of the count wheel's tooth count and the angular position of the stop boss. At the low end of the typical range — 4 turns — the watch has a short reserve and the spring sees only its strongest, least linear arc, which gives high but inconsistent torque. At the high end — 7 turns — you exploit nearly the full safe arc of the spring but you push close to the spring's set limit and rate falls noticeably in the last 8 hours. The sweet spot for railway-grade pocket watches sits at 5 to 6 turns, which keeps the spring in its linear band for the full advertised reserve.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Nturns | Permitted winding turns of the arbor before the stop boss blocks rotation | turns (dimensionless) | turns (dimensionless) |
| Zcount | Number of teeth (notches) on the count wheel | teeth | teeth |
| θstop | Angular arc of the count wheel between its open position and its stop boss | degrees | degrees |
| ptooth | Tooth pitch on the count wheel (used for cumulative error check) | mm | in |
Worked Example: Watch-winding Stop (form 2) in a reproduction railway-grade pocket watch movement
A small horology workshop in Halifax Nova Scotia is building a reproduction railway-grade pocket watch movement based on the Hamilton 992B layout. The original used Geneva stop work but the customer wants a geared form 2 stop fitted instead, with a target power reserve close to the original 40 hours. The mainspring is a 380 mm × 1.85 mm × 0.115 mm flat steel spring rated for 7.5 safe turns. The workshop needs to specify the count wheel tooth count and verify the resulting reserve at the low, nominal, and high settings.
Given
- Zcount = 6 teeth
- θstop = 300 degrees
- Mainspring safe turns = 7.5 turns
- Train multiplier (arbor turns to hours of run) = ≈ 7.0 h/turn
Solution
Step 1 — at the nominal setting, compute permitted winding turns using the design tooth count and stop arc:
Multiply by the train factor of roughly 7.0 hours per arbor turn and you get a 35-hour reserve at nominal — close to the 40-hour Hamilton target but a little short. The spring is operating between turn 1 and turn 6 of its 7.5-turn safe arc, comfortably in the linear band.
Step 2 — check the low end of the typical operating range. Drop to a 5-tooth count wheel with the same 300° stop arc:
At 4.17 turns you get a watch that runs barely a single overnight cycle. The owner notices it stopped before breakfast on weekends. The spring also delivers higher mean torque because it is never wound past its strong region, which sounds good but in practice means a faster rate when fully wound and a tendency to gain 4–6 seconds per day in the first 6 hours.
Step 3 — check the high end. Take a 7-tooth count wheel at the same stop arc:
That hits the 40-hour target, but you are now winding to within 1.7 turns of the spring's safe limit. In real service, owners do not stop winding the moment they feel the stop — they push another quarter turn. With only 1.7 turns of headroom the spring will take a permanent set within 18 months, and you will see rate drop 8–12 seconds per day in the last quarter of the reserve as the spring leaves its linear band.
Result
The nominal 6-tooth count wheel with a 300° stop arc gives 5. 0 winding turns and a 35-hour reserve. In practice the customer feels a clean firm stop at the end of winding and the watch runs from morning Monday to evening Tuesday before needing rewind — short of the 40-hour Hamilton spec but well within the spring's linear torque band. The 5-tooth low-end build cuts reserve to 29 hours and pushes mean rate up by several seconds per day, while the 7-tooth high-end build hits 41 hours but leaves only 1.7 turns of safe spring headroom and will take a set within two years of normal use. If your measured reserve falls more than 3 hours short of predicted, check three things in order: first, a peened stop boss face that has rolled the effective stop angle back by 5–10°, second, a worn rim-lock arc letting the count wheel skip a tooth under shock during carry, and third, an oversized fixed stop pin (more than 0.55 mm) eating into the angular budget at the blocking position.
Choosing the Watch-winding Stop (form 2): Pros and Cons
Form 2 is one of three practical ways to bound winding turns on a mainspring drive. The Maltese cross (form 1) is the classic alternative on wristwatches, while a slipping-bridle going barrel skips the problem entirely by letting the spring slip at high tension. Each has a different fit window depending on size, count requirement, and cost.
