Thread identification 5/16 – 38.5 TPI ??

[gerry maddenParticipant @gerrymadden53711]

Hoping someone can help me identify a thread. I have several on my OMT that are nominally 5/16 diameter but have a very fine thread which I determine using calipers to be approx. 38.6TPI. I think it must be some kind of instrument thread but cant find anything as fine as this on the web but I probably just haven't looked in the right places !. Just for comparison, in the picture below I've included a 7/32-28 BSF tap.

Edited By gerry madden on 16/11/2020 19:46:29

[gerry maddenParticipant @gerrymadden53711]

[Tony Pratt 1Participant @tonypratt1]

Isn't it annoying when you see an abbreviation you don't know [OMT], is said OMT an imperial or metric thing?

Tony

[gerry maddenParticipant @gerrymadden53711]

Apologies, its an OMT microscope.

[Tony Pratt 1Participant @tonypratt1]

If imperial likely to be 40 TPI

Tony

[gerry maddenParticipant @gerrymadden53711]

1940's British..

[Tony Pratt 1Participant @tonypratt1]

Looks a GB make so 40 TPI?

Tony

[Tony Pratt 1Participant @tonypratt1]

Could you not use the microscope itself to measure the thread pitch, a caliper won't cut it.

Tony

[Clive Brown 1Participant @clivebrown1]

Beaten to it, but 5/16" x 40tpi is a standard ME thread.

[Nigel Graham 2Participant @nigelgraham2]

It does not seem metric at all – the pitch works out as 0.658mm – so I'd go with 40tpi as others say.

If you have a set of ME taps, you could try making a 5/16 X 40 comparator in an oddment of material friendly to the component, like brass or aluminium.

[Michael GilliganParticipant @michaelgilligan61133]

My guess is that it’s a special … screw-cut to the 4BA pitch

Have a look here: **LINK**

http://pages.cs.wisc.edu/~bolo/workshop/thread.html

MichaelG.

[gerry maddenParticipant @gerrymadden53711]

Thanks all for your efforts. 40 tpi was all I could conclude too. Working backwards I only need to be a third of a pitch out with the calipers and that would be the difference. I think you are right Tony – even when I magnify the calipers what looked like sharp points in the ends of the anvils turn into very rounded off clubs and it doesnt really improve accuracy.

I dont have any ME taps but I have found a cheap one so I will buy that and see how it looks.

[JasonBModerator @jasonb]

Measure over 10 threads not one then any error will be less. You could also start the screw into a hole and measure how much it protrudes, then screw it in 10 turns and measure again, Divide result by 10 to get pitch.

[gerry maddenParticipant @gerrymadden53711]

Michael, you've just thrown a spanner in the works now ! Im wondering now as every other thread on this machine is BA…. I'll give this some thought.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Clive Brown 1 on 16/11/2020 20:11:11:

Beaten to it, but 5/16" x 40tpi is a standard ME thread.

.

That said … I believe that ME threads are only ‘standard’ amongst Model Engineers

MichaelG.

[JasonBModerator @jasonb]

There is also 5/16 x 40 UNEF to throw into the works, but given the other comments less likely to be a US thread.

Michael thought you may have suggested 4 Thury thread pitch

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 16/11/2020 20:44:15:

[…]

Michael thought you may have suggested 4 Thury thread pitch

.

I left that for Gerry to spot for himself … but knowing that it’s an OMT microscope, I dared to presume that they would have been using BA wherever convenient.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by gerry madden on 16/11/2020 20:35:07:

Michael, you've just thrown a spanner in the works now ! Im wondering now as every other thread on this machine is BA…. I'll give this some thought.

.

Easy to check … Just use a 4BA screw as a thread [pitch and profile] gauge

BA is 47.5° so it should be pretty obvious if the mystery thread is 55° or 60°

MichaelG.

[gerry maddenParticipant @gerrymadden53711]

Well, I tried the 4BA up against the grub screw and it was close… but just didn't feel quite comfortable. I then dug through my tap collection and stumbled across a 1/8BSW which is 40 TPI. Offered it up and it fitted like a glove, at least on pitch. Its just too small, or too late at night to compare the thread forms but hey for £3 I cant go wrong with the 5/16 ME and I'll order one tomorrow.

Thanks all for your suggestions and thoughts. Appreciated.

Gerry

[Michael GilliganParticipant @michaelgilligan61133]

MichaelG.

[Phil PParticipant @philp]

Dont you just hate non standard threads……..

My neighbour came round a couple of nights a go with a special bolt that he said is for holding a bed frame together.

His daughter had lost one and could I make him a replacement, he handed me one as a sample and it looked fairly straight forward so I said yes I will do it as a favour (It was dark and I didn't have my glasses on !!)

Every dimension on the special bolt was imperial, apart from the threaded end which at first I assumed must be 5/16" Whitworth. It is a good job I double checked because it was M8 x 1.5 which is very unusual.

I must have thousands of taps and dies and Coventry chasers, in all sorts of obscure sizes, but you guessed it no M8 x 1.5 was to be found.

So that 10 minute job turned out to be 1½ hours by the time I had set up the lathe to screw cut the thread, and having nothing to check it against I am keeping my fingers crossed it will fit the tapped hole in the bed frame.

Moral of this story is……..Dont let the neighbours know you have a workshop.

Phil

[Nigel McBurney 1Participant @nigelmcburney1]

Usual practice on uk instrument optical work was to use even number BA ,26 and 40 tpi whit form threads. The objective lens thread was/is .8 ins dia by 36 tpi whit form. Nowadays metricated conversion in decimal metric. The use of BA over many industries was common though even number BA become the prefered sizes and no 0 BA was generally replaced by 1/4 BSF, there with some peculiarities A lot of electrical work used odd number BA , I worked for a subsidary of Imperial Typerighters in the mid sixties,Imperial mainly used odd number BA and the subsidary where I worked which Imperial bought out used even no BA, lots of hassle from Imperial trying to get us to change to odd number,as they made all screws inhouse with well over a hundred lathes in the auto shop.

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