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Thread cutting again

Thread cutting dimensions

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Roger Hulett09/11/2018 14:38:38
131 forum posts
9 photos

I have to internally thread a brass tube. I will then fit a cap to this tube,the cap will have an external thread, and will screw into the brass tube.

Is there an easy formula to turn the diameters of the tube and the cap so that when threaded they will fit perfectly.

I am cutting 24tpi Whitworth.


Brian Wood09/11/2018 15:00:21
2579 forum posts
39 photos


Much depends on the relevant dimensions of the tube, will the wall thickness contain a 24 tpi Whitworth thread without breaking through for example?

By perfect fit, do you expect it to be oil or pressure tight? I think a bit more information will help us to help you.



Andrew Johnston09/11/2018 15:38:02
6668 forum posts
701 photos

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.


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

Emgee09/11/2018 15:38:50
2445 forum posts
291 photos

Hi Roger

All the information you need on this link **LINK**


John Haine09/11/2018 15:57:20
4715 forum posts
273 photos

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.

Roger Hulett09/11/2018 16:08:54
131 forum posts
9 photos

Thankyou for the replies. (Brian Wood), the brass tube is 3/16" thick,so should be OK for thread cutting, The cap is solid. The fit does not have to be pressure or liquid tight,but a good enough fit not to unscrew with vibration.

(Andrew Johnston), I was intending to use 55degree thread cutting inserts (from RDG) as I assumed that is the Whitworth requirement. Obviously incorrect,can you please advise further. I chose the 24tpi from the chart attached to my old S.B. lathe as it seemed fine enough to stop the cap unscrewing with vibration. Your corrected advice will be most welcome. Thankyou.

SillyOldDuffer09/11/2018 16:22:40
8876 forum posts
1997 photos

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'.


Oldiron09/11/2018 16:27:32
995 forum posts
40 photos

Many moons ago I had a South Bend 13" lathe. As far as I can remember the threading chart on that lathe which was post 1948 was for UNC & UNF threads not Whitworth.

24tpi is a standard for number 10 & 12 UNC also 5/16" & 3/8" UNF. Probably some others I cannot remember.

I stand to be corrected as my memory is not as good as it used to be.


Brian Wood09/11/2018 16:45:53
2579 forum posts
39 photos


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



Andrew Johnston09/11/2018 19:38:19
6668 forum posts
701 photos
Posted by Roger Hulett on 09/11/2018 16:08:54:

(Andrew Johnston), I was intending to use 55degree thread cutting inserts (from RDG) as I assumed that is the Whitworth requirement. Obviously incorrect,can you please advise further. I chose the 24tpi from the chart attached to my old S.B. lathe as it seemed fine enough to stop the cap unscrewing with vibration.

What I should have said is that you won't be able to get a full form insert for 24tpi Whitworth, ie, one that correctly forms the root and crest radii for a given tpi. The partial form insert from RDG will form the correct flank angle, but it won't form the crest radius, and the root radius will be too small. That leads to problems with depth of cut; since the radius is too small you need to cut a bit deeper to get the flanks correct. Of course for a one off it doesn't matter, although it it means both internal and external threads are a bit hit 'n' miss. Since the threads will be incorrect at root and crest they can't be called perfect, and will not seal properly.


JasonB09/11/2018 19:49:08
23039 forum posts
2769 photos
1 articles

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

Chris Gunn09/11/2018 20:21:04
430 forum posts
27 photos

Roger, as the components are made of brass and you want a shallow fine thread why not use BSB threads, 26tpi, designed years ago for this sort of job. this is close to your 24tpi, and you can get taps and dies cheaply in 1/16" diameter increments. Just the job for lubricators which is what it seems like you are making.

Chris Gunn

JasonB09/11/2018 20:44:39
23039 forum posts
2769 photos
1 articles

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.

duncan webster09/11/2018 21:44:51
4116 forum posts
66 photos

Why not grind a tool from a bit of HSS. Make the plug first and use it as a gauge. I've done it loads of times so it can't be difficult.

Roger Hulett10/11/2018 11:27:31
131 forum posts
9 photos

I am making a float chamber for a 1919 (unknown make) carburettor. My tube is 1 3/4" od and 1 3/8 id.. The cap is solid. The tube must remain at 1 3/4" od in order to fit in the base plate the joins the float chamber to the main body of the carb. The thread measurement of the internal thread appears to be 24tpi Whit (I have a Starret Whit thread gauge). My SB 9" lathe was made in 1939 and imported by Pidgen Bros (London). I had assumed (obviously incorrectly) that the chart attached to the lathe was Whitworth as it showed the change wheel settings for 24tpi,but I suppose it could be any 24tpi thread form. The float chamber I have (in a very sorry state of repair) has a thickness of 1/16 ",hence my query. Can I form a 24tpi thread in a 1/16 wall and then what external diam do I need the cap to be before cutting the thread on this.

If the SB chart is not specific to Whitworth,but with the correct change wheels form a thread with either 60degrees or 55 degrees (depending upon the insert) of 24tpi, does it really matter,providing the same tool is used both for cap and tube.

Tony Pratt 110/11/2018 12:35:35
2023 forum posts
12 photos


The thread form will not matter at all, obviously the same thread angle would be preferable on both male/female parts. Theoretical full thread depth for any diameter 24 tpi BSW is .0267".


larry phelan 110/11/2018 16:55:14
1191 forum posts
15 photos

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 !

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