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Balance staff pivot tolerance to jewel hole?

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LittleWatchShop Distinguished Member

LittleWatchShop

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I am turning a staff and the spec for the staff says 120um but when I measure the jewel hole in the balance cock, I get 150um.  Maybe this jewel was replaced or something...it is over 100 years old.

According to de Carle, the pivot should be polished until it "fits comfortably."  Not sure how to interpret that.

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LittleWatchShop Distinguished Member

LittleWatchShop

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15 minutes ago, nickelsilver said:

For a pivot that size give about 0.01mm clearance. When you get down to 0.07mm diameter pivots you should go to a proportionally smaller clearance, but at that size 0.01mm is fine.

To be sure about what is being measured, 0.01mm clearance would mean a pivot diameter of 0.13mm to fit a jewel hole of 0.15mm?

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LittleWatchShop Distinguished Member

LittleWatchShop

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Well, mysteries abound.

The reason I asked this question is because I measured 0.15mm for the balance cock jewel.  This troubled me because the spec for the balance staff is 0.12mm.

So, I measured the bottom jewel today and found it it be 0.13mm which comports with the guidance from @nickelsilver leaving 0.01mm of side shake.

I recently acquired a donor for this Cyma 571.  Sure enough, the balance cock jewel is 0.13mm.  Some previous watchmaker decided that a bigger hole suited his liking I guess.  That is consistent with the fact that the original roller had been pinched in order for it to fit the staff.  I doubt the staff that I removed (after I broke it) was correct either.

The original cock with the wrong jewel has a jewel diameter of 130 (hundredths of a mm).  Comparing the balance cock with the donor, it appears that the previous dude expanded the hole to fit a 130 jewel.

So, here I am...what to do.  I really did not want to replace the old balance cock with the donor because they are numbered (last two digits of the serial number, but I think that is the only straightforward choice.

I have a collection of PW plate jewels and a there are a few 130 jewels, but they all have 15 holes.

I am turning a new staff now, so I could turn the balance side larger.  LOL...what a mess.

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