Case-back threads

[Klassiker]

I have disassembled, lubricated and reassembled a number of movements and complete watches now, beginning with pocket watches and moving on to wristwatches. I have also attended some of the seminars offered by the DGC here in Germany. I would still classify myself very much as a beginner.

One of the first wristwatches I began work on was my Grandad's old Summit with an AS 1700 movement. The filled gold case was worn and filthy, and the lugs were so damaged it was unwearable, so there was nothing to lose at that stage. However, I took the decision to invest some time and money into it, with the aim of making it wearable again. If I can get it finished to a reasonable standard I thing my Dad would like to have it. I have had the case repaired and refinished, and I have serviced the movement myself. I also cleaned the dial and re-lumed the hands, put a new crystal in and replaced the gaskets.

Here are some before and after photos ( by the way, it's Dropbox. From past experience, when you follow the link, the pic is briefly displayed, then disappears. Click the button at the bottom (Oder weiter zur Webseite) to get it back). Thanks for looking! :

A dirty face:

https://www.dropbox.com/s/ujhlz1ihfv4urv2/DSC00865.JPG?dl=0

A clean face. So clean in fact, that the printing came off in the wash. Note to self: never put dials in the ultrasonic bath.

https://www.dropbox.com/s/0q6ez1ick340y9r/DSC01367.JPG?dl=0

Damaged lugs, worn plating and a very dirty movement:

https://www.dropbox.com/s/e7ijw5a0csfqpr4/DSC00877.JPG?dl=0

https://www.dropbox.com/s/d3mt0mhxf3q8bhi/DSC00879.JPG?dl=0

After refurbishing and service:

https://www.dropbox.com/s/2y8k2rbg0usbnb0/DSC01368.JPG?dl=0

https://www.dropbox.com/s/0nrbw3onyk7blz4/DSC01369.JPG?dl=0

Maybe I should write a longer post with more photos of that process elsewhere. Progress is now stalled completely as I have a problem which, despite many searches here and on other forums, I can't find a solution for. At some stage the thread on the case-back and in the watch case itself have become so worn or damaged that they don't lock anymore. In fact, they are so loose now I can snap the back on and off.

Here's a picture of the thread in the case:

https://www.dropbox.com/s/dqe0ikoaokxazq4/DSC01362.JPG?dl=0

And here's the case-back:

https://www.dropbox.com/s/949md0uwn8qindn/DSC01363.JPG?dl=0

I would appreciate any advice about what to do, and especially what not to do at this stage. It's hard to tell from these pictures, but I'm fairly sure the thread in the case itself is beyond repair. It may be possible to redress the one on the back. For sentimental reasons I'd like to preserve as much of the case and back as possible. The case-back for example has the family surname and some dates scratched in there, from the local watchmaker.

I've considered just glueing everything together and hoping it comes apart again when I do the next service, or trying to regenerate a serviceable thread with two-component adhesive. Other than that, I suppose I'll have to wait until I get my first lathe!

I'm  open to any other ideas or tips you can offer. Also, if I've missed a thread showing what to do, please let me know.

 

[rodabod]

It’s a tricky issue as the case has simply lost material due to corrosion. 

Can you try a little plumbers (PTFE) tape? Remember to wrap clockwise onto the caseback thread. 

Another alterantive could be experimenting with different rubber gaskets to see if any can help to form a reasonable seal. 

[Klassiker]

Thanks. I will try the PTFE tape tomorrow.

[Klassiker]

I tried the PTFE tape, and it worked well enough so that the threads engaged and I could screw the back on. I gave it a little tweak with the wrench, but it's only slightly more than finger-tight. It was on the wrist today and didn't work loose, so.....success, I guess. Thanks very much for the suggestion. Certainly for the moment that has saved me a lot of heartache. Maybe beginners luck, or maybe with a few more attempts I can get it to grip harder. I think I'll quit while I'm ahead though (for a while anyway).

However, I fear it is only a matter of time before the threads part company again, with a further loss of profile. Anyone got a suggestion for a more permanent solution?

[TexasDon]

This is such an unusual failure that there really isn't much information available. IF you have access to a lathe, the threads can be recut in both the case and on the back plate. It would take a good skill level and an even better eye. Do you have any idea of the thread diameter and pitch? 

Additional wraps of the PTFE tape may make the back more secure for a short term repair. Please keep the forum posted on what you do.

[Klassiker]

Hi Don,

yes, so unusual that I haven't been able to find any useful tips anywhere else. Not even in the best books I have. So a big thanks to you and rodabod for giving me the benefit of your experience.

I don't have a lathe, and at the moment I've nowhere to put one. Otherwise I'd be tempted to use this as an excuse to go shopping. I could measure the thread, but for the time being I'll just have to be content with the PTFE tape.

[rodabod]

The issue is though, you can only cut the case-back thread smaller, and the case thread larger......

[Klassiker]

 

On ‎2‎/‎22‎/‎2019 at 12:29 PM, rodabod said:

The issue is though, you can only cut the case-back thread smaller, and the case thread larger...... 

I would start by measuring what's left of the original threads, and decide if the one on the case back can be used with just a clean-up, to tidy the flanks without removing metal from the crown, or cutting the root deeper. Best to keep it to a standard size if possible. Then I'd cut the thread in the case away completely by boring out to the case back o.d. and make a  ring to fit inside. (Material? Push-fit, glued or soldered in?) Then I'd cut a new thread into the i.d. of the ring. Does that sound like a cunning plan?