We talked about positional accuracy several times in the other two articles about regulation, so let’s dive into that concept.
Positional accuracy is important because a watch’s rate will vary as it moves around; unlike a clock, which is bolted to the wall, watches are constantly in motion, and the balance is affected by its relationship to the ground by gravity.
Typically, a balance will keep relatively similar time in its “dial positions,” which is when the watch is dial-up (wrist toward your face) or dial-down (wrist turned away from your face). In these positions, the balance and hairspring are parallel to the ground and relatively unaffected by gravity. The balance is also rotating on the tip of its staff (axle), which is a rounded pivot that rests on a perfectly-polished ruby jewel—very low friction.
These different positions will vary from each other, and can vary from the dial positions as well. In the stem positions, the balance and hairspring are perpendicular to the ground, so gravity has a greater effect, and the balance is moving on the side of its axle, which has more friction than the tip.
The Sellita SW300 in our Olmsted is required to be adjusted in 4 positions with maximum/minimum rates of +/- 5 s/d and a total delta of 20 seconds, but we typically try to do better than that, with an average rate of +0 to +10 s/d. As a watchmaker, I like to adjust to 5 positions to ensure proper timekeeping in use, as well, so any watch that comes through for service will leave my bench adjusted to that standard.
Now let’s dig into why a positional errors occur, and how to fix them.
If the wheel turned fully, a heavy point would spend equal amounts of time at all points around the circumference of the wheel. But since the wheel doesn’t fully turn, some part of the wheel is “up” and the opposite is “down” for most of the rotation, so a heavy spot can start to have a pendulum effect.
Small irregularities in the wheel are always present. Try as we might, no component is perfect, and heavy spots can even occur because of tiny variances in the density of the metal used to form the wheel. That’s why, if you flip a balance wheel over, you’ll see some tiny slots or holes.
Static poising like this makes a huge difference, but isn’t the end of the story.
This is all very complex, and it’s why regulation is one of the most challenging and critical parts of a service. Assembling a movement, after all, is delicate but straightforward. Regulation requires an entirely different state of mind and activity.