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How do I identify the thread size on a screw that I want to fabricate?


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I have a watch with a set lever screw that has a very chewed up head, and it's difficult to get a screwdriver in there.  I'm considering fabricating a new one if I can't refurbish the head.

However, I have no idea how to tell what the thread pitch is.  If there are two dimensions to a screw (diameter and pitch), I'd need a testing jig that has every common combination of thread pitch and screw diameter, which as far as I'm aware doesn't exist.

I have one of those cheapo Indian tap and die sets (https://www.cousinsuk.com/product/0070mm-to-0200mm-indian), but that doesn't tell you thread pitch, only diameter.

Appreciate the help, thank you.

 

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The screws used in Swiss watches (since the last 100 years or so?) are standardized, to NIHS norms. If you have a look at the Asco-Schurch catalog in the cutting tap section they show all the thread diameters and screw pitches (and tap drill sizes, really handy catalog). So if it's a Swiss watch, once you know the diameter, the pitch takes care of itself. I don't know about Japanese watches, and American stuff is all over the map. Antique stuff will often correspond to a Martin screwplate thread, which for the most part, in small sizes, tend to actually be quite close to the modern Swiss standard.

 

Asco-Schurch

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1 minute ago, GregG said:

Thank you for the link, that is helpful to know.

What if the watch is not Swiss and/or is it over 100 years old?

You'd have to measure, would really need a profile projector or toolmaker's microscope for watch size screws. Then, you'd have to make the die, haha. For antique stuff I've always managed with a Martin screwplate (there's L and B plates, Latard and Bourgeaux, slightly different pitches and diameters, B seems closest to modern metric); there's also Marin G, for gauche or left hand. Then there's Thury which was also Swiss, 19th century, and was adapted in the British Association threads, with a nice thread angle of 47.5 degrees. The Germans had other threads (I have a Ludwig & Fries plate here), the French had theirs, for antique stuff it's a huge subject. A lot of really old threads have quite odd forms, rounded crests and almost straight flanks, odd flank angles like the Thury. I know a world class clock restorer who has over 500 different screw plates, all classified, and can replicate just about any screw for any clock with those.

 

Nowadays you could make about anything you want with a relatively cheap Sherline CNC lathe I suppose.

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