That screwplate appears to be metric. So the sizes listed are the thread size, 20 is 2mm, 13 would be 1.3mm. So you would want to start with 1.3mm as your diameter. But- it might (probably) not cut the thread so much as cut and form the thread by displacing metal. I say that because it looks a little dubious in quality. So try a little smaller and see if you get a good thread form. You can turn a taper from say 1.15 to 1.3, and then thread that, and see where along the taper you get a full thread. Make a note of it for the next time you need to thread that size.
The old Martin screwplates had a screwy (haha) numbering system. There were two main designations from Martin, L and B; both seem to have the same thread diameters regarding their numbering, the difference is the pitch of the thread. Martin G plates were for left hand threads. Funnily enough, the "backward" numbering system for watch stems and crowns corresponds to the Martin sizing, I guess it was taken up at a time when Martin screwplates were the primary threading tool for watchmakers. 99% of the time I use industrially made metric taps and dies for threading, but sometimes for an old piece I will use a Martin plate. The threads are a little more rounded on their crests. I find that for a given size I need to turn a little undersize, as mentioned above. With new "real" metric stuff I turn to the nominal diameter.
Watch was working fine until I tried the Chronograph function. Everything works fine but the chronograph. It won’t reset back to 12. Just lingers around wherever it wants to go back to. Logic tells me it’s grease or broken/damaged spring or both. Found a bit if a rusty portion on the top pusher. Gonna have to try and take this boy apart.. any help will be greatly appreciated.
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So IMHO it is not Shellac you have to worry about. In my experience, there were no problems with the shellac (1-2 minutes in IPA in ultrasonic and immeadiatly air blown after). BUT you need to be careful with other glues used, like in the japanses movements - the stud is not glued with shellac, but something else, that I found it softens a bit... For those, I would just rinse by hand in IPA and blow-dry immediatly.
LAtely I am using a hairdryer to blow hot air - I find it more reasuring...
The length of an extended pivot whether to take a hand or a driving wheel really depends on the design, but 1.5-2mm is reasonable. I sometimes add a little Loctite 648 or 638 when friction fitting the new pivot, but I think it's mostly psychological, as there really isn't any room for it if the pivot is correctly sized for a friction fit. If the pivot is loose enough in the drilled hole for the Loctite to have room, then it won't hold anyway, there just isn't enough surface area for it to do its magic.
You really have to watch the size of the new pivot- often the wall thickness of the drilled arbor is quite thin, and the interference fit is on the order of a few microns. Too much and you can split the arbor... and of course too little and it doesn't hold.
To tap a hole for a screw to fit in it, then you use the 'thread pitch minor diam' for the drill diameter. This will drill a hole the size of the bottom of the threads of the screw. The tap will then take out the metal between this diam and the top of the thread in form of the thread. A larger diameter drill will mean the thread depth is not full and so may strip easy if the screw is tightened too much or just be too loose a fit. A smaller diameter drill will mean the tap is cutting into more metal and if care is not taken will jam the tap and snap it. Normally most screw threads will have tables that show the tap drill diameter. The smaller the screw size then the closer the tap needs to match the pitch minor diameter.
For making a screw, then you use the 'thread pitch major diam' for the blank rod/wire diameter. The rest is similar to above, but too big diam will mean more metal removal and possible breaking of the blank in the die, and too small diam will result in a loose fit and possible stripping when tightened.
There are basically 2 forms of tap/die for each size. One is tapered to allow easier starting of the tapping stage but will not tap all the way to the bottom of a hole or all the way to a shoulder on a screw. The second will do this and is normally used after the tapered one.
The secret is to use some oil or grease on the tap/die when working steel and tougher metals.and ensure the tap/die is fully square to the work-piece. If you don't have tapping paste/oil then try and use one with EP (ie motor EP gear oil or Moebius EP 1300 etc,) even engine oil will do as these have additives that work to reduce friction at the cutting edge of the tap/die and only come into effect at those local high temps. Brass, copper and aluminium can mainly be cut without a lube or just use paraffin oil.
Take it slow and just cut a very small bit at a time, backing off every now and then to clear out any swarf, don't force the tap/die. When following a tapered tap with an end tap always clear out the swarf so you have a clean start with the next tap.
The pitch is simply the distance between adjacent thread peaks and the form/size based on several industry standards.