Type of thread on screw of sash cramps?

[Arty Flinders]

I recently bought a couple of sash cramps from ebay. They're pretty much the same in structure except for the thread of the screws. One is a very coarse thread, the other seems unusually fine. I can see that winding them both up together on a delicate job is going to be a little awkward because of this. So, I intend to buy a third, two of which will then, I hope, make a matching pair. Can anyone say what the relative merits of these different thread types are, to help me decide which type of thread to go for?

[mrcheerful]

You can apply more pressure with a fine thread. The coarser thread is less easy to jam up with saw dust. Either thread will give good results as you don't generally need the extra pressure the fine thread can give.

mrcheerful

[Jerry Built]

Hah!

The "coarse" thread should hopefully be a "square" thread - this is properly called an "Acme" thread, and is the proper one to go for. I prefer it when the threads are not chrome plated, as the chrome can flake leaving sharp edges.

J.B.

[Jerry Built]

That, dear boy, is why I put "square". Note the inverted commas.

J.B.

[Andy Dingley]

Acme isn't a square thread. Although it's similar (they're both flat topped), Acme is cut with 29° between the thread faces. This makes it much easier to cut than a square thread, and true square threads pretty much vanished before the war. Neither is easy to cut with a diestock, they're usually turned or ight be rolled in a small diameter.

[Arty Flinders]

Considering the weight of the advice and being just an old "square" myself, that's the thread I'll go for. Thanks,