Cutting an external thread

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Cutting an external thread

This topic has 8 replies, 6 voices, and was last updated 7 December 2023 at 16:51 by Howard Lewis.

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5 December 2023 at 15:01 #697545
Peter Simpson 3
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@petersimpson3
I’m making some 7/32″ BSF screws. The die I’m using is new and cuts a nice thread. How far from to a shoulder would you expect the last thread to be. There appears to be two thread missing as the die touches the shoulder is this normal. The screw I’m copying has threads touching the shoulder.
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5 December 2023 at 15:08 #697546
Clive Brown 1
Participant
@clivebrown1
I think that it varies with the die. They will have a taper on one side, normally on the side which is marked with size etc. If I want to thread as near as possible up to a shoulder I try reversing the die in the holder after the first cut. This often allows an extra turn or so to be cut.

Commercial threads may well be made by a different type of tooling, hence threaded up to shoulders as you observe.
5 December 2023 at 15:30 #697547
DC31k
Participant
@dc31k
Another approach is to look at where the finished screw is going and ask yourself whether it actually needs to be threaded to tight under the head.

There are places where an exact facsimile is necessary, but there are also places where the same functionality can be provided with a lot less effort.

If you need the threads even tighter under the head than the reversed die provides, you can put the die on a surface grinder (writing side downwards) and thin it down until it cuts to its face.
5 December 2023 at 16:08 #697557
JasonB
Moderator
@jasonb
You can also consider cutting a relief just under the head to the minor diameter of the thread and just over 1.5 pitchs wide so the full thread runs into that. Or another alternative is to drill the top of your threaded hole 7/32″ or just over by a couple of pitches deep. Both will allow the screw to go in until the underside of the head touched the tapped surface.
5 December 2023 at 16:41 #697562
Mike Hurley
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@mikehurley60381
As Clive Brown says, cut as normal with the tapered side leading and when as far as you get to the shoulder reverse the die and cut the final bit. Will give a thread that is quite sufficient for all practical purposes. Have done it that way for years on odd occasions when needed

Regards Mike
5 December 2023 at 19:05 #697596
Peter Simpson 3
Participant
@petersimpson3
I have reversed the die in the holder and cut threads from both sides with no change in thread form. Looking at the die under a magnifying glass both ends have the same tapper. The die is a “cheap” carbon steel one. Would the likes of a HSS quality die be any better ? The screws are for old Webley air rifles so I’m sure the finished screws will be OK. When making hex headed screws I have used the undercut method to get a close fit when required.
5 December 2023 at 19:15 #697599
JasonB
Moderator
@jasonb
A lot of new dies seem to have a similar taper both sides even good ones so unles syou can see before you buy you may not gain anything.

I do have a couple where I have hollowed out the backs with a dremel but still left a solid “ring” around the edge so they still sit true in a die stock but will go right up close to a shoulder provided said shoulder is not too large. Should not be hard to do on carbon steel ones.
5 December 2023 at 20:40 #697637
Peter Simpson 3
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@petersimpson3
Jason I had considered, would it be possible to reduce the depth of taper to get the cutting edges closer to the shoulder. Your idea is very interesting.
7 December 2023 at 16:51 #697940
Howard Lewis
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@howardlewis46836
Is it absolutely necessary for the therad to run right up to the underside of the head?

Often, the hole has a chamfer, or there is enough thread engagement for an undercut on the screw not to damage security.

Ideally, as long as you can get 1D (one diameter) of thread engagement, sufficient clamp load can be exerted.

You are cutting the thread with a die; the original screw was probably mass produced on a machine specifically designed to produce the screw in its entirety, by the thousand. (Bar in, finish machined screw out)

A Cri Dan screw cutting machine would produce a perfect 1″ BSW thread, 8 or 10 inches long in under a minute, in several passes! So 7/32 BSF screws would be even quicker.

At the Bell and Howell cine camera factory in Gloucestershire, the machines there would produce a completely machined small screw in less than a minute!.

Probably you won’t have a problem.

Howard
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