BA threads. Why the tpi?

[jimmy bParticipant @jimmyb]

[jimmy bParticipant @jimmyb]

I’m looking to make up some extension rods for a gauge. The thread is 2BA.
This where the fun starts for a Sunday. I’ve got taps and dies that size, but thought I’d screwcut them, to get threads perfectly in line. 31.35 threads per inch? Converts to .81mm pitch?

Who thought that was a good idea?

Jim, (now spending the rest of Sunday reading about threads)

[JasonBModerator @jasonb]

It is quite logical.

 

0BA is 1.0mm pitch you then just keep multiplying by 0.9 so

1BA 0.9mm pitch

2BA 0.81mm pitch

Playing about with your change wheels should get you close enough

Edited By JasonB on 22/10/2017 13:33:23

[NJHParticipant @njh]

Hey Jim

BA stands for BRITISH Association none of yer newfangled European stuff here so forget the .81mm and embrace the (almost) perfect symmetry of 31.35!

Norman

 

Edited By NJH on 22/10/2017 13:42:41

[Anonymous]

Posted by jimmy b on 22/10/2017 13:27:44:

Who thought that was a good idea?

The British Association.

Mathematically the pitches follow a strict geometric progression, rounded to 2dp.

Andrew

[jimmy bParticipant @jimmyb]

I just don’t see a rhythm with them. Weird tpi and doesn’t convert to milimetric clean…..

[Rick Kirkland 1Participant @rickkirkland1]

Mmmmmmm, BA. Strong threads in small sizes. I remember learning about the genius of this geometry probably 50 or so years ago. None of yer stupid weak knee’d metric ISO malarky and all that sails with it. Viva le BA.

[Anonymous]

Posted by jimmy b on 22/10/2017 14:02:51:
I just don't see a rhythm with them..

It's there in the mathematics; rather more logical than the arbitrary tpi values used for other thread series.

Andrew

[jimmy bParticipant @jimmyb]

I’ll do some research……

[jimmy bParticipant @jimmyb]

Not metric like you might expect, but with diameters determined by a factor proportional to a power of the logarithm to the base 10 of the thread pitch in millimeters.

I think it’s time to move on…

[MikeParticipant @mike89748]

When I were a lad… we regarded 2BA and 3/16 BSF as interchangable. Not quite true, but in many practical situations it does well enough.

[Andrew TinsleyParticipant @andrewtinsley63637]

Jimmy,

It is perfectly logical, in fact if you care to research, you will find that BA is a superior thread standard than the Metric ones.

I appreciate that your comfort zone is Metric. I matured on Imperial measurements and then had to use Metric for my scientific career. So I am ambidextrous as it were. Please don't knock what is a perfectly logical imperial system, just because you are not comfortable with it!

Each to his own and I would not dream of knocking the metric system, so give Imperial people the same courtesy. each system evolved to accommodate specific problems and they both work.

Andrew.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by jimmy b on 22/10/2017 14:29:04:
Not metric like you might expect,

They are metric in that they are all based on a 6mm diameter 1mm pitch thread. All is explained here. One of the first metric thread standards in the world.

Russell

[Neil WyattModerator @neilwyatt]

People are missing the point off Jimmy's original question: 'WHY'

The 'why' is because the Whitworth series screws only had a few smaller than 1/4" and they were all a bit coarse for instrument work.

It was decided to have a series of small screw sizes that would suit almost any use. After looking at examples from a around the world a Swiss system was adopted and (supposedly) tweaked a bit.

By making each size exactly 0.9 of the previous size everything stays in proportion even though the actual numbers are a bit odd. This didn't matter as they expected them to be made with taps and dies, not screwcutting.

Although 0BA is 6mm diameter and 1mm pitch, the British Standard is given in inches rounded to 0.0005" whatever Wikipedia says about the nearest 0.01mm!

In practice you will probably only ever need to measure one to check which BA size it is.

Despite the passage of time and the appearance of small metric threads, BA is still very useful, especially for modellers who want small, well-intentioned threads.

[jimmy bParticipant @jimmyb]

Thank you for all the input.

