Hi Chris,
The screws on the balance are actually for poise in this case - before the advent of modern manufacturing and poising methods, screws were the best method of providing the adjustability needed to eliminate the initial manufacturing errors.
The temperature compensating balance would be a different animal to the one you show, as it would be bi-metallic construction, and split in 2 places near the arms. It's those two features that make the arms grow or shrink as the temperature varies, changing the moment of inertia of the balance and the timing accordingly. Here is one (on the left) and you can see that the rim of the balance is made of 2 different materials, and the rim is also split:
The temperature compensating balance was replaced by the solid balance (no split rim) when they came up with alloys that were more resistant to thermal expansion, such as the Glucydur material used in most higher end watches these days. Advances in balance spring materials also helped make watches much more stable across a wide range of temperatures.
Balances sometimes still use screws on higher end watches - for example the Cal. 65 in my GO has them:
Today they are more a sign of a high end or traditional maker than a necessity. Of course some balances have screws only for timing, and not for poise, so Microstella screws on Rolex balances, the Gyromax weights on PP balances, and the screws on the various Omega free sprung balances.
How you poise a balance depends on the type. On a smooth balance with no screws removing material is really your only choice. If the balance has screws that can be removed, then washers can be added under the screws to add weight in the light area, rather than removing it from the heavy area. I have several selections of timing washers, and they are sorted by the ligne size of the watch you are working on, then in how much time they will change the rate by. In this photo each vial has what look like small specs of dirt in them - those are the washers:
Here is a balance screw I have removed and have added a timing washer to, and it's shown next to a ball point pen for scale:
I will add weight whenever possible because it's easily reversible, but often when you get down to the very fine poising there aren't washers small enough to make the final adjustments, so removing weight is the only way. Adding or removing too much weight can get you into trouble, so slow and steady steps are the best approach, especially when you are just starting out and don't yet have the feel for how much to add/remove.
How you remove the weight can vary also. For a smooth balance, I use a small pivot drill in a pin vise, and simply use that to remove some material on the underside of the balance rim. For removing weight from screws, the style that removes weight from the underside of the screw is preferred by many, rather than filing the screw down or using the screw cutters are essentially hollow out the head of the screw (and if used a lot will remove the slot from the screw). So this is the preferred technique:
When doing the static poising, the tool has to be level, no drafts in the area (including your breath), and don't level everything up and then lean on the bench while doing your tests. Oh and don't assume there's just one heavy spot!
It's fine work that takes some patience to learn and master, but it's rewarding when you get it done.
Cheers, Al
Click to expand...