Thread: A threaded, tapered thumbscrew...

Tapered threads are very unusual, I would hazard a guess that yours just looks that way due to wear.

5/16” thread fits between 1/4” and 3/8”, but it could be a Unified Coarse (UNC) or a Whitworth (BSW or BSC) thread form depending whether or not the brace came from the US or the UK. The difference is in the thread angle as both of them have the same pitch of 18 threads per inch.

The chances of it being any form of metric are very slim unless it is of French or German manufacture.

Hi SparkG,
I'm pretty sure I've never seen a tapered thread like that, how would it even work? A lot of the old tool makers used their own thread design, your best bet might be to find someone with a lathe who can turn a replacement for you.
Cheers,
Geoff.

Thanks fellas...i didn't think it was metric either ChiefTiff, and I will look into what you said about 5/16" (I thought I tried that, though) and the other thread types.
Geoff, also a good idea to have one turned, actually I know a guy with a very fine lath and knows how to use it well.

And I couldn't figure how a tapered thread would work either

Pipe threads are tapered, but I've never seen them used for thumbscrews. The sizes you tried might correspond to nominal pipe size of 1/16, with an OD of 5/16 inch and 27tpi. With those specifications, your mate can probably make one.

Cheers,
Joe

Originally Posted by joe greiner

Pipe threads are tapered, but I've never seen them used for thumbscrews. The sizes you tried might correspond to nominal pipe size of 1/16, with an OD of 5/16 inch and 27tpi. With those specifications, your mate can probably make one.

Cheers,
Joe

Looking at the photo of that thread, it doesn’t look like 27tpi to my eyes.

Originally Posted by Colin62

Looking at the photo of that thread, it doesn’t look like 27tpi to my eyes.

Mine either, it looks quite coarse relative to its diameter. In about the same depth as the diameter of the hole, there are only 4 or 5 thread lands, so if that's a 5/16" hole, it would put the pitch somewhere around 14-16 tpi, which is a good deal coarser than NC for that diameter. I strongly suspect it's an 'orphan' pitch. The thread looks like a rounded-over square form, but that may be an illusion too. However, best way to figure it out is with a thread gauge, then hope the gears on your friend's lathe can match it.

Like the others, I doubt it's intentionally tapered, while taper on a pipe plug makes sense, I fail to see what advantages it might confer in this situation, methinks it would be more of a nuisance.......

Cheers,

Originally Posted by IanW

, methinks it would be more of a nuisance.......

Cheers,

Me too.

I'm also of the opinion that any taper is more likely a sign of much use (and maybe abuse.)

I'd either drill it out and re-tap and/or, if not concerned about maintaining originality, fitting an easy-out helix of appropriate size, possibly using a dab of liquid metal to fill any slop from the taper.

The latter'd give you a durable thread of known size. If the taper is from wear, I'd be concerned about the hardness of the metal and thus the durability of any retapped threads...

If you can get a thread guage beg or borrow it may give you an inkling as to the thread type ,therefore starting you on a good means to determine where to go next.

Failing that an engineering shop might sort it out for you.

Well, I do have a thread gauge, it seemed like 18 but I can't see enough in that hole to tell for certain, so I bought a couple thumb screws of 5/16-18. But no go, jams in the threads as did the 3/8". I'll keep searching. I'm afraid if I drill it out and rethread, it might crack.
Thanks everybody...

Just to close this out, I had a friend drill it out and retap it to use a 5/16-18 thumbscrew. Works perfect, on to the next problem!
Thanks again everybody.