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ETA 6498- Riveting and Poising a balance wheel

We continued our preparation for the Escapement exam, learning how to rivet a 6498 balance staff and how to poise a balance wheel. We would also learn how to remove the balance staff from the wheel and how to fit/remove the roller. All of the above are just a few of the things that will be assessed during the exam so its vital to have a good understanding of it all and get lots of practice.

Our first task was to learn how to rivet the balance wheel onto the balance staff and our previous experience riveting train wheels would definitely come in handy. As I mentioned while making balance staffs, the staff has a step with an undercut, this part forms the rivet required to hold the balance wheel. This requires a staking set to be used once again in a similar fashion to when we riveted train wheels. The rivet is labeled as the balance wheel arbor in the diagram below.

Diagram of a balance staff…

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The balance wheel and balance staff are placed on the staking set, a round-nosed punch with a hole just large enough to fit over the staff (down to the rivet) is selected, this along with a few taps of a watchmaker’s hammer spreads the rivet out. Some watchmakers prefer the single, good-strong-whack approach but I choose to give it a series of taps while rotating the balance and punch in between taps. This way I’m less likely to destroy the staff and/or the balance with to strong a blow. Once the rivet has been pushed out enough to securely hold the balance, a flat nosed punch is used to flatten the rivet in place.

Bergeon Staking Tool…

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With the balance riveted onto the new staff, the balance must be checked for trueness, just as train wheels are. Subtle corrections can made to insure that it is absolutely flat at the rim. The roller can then be fitted and the balance is now ready to be poised.

Here is the listed full process:

1) Push balance wheel onto staff with flat punch

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2) Open rivet with a domed punch (Turn the punch while riveting)

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3) Finish rivet with a flat punch

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4) Check for trueness and flatness in true poise

5) Fit the roller (A punch is used that won’t cause any damage the edge of the safety roller)

6) Riveted balance now ready for static poising

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Static poising is carried out by placing the balance on a perfectly level poising tool with the cylindrical parts of the pivots resting on the ruby jaws. If the pivots are perfectly clean and the ruby jaws of the tool are perfectly level and clean, the balance will come to rest with the heavy spot at the bottom. A tiny amount of material can then be removed from the underside of the rim at the heavy spot with a small tungsten three corner cutter. The balance is then checked again for poise and the process repeated until the balance will stop freely in any position, indicating that its in poise.

Balance being poised…
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While the poising process can sometimes result in a number of small holes in the underside of the rim, with some care and a lot of practice, a balance can be poised with a maximum of 2-3 holes. By removing the largest amount of weight from the hole closest to the heavy spot and a smaller amount from the holes next closest, the balance can eventually be brought to poise without looking like a piece of swiss cheese!

Once we had poised the balance, it was time to learn how to remove the balance staff from the wheel but before that we had to learn how to remove the roller. The easiest way to remove the roller is with the Bergeon 2810 tool, its a simple piece of nickel with a slot and blades on either side of increasing thickness. To remove the roller, you simply slide the balance into the slot with the blades under the roller until it fits snuggly and then squeeze the open end of the tool together until the blades pop the roller off. The tool makes this process effortless and prevents damage to the roller during removal.

Bergeon Roller Remover…
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With the roller removed, the staff can be removed from the balance wheel. The safest way to do this is it to cut the staff out on a lathe. Although some people recommend just cutting the small rivet out from the top of the staff, we were shown to cut through the entire hub from the bottom. The advantage to this being that you have more material to cut through and a larger diameter to keep you from slipping and gouging the arms.

In similar fashion to the oiling plates, we had to rivet and poise a balance wheel five times with it being checked by our tutor each time. The balance would be checked for poise and then we would have to remove the staff ready to use the balance wheel again. After completing the process three times we were given a new wheel and tasked with completing the last two go’s in less than 3 holes. Thankfully I completed all five, getting faster with each go and completing both the last two in 3 holes. Again the whole process has given us much needed practice and we can move on again to the next part of the exam to prepare for.

Posted in The British School of Watchmaking and tagged , , , .