| Property | Form 2 geared stop work | Form 1 Maltese cross stop | Slipping-bridle going barrel |
|---|---|---|---|
| Maximum count before reset | 4 to 7 turns (set by tooth count) | 4 to 5 turns (set by finger geometry) | Unlimited — no hard stop |
| Angular accuracy of stop position | ±0.5° at the blocking face | ±2° due to finger entry geometry | Not applicable — torque-limited not turn-limited |
| Manufacturing complexity | High — count wheel, rim lock, finger all to <0.02 mm | Medium — single Maltese cross plus finger | Low — bridle is a stamped strip |
| Suitability for size class | Pocket watch and chronometer (≥30 mm movement) | Wristwatch (20–35 mm movement) | Automatic wristwatch and small clocks |
| Service life of the stop interface | 20+ years if stop boss hardened to 60 HRC | 10–15 years before Maltese finger wear opens stop angle | Effectively unlimited — bridle just slips |
| Effect on rate stability | Excellent — keeps spring in linear band for full reserve | Excellent — same principle, narrower range | Poor — spring runs into non-linear top region during overwind |
| Typical cost premium over no stop work | High (additional jewelled wheel and bridge) | Moderate (one star plus finger) | Low (bridle adds pennies in stamping) |
Frequently Asked Questions About Watch-winding Stop (form 2)
That is almost always cumulative tooth pitch error on the count wheel, not the stop boss. If your notch pitch tolerance opened past ±0.03 mm during manufacture, the cumulative error across 6 teeth reaches 0.18 mm — enough that the rim-lock arc no longer holds the count wheel at the right angle when the stop boss approaches the fixed pin. The first sign is exactly what you describe: the watch wound to a hard stop when new, then gradually allowed extra travel as the rim lock wore in.
Pull the count wheel and measure pitch on a stage micrometer. If three teeth in a row show pitch over 0.45 mm in a 6-tooth wheel intended for 0.42 mm, scrap it. Filing won't recover it.
Use 6. The rule of thumb in chronometer practice is to keep at least 1.5 turns of mainspring headroom above the stop point so a heavy-handed user cannot set the spring. A 5-tooth wheel at 300° gives 4.17 permitted turns and 3.3 turns of headroom — safe but the reserve is short. A 6-tooth at 300° gives 5.0 turns and 2.5 turns of headroom — the sweet spot. A 7-tooth gives only 1.7 turns of headroom, which sounds adequate on the bench but fails in real service because users always push past the felt stop.
It comes down to module and impact load. The form 2 count wheel needs roughly 8 to 10 mm of diameter to keep tooth pitch above 0.4 mm and locking-rim clearance manageable. A 30 mm marine chronometer has the real estate. A 28 mm wristwatch movement does not — you end up with a 4 mm count wheel where pitch errors of 0.01 mm scale to angular errors of 0.6° at the stop, and the stop position drifts visibly across temperature.
Maltese cross form 1 needs only one star wheel and a finger, all of which scale down cleanly to wristwatch sizes. That's why almost every haute horlogerie wristwatch uses form 1 and almost every chronometer-grade pocket movement that bothers with stop work at all uses form 2.
The 3-hour shortfall is consistent with the user not getting the full 5 turns into the spring at winding. Check the engagement window of the stop-finger first — if the leading face is rolled or the count wheel's first notch is peened, the user feels the stop coming on roughly half a tooth early and stops winding. That alone takes about 0.4 turns off, which works out to roughly 2.8 hours of reserve.
A second common cause is a sticky rim lock dragging the count wheel forward by friction during the non-engagement portion of each turn — you wind 5 times but the count wheel sees 5.3 indexes, hitting the stop early. Pull the bridge and check that the count wheel spins freely with no axial preload from the cap jewel.
Replace it. Dressing the boss back to a square face changes the angular position of the stop, which directly changes Nturns. Take 0.1 mm off the boss on a 6 mm count wheel and you have shifted the stop angle by roughly 1.9°, which subtracts about 0.03 turns of reserve — small but cumulative if the watch is serviced multiple times.
The deeper problem is that a face soft enough to peen in the first place will peen again within a year. The right repair is a new count wheel with the boss heat-treated to 60 HRC minimum.
Rim-lock clearance has opened past 0.03 mm. The whole point of the locking arc is to hold the count wheel motionless against shock for 325° of every arbor turn. Once the radial clearance grows past about 0.03 mm, a sharp shock translates into enough angular play that the next tooth on the count wheel can rotate past the stop-finger and lock in the wrong position — costing you one full turn of reserve every time it happens.
The diagnostic is straightforward: with the bridge off, push the count wheel radially against the arbor pinion. If you can see daylight or feel measurable side play, the rim is worn. New count wheel, or a new pinion if the wear is on the arbor side.
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