Longines Balance Stud Screw

[StuartBaker104]

Hi all,

I'm looking for some advice...

I am working on a Longines cal 23m, which is a very thin movement at only 3mm high. All is clean and back together, with one small but significant problem... the hairspring stud screw thread in the balance cock is worn / stripped. The screw will run down, but not quite grip well enough to be tightened. Since everything is all very close together I really need to get the fine adjustment right on the stud height - maybe someone else had this problem with it before and its just seen too much excitement over the last 70 years.

The screw diameter is 0.3mm and the stud holder is only 0.56mm thick so not a lot to play with, but the screw hole is pretty well centered.

Parts are in pretty short supply and I feel that short of tracking down a donor movement (ebay has one for £75 incl postage from USA), my best option is to fit a larger screw. Cousins has selections for £13.95 starting at 0.35mm which might be good if long enough. However the smallest tap they have is 0.4mm in a set costing £375. My luck without tapping out the hole would be a broken or jammed screw no doubt .

Does anyone have any bright ideas on how to proceed? Use a larger screw but dress the o/d maybe? Any clever ways to repair the thread in the balance bridge? Hope the selection comes with more than one 0.35mm screw and make a tap out of one (might be easier said than done)?

This job has a deadline (70th birhday gift) so all other playing is on hold till I sort it.

Thanks in advance.

S

Edited January 18, 2016 by StuartBaker104

[dadistic]

Your question prompted me to look up miniature screw threads in a copy of Machinery's Handbook, very interesting stuff. These little buggers were standardized in '58 as the American Standard for Unified Miniature Screw Threads. Interestingly, these threads are interchangeable with the ISO 1501 metric standard. Both are defined using the major diameter in mm. 

 

0.30 UNM is 0.080 mm pitch

0.35 UNM is 0.090 mm pitch

0.40 UNM is 0.090 mm pitch

 

It looks to me like one or two steps larger might just "go" without having to re-tap, but I wouldn't force it :-)

I also looked up some UNM taps, and boy are they expensive! ~ $130 US for a single tap! And I bet I would break a couple before I got the hang of using them!

[StuartBaker104]

Hi David,

Thanks for this. I ordered some screws which are being held to ransom for my signature at the post office, so hopefully will be able to see if I can just get away with fitting a 0.35 over the weekend. Hopefully the pitch difference won't be too much of an issue...

If not, I'm thinking of opening up the hole, filling with epoxy putty and using a screw as a tap.

S

[StuartBaker104]

Good news is that I now have a .33mm screw which fits snugly. Still need to strip it down and wash out any swarf but no time to play tonight .

I assume I also need to round the end of the screw as the stud is triangular and therefore has no groove, so a pointy screw will make holes which will make it difficult to adjust.

S

[dadistic]

That I don't know about in such a small screw, set screws are often concave at the end, but trying to do such at 0.33mm is something I don't want to think about :-)