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AP1875

Amplitude drops or watch stops with crown screwed down

Question

I serviced a Chinese movement, has good results on the timegrapher. 

Installed it in the watch case and when I screw the crown down the amplitude drops off dramatically to 170-200. 

I assume this is due to the stem? However I'm not sure how the stem can affect amplitude and cause the watch to stop? 

Time grapher results without crown screwed down. 20190917_224559.thumb.jpg.5795cd5ad9aff1c1214b71b7ec607ac2.jpg20190917_224644.thumb.jpg.cc2f8dc80d4434157503212c10911076.jpg20190917_224620.thumb.jpg.7d9ad2354b3a2a91cc0efb363d23e936.jpg

 

Edited by AP1875
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Could it simply be pushing the movement against the movement ring, and thus fouling the balance, or some other part?

Alternatively it may cause the movement, or one of the hands to press hard against the face, or push the movement up so that the second hand touches the crystal.

A few pictures might help us decide on the cause.

Edited by AndyHull

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35 minutes ago, AndyHull said:

Could it simply be pushing the movement against the movement ring, and thus fouling the balance, or some other part?

Alternatively it may cause the movement, or one of the hands to press hard against the face, or push the movement up so that the second hand touches the crystal.

A few pictures might help us decide on the cause.

Thanks for your reply, 

There's no movement ring, so that's ruled out. 

I can remove the glass and see if it has the same problem ruling the hands out but the movement is held by clamps so I don't think it's anything I can see. Which is why I wanted to question the stem causing the issue inside the movement. 

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Given that the quality control of some of these Chinese movements is marginal at best, and woeful at worst, it may well be the case that pressure against the stem is causing it to jam somehow. Exactly how is the question. The stem itself may be too long, or machined out of spec., or the tolerances of the case or the screw in crown may be wrong, or the stem may be too short, and screwing it in, may cause it to pull out slightly, jamming up the keyless work.

If you manually push the stem in with the movement uncased, do you get the same effect?

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Hi  I think Andy is on right track regarding tolerences, When assembled and tested the movements would not be cased so a problem of this nature wouldn't show up untill cased,    My guess would be an over length stem pressing on the castle wheel and interfering with the movement .

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So, I removed the movement again and reinstalled it. I ran it without the case clamps tightened down that was fine so tightened them slightly. Seems fine upto now. 

So it must've been something inside the keyless/stem? 

How does a keyless/stem affect amplitude? I can't visualise what's happening inside the movement. 

I'm not familiar with the term castle wheel, are you referring to the winding or sliding pinion? 

49 minutes ago, watchweasol said:

Hi  I think Andy is on right track regarding tolerences, When assembled and tested the movements would not be cased so a problem of this nature wouldn't show up untill cased,    My guess would be an over length stem pressing on the castle wheel and interfering with the movement .

 

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"Castle wheel" is probably another term for the sliding pinion since it looks like a little castle turret.

 

 slidingpinion.jpg

 

If the case alignment with a screw-down crown isn't perfect between the stem and the movement, the stem can act like a spring when the crown is tightened and exert a force on the movement in an up or down direction. If that makes any part of the movement touch a part it's not supposed to (fouling), it can have a tremendous effect on the performance, including amplitude.  

 

You may never be able to find out exactly what's touching what because as soon as you release the stem, the pressure goes away. You might be able to put it all together and let it run for a month or so and then take the movement apart and look for wear marks on the pieces, but I wouldn't recommend that. It's better just to use a spacer to make sure the the case, dial, movment and stem are all in alignment.

 

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8 minutes ago, eccentric59 said:

You may never be able to find out exactly what's touching what because as soon as you release the stem, the pressure goes away.

With ingenuity you could be able to. Plastigauge was invented for that. Now, I've never heard of it being uses on watches, but one could use aluminium foil stacked in different places to find where and how much shimming is needed.
Philippe Dufour once saidn that case makers "are not very precise people", maybe the OP found himself in the same predicament :)

 

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8 minutes ago, Watchtime said:

maybe the length of the stem is the problem?

The watch seems fine now. Just so I know for the future if it was the length of the stem what is the most practical way of shortening it? 

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45 minutes ago, AP1875 said:

The watch seems fine now. Just so I know for the future if it was the length of the stem what is the most practical way of shortening it? 

I'm also interested in knowing how to cut a stem, because I have a project that will involve to adjust the length of the new stem I bought. I think the final fine tune is done with a small grain file, but I don't know what tool to use for the first "big" cuts (I'll have to buy that tool, I don't think I have anything for that).

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I'm also interested in knowing how to cut a stem, because I have a project that will involve to adjust the length of the new stem I bought. I think the final fine tune is done with a small grain file, but I don't know what tool to use for the first "big" cuts (I'll have to buy that tool, I don't think I have anything for that).

Any hand held wrench cutter will do. The file the end to a dome.


Sent from my iPhone using Tapatalk Pro

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Great! I'm keeping this info to use it when I get to that part of the prj. I'm still fighting with the balance wheel, but that's another story,

Edited by aac58

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21 minutes ago, jdm said:

Stem cut a bit too short? No problem, drop a minuscule piece of wire in the crown, apply locking agent and screw down well. Shhh don't tell anyone! 

You will get away with about two thread turns of missing depth that way (allegedly.. not that I would know of course) any more than that and you might have to resort to a stem extender or a new stem.

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9 minutes ago, AndyHull said:

You will get away with about two thread turns of missing depth that way (allegedly.. not that I would know of course) any more than that and you might have to resort to a stem extender or a new stem.

That's why a wrote "a bit". Anyway, a 1.0mm stem can have a 0.2 or 0.25 pitch, which means a crown tapped 3mm deep takes like 15 or 20 turns to full screw. One can have undercut 2mm and still have enough threads to hold safe.

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That's why a wrote "a bit". Anyway, a 1.0mm stem can have a 0.2 or 0.25 pitch, which means a crown tapped 3mm deep takes like 15 or 20 turns to full screw. One can have undercut 2mm and still have enough threads to hold safe.

Too much math. I would just make another stem on a lathe

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