Screw cutting BSP threads

[Adrian 2Participant @adrian2]

My Myford super seven is an export model and the gearbox does not have a 19 TPI setting (1/4 and 3/8 BSP ) .

My question is, can this pitch be arrived at by using change wheels and or the metric conversion set?

As a new comer to the delights of a gearbox, I am trying to get to grips with what can be achieved .

I do not have an immediate need for this thread but would like to know should I require it again later on.

Thank you.

Adrian.

[Adrian 2Participant @adrian2]

[JasonBModerator @jasonb]

discussed the other day re a Boxford but same principal applies eg play about with teh gearbox input

[Simon Williams 3Participant @simonwilliams3]

Well, according to Brian Wood (Gearing of lathes for Screwcutting) all you need to do is to fit a 29T gear to the mandrel gear, and set the American gearbox to 23TPI. This combination will cut 19.04 TPI, but don't disengage the half nuts once you've started. I'm assuming as you don't have a 19TPI setting you do have the alternative 23 TPI.

So the problem transfers from not having a 38 T in the gearbox to needing a 29T.

No doubt Brian will chip in with some other suggestions?

Rgds Simon

[Brian WoodParticipant @brianwood45127]

Hello Adrian,

Chipping in as anticipated!

​The export gearbox was given a 23 tooth wheel in the gear cone to suit the North American 23 tpi thread pitch common on their own make of lathes. To do that, Myford took out the 19 tooth wheel in the domestic version of the gearbox and shuffled the gears together again.

​I'm afraid the 29 T wheel combined with 23 tpi setting of the gearbox gives the closest approach at 19.04 tpi.

​Regards Brian

[Adrian 2Participant @adrian2]

Thanks to everyone, quick response. I am not clear on where to fit the 29T wheel. I have the 23T in the gearbox as Simon assumed.

19.04 TPI sounds close, could that discrepancy be considered of no consequence in practical terms?

Adrian.

[Brian WoodParticipant @brianwood45127]

Adrian,

​Fit the 29 wheel in place of the 24 tooth mandrel wheel that engages with the first of the cluster gears and don't forget to change it back again afterwards or the gearbox will turn out some very non standard pitches in the other settings.

​19 tpi is a pitch of 0.05263 inches—-19.04 tpi has a pitch of 0.05252 inches, about 1/10th of a thou per inch of thread in error- I imagine it is close enough for most purposes.

Regards
​Brian

 

 

Edited By Brian Wood on 22/07/2017 16:30:34

[Adrian 2Participant @adrian2]

Thank you for your help Brian,

It is good to know I can cut 19TPI if I need to. I don't think I will let 1/10th of a thou bother me!

Kind regards,

Adrian.

[Clive FosterParticipant @clivefoster55965]

Myford leadscrew accuracy is said to be 2 thou per foot or better. 1/10 th per inch is same order of magnitude so probably of no great importance in practice. Especially if the errors subtract.

Clive.

[Simon Williams 3Participant @simonwilliams3]

Adrian – If you search for "Myford Super 7 screw cutting gears (metric)" using the "Keyword" field at the top of these postings, you'll find yourself being taken to (one of several) threads on this subject. It's actually about metric threading, but that's irrelevant as metric threads on a lathe with an 8 TPI lead screw are only imperial TPI's with funny numbers, the principles of working out the actual TPI being cut is the same. The main attraction of this thread is that the OP (Allan) put up some annotated photo's which explain which gear is meant by Brian's phrase "the mandrel wheel"

In case you didn't know, Brian has written the definitive book on the subject – his "Gearing of Lathes for Screwcutting". It should have been subtitled "All you need to know about making your lathe cut threads it's never heard of!"

Best rgds Simon

[Anonymous]

Err, surely the error is about a tenth of a thou per pitch, after an inch you'll be out by 19 times that, or roughly 2 thou. Still not particularly significant, unless you've got a long thread engagement.

Andrew

[Adrian 2Participant @adrian2]

Simon,

Thank you for the link to Allan. Interesting reading just what I was looking for.

Regards Adrian.

[Neil LickfoldParticipant @neillickfold44316]

Brian Wood, How do I get you book of screw cutting gear arrangement , I have a Myford S7 here in New Zealand. The quickchnage box is green and is an add on with my lathe I think, as the Lathe is painted Grey.

Neil Lickfold

[MonomanParticipant @monoman]

While on the subject of BS pipe threads can I ask if anyone has a technique or ideas on making BSPT (taper) threads? I am mainly interested in external but any suggestions also for internal.

The sizes are given are all over the text books and other tables but its the concept of turning a taper and a thread without a taper turning attachment that fascinates me

This is not just a theoretical interest, I have a need for some 3/8" and 1/4" taper plugs and would rather make my own than buy ready made.

