BA threads. Why the tpi?

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

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

The British Association.

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

Andrew

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

Andrew

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.

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.

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

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.

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

.

What a delightful concept !!

MichaelG.

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

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

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.

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

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.