Clockmaking Excellence

This page is a visual dictionary of the standards expected of twenty-first century clockmakers. These images are intended to be used for clockmakers and consumers alike to be able to judge the quality of clock repair work. For each topic an exemplary example is presented along with some photos of unacceptable work and a short text description of the standard.

Pivot Burnishing Standards

Lantern Pinion Depthing

Pinion and Wheel Engagement

Clock Bushings

 

 

 

Pivot Burnishing Standards

In recent years there has been a lot of talk about pivot burnishing. This is a skill of using an oiled, hardened polishing lap (burnisher) to resurface and render the pivot operating surface bright, flat, smooth, and toughened. Note the reference here is to “toughen,” as the process will not harden the pivot but toughens the working surface and renders it longer lasting, more resistant to wear over time, and, most important, it causes the oils used in normal operation to stay at the point of pivot contact with the bearing hole. Older techniques using powders, files, etc., along with buff sticks and diamantine, too often left residues that can get into the new, cleaned bearing surface if extreme care is NOT used. After they are burnished, the finished pivots need a quick rinse, and they and their wheels are ready for service. What is the standard of workmanship expected in the service of clocks? Pivot Y and Z are NOT acceptable. Pivot X is the minimum accepted quality, and pivot W shows the desired result to achieve EVERY time you burnish a pivot. The pivots below were all done with an oiled, carbide burnisher, then followed by the use of an oiled high-speed steel burnisher. The carbide burnisher was prepared with a coarse diamond lap on one side and an extra fine lap on the other. The high speed burnisher (the second lap) was surfaced with fine emery cloth on one side, then crocus cloth to finish the other side. When mastered, it takes about 20–30 minutes for a successful bench person to complete a set of an American time/strike mantel-movement pivots.

Pivot Burnishing-Excellent through Poor.

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Lantern Pinion Depthing
Correct engagement of a wheel and lantern pinion.

One of the most important skills in horology is correcting/resetting the engagement of wheels with their respective pinions. It is one of the most critical issues necessary to insure the smooth running of any train of gears.

The depthing of a wheel and lantern pinion is measured at the moment where the point of contact of the wheel tooth and the trundle crosses the line of centers (the moment of deepest engagement). The point on the tooth that falls on the pitch circle should be touching the point on the trundle that falls on its pitch circle (see illustration). Another way of saying this is to express it in geometric terms: the distance between the centers of the wheel and pinion should equal the sum of the radii of the two pitch circles. The pitch circle for a wheel is the circle that traces the points where the addendum rises from the dedendum. The pitch circle for the lantern pinion intersects the center point of all the trundles. If the engagement is too shallow, these points will never touch. If the engagement is too deep these points will align before the tooth crosses the center line and again afterwards. A worn hole in a plate or a worn bushing can change the depth of engagement of the wheel and its pinion. Bushing is not a “mantra” for our profession! Setting a depthing to its correct pitch point is!

Wheel tooth engages too deeply with the lantern pinion. Wheel tooth engages too shallowly with the lantern pinion. Diagram of proper engagement of a wheel and lantern pinion.

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Pinion and Wheel Engagement
CorrectEngagement-Wheel-Pinion One of the most important skills in horology is correcting/resetting the engagement of wheels with their respective pinions. It is one of the most critical issues necessary to insure the smooth running of any train of gears.  
    The depthing of a wheel and pinion is measured at the moment where the point of contact of the wheel tooth and the pinion leaf crosses the line of centers (the moment of deepest engagement). The point on the tooth that falls on the pitch circle should be touching the point on the leaf that falls on its pitch circle (see illustration below right). Another way of saying this is to express it in geometric terms: the distance between the centers of the wheel and pinion should equal the sum of the radii of the two pitch circles. The pitch circle for a wheel or pinion is the circle concentric to its axis that traces the points where the addendum rises from the dedendum. If the engagement is too shallow, these points will never touch. If the engagement is too deep,these points will align before the tooth crosses the center line and again afterwards. Both faults will have a negative impact on the running of the clock.
    A worn hole in a plate or a worn bushing can change the depth of engagement of the wheel and its pinion. Bushing is not the “mantra” for our profession! Setting a depthing to its correct pitch point is!
WheelToothEngageTooDeeply WheelToothEngageTooShallow DiagramofProperEngagement

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Clock Bushings

Fig1-ProperBushing

Fig2-ProperBushing

Fig3-Caution

Fig4

Fig5

An excellent bush is one that realigns the depthing of two wheels correctly. The wheel and the pinion pitch circles touch each other as close to their line of centers as possible. The excellent bush is always flush to the inside of the plate, Figure 1, and stands flush, Figure 2, or no more than 10% higher than the front of the plate, Figure 3. Any higher and the likelihood of the bush coming loose in preparation or usage increases markedly, Figure 4. Any lower than the plate and long-term support is compromised, Figure 5. Should the bushing need smoothing on the inside of the plate to bring it flush, the surface must be finished smooth. If it is sanded or filed, it will wick the oil from the shoulder and add drag to its motion, Figure 6. It is usually better to cut a bushing to size on the lathe rather than having to sand it down in place. The bush should have a bright, burnished hole whose edges are smoothly deburred on the back side of the plate and countersunk slightly on the front side, but only enough to allow for better oil retention and/or as close a match to other plate pivot holes, Figure 1.
    A faceted pivot hole often results from a dull or worn cutting broach. Cutting broaches need to be reviewed regularly for wear and replaced as needed. These are not lifetime tools but simple cutting devices that wear with use. Smoothing broaches also need touch-up maintenance regularly. As their surfaces become uneven, scratched, or mottled, they must be re-treated (like burnishers—but round) to keep their working surfaces uniform for the best finish. Smoothing broaches, like their relatives the pivot burnishers, should be oiled when used with the same oil types used to lubricate the movement. This reduces the potential for cross-contamination if any oil residue gets through the final cleanup processes.
    Expecting a cut but unburnished hole to lap itself smooth is also incorrect. A newly cut hole should be cut and then smooth broached to the exact size needed. A hole cut to size but left unburnished will wear more quickly, as the particles from the ensuing lapping process add to the slurry and cut the new bearing more quickly out of size!
    Pivot fit in its hole (sideshake) has several variables to consider as the new hole is sized. The general reference has been a 7º motion in any direction from straight up. This is more of a guideline rather than a hard-and-fast rule. Consider plate thickness, proximity to the main power source, solid- or lantern-pinion engagement, tooth quality, temperature, etc. If the teeth are stubby (or long and thin), extra sideshake can cause poor depthing during performance. With lantern pinions, there is more room for variation. When these types of movements are under load, they can have a subtle flex in the plates that causes the clock to bind—even though when not wound, the sideshakes look fine. When close to the power source, a bit more sideshake may be desirable to ensure the best power flow. If the plates are thick (e.g., Herschede clocks), an allowance must be made for just a bit more sideshake. If the bearings are too tight, it may cause the movement to bind up under load. Ever had a clock or watch leave the shop in the winter and six months later was a come-back? How about the reverse of that problem—summer to winter? For reasons like these, it’s always better to get depthings a bit close and then open them up as needed. A loose hole that, under load, still keeps the depthing on line of centers is still acceptable and quite functional.
    One last point, if the plates are flat and the bush work is perfectly flush, the endshakes should be adequate as was the original. However, always check the work, as plate deformity during bushing installation can cause a loss of endshake, and the product can bind up or fail to run properly.

Fig6

 

Fig7

 

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