Jerry

[Swarf, Mostly!Participant @swarfmostly]

Posted by Andrew Johnston on 22/07/2017 18:15:02:

Err, surely the error is about a tenth of a thou per pitch, after an inch you'll be out by 19 times that, or roughly 2 thou. Still not particularly significant, unless you've got a long thread engagement.

Andrew

It seems relevant to point out that BSP threads on unions aren't very long – i.e. they're not long enough for the cumulative pitch errors discussed here to get big enough to be a problem.

The shanks of domestic sink & bath taps might, of course, be a different matter. But then, it's not all that many years since they were cut with hand chasers! (Off topic but research 'Barking Brassware'. )

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 23/07/2017 09:44:47

[Simon Williams 3Participant @simonwilliams3]

Adrian – playing with the numbers I've found a couple of alternative answers which aren't as accurate as the 29 t gear solution, but they would save you having to buy a 29 t gear.

Option 1 is to fit a 25T gear on the mandrel wheel, select 20TPI on the gearbox, and actually cut 24/25 x 20 = 19.2 TPI. Pitch of 19.2 TPI is 0.05208 ins, pitch of 19 TPI is 0.05263 so pitch error is 5.5 tenths per rev. Over a typical 1/2 inch long thread this error multiplies, so you get a cumulative error of say 6 thou. That's only accumulated an error of 10% of the thread pitch, so In a tapered thread I don't think that would matter, tapered threads are designed to jam up! I'd still be OK with this on parallel threads provided the pressure was moderate.

Option 2 is to buy a 33T gear, pop it on the mandrel wheel, and select 26 TPI. Lathe now cuts 24/33 x 26 tpi =18.909 TPI, pitch = 0.05289 ins. Pitch of 19TPI = 0.05263, so error is approx. 2.6 tenths per revolution. So error of a 1/2 inch thread engagement is about 3 thou, so half the error of option 1.

So why is buying either of these gears better than buying a 29T?

25T is a standard size, you might have one, certainly easily available. For the purposes of this you could make one by hand – turn up a blank 1.35 ins OD and divide the rim into 5, then each section into 5 again gets you 25 segments, as evenly as possible. File 'em out by hand, any irregularities will come out in the wash!

33T isn't a standard size, though you will have realised by now that this and a 34T get you most of the metric threads, so buying one or both has a wider application.

Hope that helps,

Best rgds Simon

[Adrian 2Participant @adrian2]

Thank you Simon for the trouble you have gone to. Much appreciated, plenty of food for thought their.

Kind regards,

Adrian.

[Brian WoodParticipant @brianwood45127]

Gentlemen,

Some replies are due I think

​Andrew Johnson ​You are indeed quite right, the error is per pitch, a silly oversight of mine. I hope it hasn't mislead others, thank you for pointing it out

​Neil Lickfield If Crowood Books have an agent in NZ they are the publishers, otherwise Amazon have them on sale, as do Arc Eurotrade. I don't know if Arc post to NZ The ISBN number is ISBN 978-1-78500-250-2

​Simon Williams 3 There are of course many other wheel/gearbox combinations that produce results of larger error, much depends on what the user is prepared to accept. My book lists results which fell within defined limits which I considered acceptable.

Best regards
​Brian

[Neil LickfoldParticipant @neillickfold44316]

Brian, I sent you a PM message on the forum here.

To cut tapered threads, you either need a thread cutting lathe that has the adjustable pivoting headstock, or else like on a Myford S7, You need a taper turning attachment, and the compound slide to be rotated around so that the cut can be applied.

Neil

[Anonymous]

Posted by Neil Lickfold on 23/07/2017 11:42:28:

To cut tapered threads, you either need a thread cutting lathe that has the adjustable pivoting headstock, or else like on a Myford S7, You need a taper turning attachment…………

When I cut the 3/8" BSPT threads for my traction engine boiler drain plugs I plan to use a hydraulic copy unit following a tapered pattern. As far as I can see that should work in conjunction with normal screwcutting procedure. The hydraulic copy unit tool is behind the work, but I run with it upside down and the lathe running in the normal direction, so I should still be cutting a RH thread moving towards the headstock.

Andrew

[John HaineParticipant @johnhaine32865]

Posted by Neil Lickfold on 23/07/2017 11:42:28:

Brian, I sent you a PM message on the forum here.

To cut tapered threads, you either need a thread cutting lathe that has the adjustable pivoting headstock, or else like on a Myford S7, You need a taper turning attachment, and the compound slide to be rotated around so that the cut can be applied.

Neil

Or CNC – standard feature in Mach 3 IIRC. No change wheels needed either…

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