The theoretical thread depth for the Whitworth thread form is 0.64032738p, where p is the thread pitch. So start with whatever is the ID of the tube and cut the internal thread to the calculated depth. Then start with the OD of the plug at the ID of the tube plus twice the thread depth. Cut the external thread, using the tube as a gauge during the final cuts.

That's the theory, the practise is rather messier. For a start 24tpi isn't a standard Whitworth pitch, so it's unlikely you'll be able to buy inserts. So that'll mean grinding ones own tools. Inevitably that'll mean that the crest and root radii won't be precise. So the threads won't be perfect. If the OP is expecting the threads to be gas/liquid tight without sealant, dream on! For that you'd need to be using BSPT threads.

Andrew

Yes, I know 3/16" and 7/32" Whitworth are 24tpi, but they're not in the original standard

Based on my experience with thread milling, one possibility for the internal thread at least would be to use a 24 TPI tap of significantly smaller diameter than the ID you are threading and grind off all but one of the "flutes" and grind the point back until it is square. Then mount in the toolpost with the cutting edge of the remaining teeth at centre height.

I'd proceed exactly as described by Andrew except I'd dump the Whitworth form. Unless of course what you're making has to be authentic and Inspector Meticulous is on the case!

Noting that 24tpi is close to M1.0, and guessing that the important thing is that the two made parts fit together rather than having to conform with genuine Whitworth threaded items, I'd substitute a metric thread-form tip. Apart from standardisation, there's no particular reason why one shouldn't cut a metric form thread at 24tpi to match a plug in an odd diameter tube.

By using the internal thread as a gauge, you can cut down towards the plug's theoretical dimensions and deliberately leave the thread slightly oversized to make it a tight fit. Assuming tight is what you mean by 'perfect'.

Dave

Roger,

I think Dave's [S.O.D.] suggestion is a good one, the two items are to fit together and if they do that to your satisfaction then the thread angle chosen doesn't matter in the least.

Proceed as he suggests

Regards

Brian

SOD won't using a 1.0mm pitch full form metric cutter leave overlength crests and valleys when used at 24tpi and if you go to partial form you are left with fiddling about to get the right DOC.

Also 24tpi is not the same thing or even close to M1, on the other hand it may be close -ish to 1.0mm pitch

Edited By JasonB on 09/11/2018 20:00:34

If you want to stick with 24tpi and keep the maths simple then get a 24UN full form insert.

That is 60deg not 55deg but you will be able to look up the DOC for a 10-24UNC and apply that to your pipe/plug whatever diameter that may be.

I seem to remember that Sparey covered all that ground in his book,years ago. Yes,I know he,s regarded as "Old Hat" these days,but there,s still a great deal of information [and in simple terms ] to be found there. Always worth a read.

I think your question has been answered,and explained,in that book.

As others have said,what difference does it make what thread you use,provided it,s the same on both parts.

Remember that with tubing,the wall thickness plays a big part when it comes to threading !