I had never before considered threads to such depth. I content myself with a little extra knowledge!

cheers Jim

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 22/10/2017 16:28:19:

… small, well-intentioned threads.

.

What a delightful concept !!

MichaelG.

[jimmy bParticipant @jimmyb]

Thread problems at work are usually either identifying an unusual thread or simply finding the spec. This is the first time I've ever considered the "why".

NPTF, MJ and UNJ are a few of the specs that tend to trip people up. Identifying threads can also be fun. For example finding the spec for a Morgan wheel nut thread……easy when you know

Jim

[jimmy bParticipant @jimmyb]

One of my late shift searches for a thread spec did lead me down a rabbit warren, it was whilst going down a blind alley that I found all about the "German bottle closure thread"…

 

jim

 

 

Edited By jimmy b on 22/10/2017 16:44:19

Edited By jimmy b on 22/10/2017 16:50:28

[richardandtracyParticipant @richardandtracy]

How about the plastic bottle thread spec for Coke, Pepsi etc. That is fun when you need to interface with them, but know so little you don't even know what questions to ask. I think I found them as a freebie Indian National Standard, which was/is equivalent to an ISO.

Regards

Richard.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Tinsley on 22/10/2017 15:30:41:

 

…

Please don't knock what is a perfectly logical imperial system, just because you are not comfortable with it!

…

Each to his own…

…

Andrew.

I hope I'm still allowed to criticise the imperfectly logical aspects of Imperial measure? After all there are plenty of examples. Remind me again why there are 14lbs in a Stone?

The current state of the Imperial system is the result of much tidying and tweaking. But it's internally inconsistent. The inconsistencies are historic and nothing can put them right. MKSA also has imperfections but at least it hangs together logically and has room to improve. That's why, taken as a whole, 'metric' is superior.

I'm curious as to why any system of measurement should gather a fan base, become a national icon, or need defending. Engineers and scientists surely understand that measurement is utilitarian, not emotional. The best system is the one that's most useful and straightforward. When I point out that Imperial has more defects than the Metric system, I'm not attacking a religion, personal beliefs, or an individual, its just the truth. I hope we all welcome that!

Equally I'm not offended when the various Metric kludges are pointed out. Bring 'em on, it's all educational. And you never know, they might fix it.

Dave

 

 

 

Edited By SillyOldDuffer on 22/10/2017 18:13:14

[thomas oliver 2Participant @thomasoliver2]

10 UNF thread is a few thou bigger than 2 BA and 5mm is a few thou bigger again, but they all have virtually the same tpi.. Thus a 10 UNF screw will fit a slack 2BA thread etc.

[duncan webster 1Participant @duncanwebster1]

Face up to it, Imperial is dying (and the sooner the better). As SoD says, who wants to have to remember the really odd conversion factors such as 550 (ft.lbs/sec to horsepower), 778 (BThU to ft.lbs) etc etc. For most jobs whole numbers of mm are quite good enough, not messing about with 1/64 and 1/32. My son is doing very nicely in real engineering (rather than hobby), and works entirely in SI. Like Andrew Tinsley I'm bi-lingual, but I always preferred to do calculations in SI, much more logical.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

… Remind me again why there are 14lbs in a Stone?

.

Sheep [or more specifically, Wool]

We adopted the 'Wool Stone' as a standard … and that was 2x 7pounds

7pounds being the accepted weight of a typical sheep clip.

Shear genius

MichaelG.

[Mick B1Participant @mickb1]

Posted by SillyOldDuffer on 22/10/2017 18:12:25:

I'm curious as to why any system of measurement should gather a fan base, become a national icon, or need defending.

Dave

The answer to that is that people who've made substantial intellectual investment learning idiosyncratic systems as schoolchildren are naturally protective of the effort it cost them.

Additionally, a great deal of the documentation and drawings for model engineering projects and mechanisms dates from a period when that system was in the ascendant, so it makes sense for model engineers to be fluent in it and conversions between it and the currently-favoured system.

[Brian HParticipant @brianh50089]

As depressingly usual, the topic has drifted off into something quite different. The original question as to why the apparently odd TPI in BA question has been adequately and comprehensively answered, so why the drift off into the sterile argument about metric and Imperial?

Brian

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