When adjusting to 3 or more positions, our first goal is to get the dial up (DU) and dial down (DD) positions to agree. How close is “close enough” depends on the age, grade, and condition of the watch. For the Hamilton 950B, a classic railroad pocket watch, Hamilton specified a difference of no more than 6 seconds between the DU and DD positions, and I tend to use 6 seconds as a goal. For lower-grade watches, we could probably live with 10 seconds, but we’d rather not.
But sometimes a watch isn’t even close to 10 seconds. Many, many factors cause a watch to have different rates when DU and DD. As an earlier post discussed, anything that changes the relative friction will make the DU and DD rates differ.
The most common causes, in my experience, are differences in oiling and jewel cleanliness. Usually, scrubbing the jewels, cleaning the balance staff pivots, and oiling again aligns the DU and DD rates.
Pivot Shape and Friction
You’ll occasionally, however, have to reshape the pivots. A balance staff’s pivots should be, in Jendritzki’s phrase, “flat, domed surfaces” (p. 3), as shown in the image.
Let’s think about how the shape of a pivot affects a watch’s rate. Imagine two different shapes: a pivot with a pointed end, like a cone, and a pivot with a completely flat end, like a tube.
The “contact patch” of the pivot is much greater when the pivot is flat. This increases the friction between the pivot and the jewel. If friction is greater, then amplitude is lower. And if the amplitude is lower, as a general rule, the rate is faster. (This happens when a watch runs “fast in the short arcs,” i.e., at low amplitude—generally true, but not always, for good-quality vintage watches.)
So, in short, flatter pivots increase friction. If one pivot is flatter than the other, the DU and DD rates will differ—one end will have more friction, thus changing the rate.
Pivots should be “flat, domed surfaces” so that the horizontal and vertical positions are similar. Friction is higher in the vertical positions because the balance staff contacts the sides of the jewels at 2 points. Flatter pivot ends increase friction in the horizontal positions, thus aligning the horizontal and vertical rates.
Why Would One Pivot Get Too Flat?
When a watch is dial up or down, balance pivots rub against their end stones. If the jewels are clean and oiled, all is well in the world.
But many old watches were left running for years without proper cleaning. The oils gums up, dust collects in the jewel, and the gunky paste grinds away at the pivot, flattening it.
For pocket watches, I see flatter upper pivots (on the hairspring side) much more often than lower pivots (on the roller side). People tend to leave their watches dial-up overnight or when displayed. As a result, the upper pivot rests against the jewel and grinds away against it. (These jewels are often harder to clean, too.)
How to Round a Pivot
You have a few options for rounding a pivot, such as a Jacot tool, lathe, or pivot polisher. Here, we’ll describe how to use a pivot rounding tool. These tools, like the Bergeon 5482 shown below, offer a quick and practical way to round an overly flat pivot.
The end of this tool has a concave jewel. The flatter pivot is inserted into this end, and the contour of the jewel aids in reshaping it.
You’ll need an abrasive to reshape the steel pivot. Here we have some basic clock oil and diamantine powder. Diamantine is a common abrasive used in polishing steel parts (no, it isn’t made from powdered diamonds).
Mix some diamantine with the oil to form a thin paste and insert it in the concave jewel. Then insert the flatter pivot and rotate it, supporting the other pivot with your finger. You needn’t remove the hairspring or roller.
It is better to round in stages, testing the rates in between, than to round too much.
Needless to say, cleaning the pivot thoroughly afterward is essential. Otherwise, your diamantine powder will end up in the jewel, happily grinding away as the watch runs.
Round or Flatten?
Technically, you could align the DU and DD rates by flattening the rounder pivot instead of rounding the flatter pivot. In most cases, though, you would rather round.
Kleinlein, in his book, points out that we should seek to improve a poor condition than to reduce a good one. In general, the flatter pivot has worse amplitude, so rounding it will improve amplitude, whereas flattening the other side will reduce it.
In addition, for these old vintage watches, it’s much more likely that a pivot is too flat than too round, given how often they were run without proper cleaning and oiling. Rounding the flatter pivot is more likely to bring the watch back to its factory performance.
Rounding a pivot should be one of your last resorts. It isn’t reversible, so all the other many causes of DU/DD errors should be ruled out first. Be sure to eliminate differences in cleanliness and lubrication, chipped or pitted cap jewels, hairspring rubbing, and proper regulator pin spacing before grinding away on your pivots.