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BA threads

why 47.5 degees?

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Anthony Knights31/01/2021 13:40:12
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I am puzzled by the 47.5 degree angle between the flanks of BA threads. Was there some obscure mathematical reason for this value, or were the design committee split as to whether to use 47 or 48 degrees. Whatever the reason, I don't see how half a degree would make any difference one way or the other. If it was up to me, I would have chosen 48 degrees, purely on the KISS principle. 48 factorises to 3 x 2e4 and would be really easy to set up for grinding threading tools. Forgive me for asking stupid questions, but I spent my life as an electronics engineer, where one learns to think logically and learns something new every week.

not done it yet31/01/2021 14:03:01
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I don’t think it was mathematical, particularly. The Swiss watchmakers were already using that angle, so perhaps best to ask them!?🙂

Tim Stevens31/01/2021 14:18:55
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A] it is not a stupid question

B] all engineers learn to think logically, sooner or later. Not all of them realise this.

C] I have been unable to find a helpful answer to your main question. There is certainly an obscure mathematical reason for the BA sizes, but I cannot remember the formula they used, or why. The same applies to the spanner sizes - few of which match anything in the Whitworth, BSF, USS, SAE, German or French spanner sizes. BA is described as 'developed from the Thury thread series', so Prof Thury must be your contact for the thread details - but bear in mind that 'no authorised standard exists for the Thury series' and his detailed specification figures are 'geometrically inconsistent'. Both quotes from Machinery's Screw Thread Book. And he died in 1938 - sorry.

The Wiki page on Professor Thury has further details.

Cheers, Tim

Cheers, Tim

Edited By Tim Stevens on 31/01/2021 14:19:45

Michael Gilligan31/01/2021 14:51:23
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Posted by not done it yet on 31/01/2021 14:03:01:

I don’t think it was mathematical, particularly. The Swiss watchmakers were already using that angle, so perhaps best to ask them!?🙂

.

If I recall correctly, there are two reasons why they settled upon that angle:

1. It is easier to tap

2. It 'holds' better [something to do with vectors]

... If I can locate my reference, I will post it for discussion.

MichaelG.

SillyOldDuffer31/01/2021 14:54:41
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I asked much the same question a few days ago. In it I suggested there's a relatively narrow range of useful thread angles and that 60° (Metric & UN) provides the strongest thread whilst 47.5° provides the best frictional grip. That makes 55° Whitworth a compromise between strength and grip.
JA said it's the other way round! It's not that I don't believe him, but it would good to see a solid reference.

Whitworth sampled what British Industry were doing at the time and standardised on an average. His thread is based on experience rather than science. 40 years later, Thury seems to have done the same, except his samples came from electrical and instrumentation makers rather than mechanical engineering and construction.

The Lowenherz thread was invented by a metrologist rather than a practical engineer. He went for 53° 8′ ( 53.1333° ), and there is an explanation. According to Wikipedia:

The unusual angle was chosen so that the pitch would be approximately equal to the thread's triangular height; however, the design was later truncated (flattened) at the roots and crests of the thread by a factor of one-eight the pitch, so the pitch is about 25% larger than the height, and the thread's depth is about 75% the length of its pitch.

Dave

Edited By SillyOldDuffer on 31/01/2021 15:01:54

Stueeee31/01/2021 15:07:05
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75 forum posts

I do remember reading somewhere that the BA series was based on the Thury threads. From a quick compare it looks like those responsible for the BA series just did some rounding on the Thury diameters and pitches e.g. No. 10 Thury .349mm pitch, 1.64mm dia; No.10 BA .35mm pitch, 1.7mm dia.

Thury thread table

JA31/01/2021 15:26:27
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Dave

I will reply to the comment about locking with analysis. It is going to take time since I will have to do a drawing and type out some maths. This was the sort of analysis we did in HND.

At present I am deep into looking at the Stephenson reversing gear (very much more complicated) so it may be a couple of days before I reply.

I will try to remain quiet until then.

JA

SillyOldDuffer31/01/2021 15:43:16
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Posted by JA on 31/01/2021 15:26:27:

Dave

I will reply to the comment about locking with analysis. It is going to take time since I will have to do a drawing and type out some maths. This was the sort of analysis we did in HND.

At present I am deep into looking at the Stephenson reversing gear (very much more complicated) so it may be a couple of days before I reply.

I will try to remain quiet until then.

JA

Many thanks! I collect second-hand technical books and have one that probably explains all. Unfortunately it's well out of my league. The reader is expected to follow the maths, which I don't! All help gratefully received.

Cheers,

Dave

Nick Clarke 331/01/2021 16:04:03
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Two extracts from the Wikipedia article on the Thury thread:

  1. The Thury thread is unusual in having a comparatively small 47.5° thread flank angle, which was chosen to make fabrication easier and to achieve greater holding capacity than screws with larger flank angles.
  2. The committee recognized the validity of many of the design aspects of the Thury thread, and a mere two years later published their specifications for the British Association (BA) thread. The BA committee made only slight modifications to the rounding radii of Thury thread and gave specifications rounded to the nearest thousandth of an inch.

But no calculations I'm afraid.

Martin Kyte31/01/2021 16:17:36
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What makes you think that setting to 47.5 degress is any more or less demanding than an to an integer to the same degree of precision. ?

regards Martin

Ian Johnson 131/01/2021 16:35:01
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Posted by SillyOldDuffer on 31/01/2021 14:54:41:

The unusual angle was chosen so that the pitch would be approximately equal to the thread's triangular height; however, the design was later truncated (flattened) at the roots and crests of the thread by a factor of one-eight the pitch, so the pitch is about 25% larger than the height, and the thread's depth is about 75% the length of its pitch.

Dave

Edited By SillyOldDuffer on 31/01/2021 15:01:54

A lot of 'approximately and aboutery' here! If that explanation is true (wikipedia) then all thread angles are based on guesswork and experience, with companies and governments wanting to protect their product and standards with their special and unique thread forms.

IanJ

Bazyle31/01/2021 17:12:23
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5831 forum posts
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Was it on here recently that someone said it is more difficult to cross-thread a BA screw than other threads? Is that the angle or the crest rounding?

If doing calculations remember that when this was worked out they didn't have excel and typically used 4 figure tables if they neede to be really accurate.
I came usntuck a few years ago trying to get something to match round figures using a computer but when I looked up the relevant cosine in old tables the 'error' made it fit round figures just like they might have done 100 years earlier. Further thought on how and why the item had been drawn indicated how the angle came to be and it was probably pure luck that it was an integer.

peak431/01/2021 20:02:59
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Here you go folks, some interesting reading for your next tea break; link to part 2 at the foot of the page
https://www.sizes.com/library/technology/thread_BA1.htm

From the 2nd page section 9
"For, as has recently been pointed out by Mr. Bosanquet,5 it is easy to cut a thread, whose pitch differs from one millimetre by an amount which may for all ordinary purposes be neglected (1/155300th), with a guide-screw based on the inch by the addition of a wheel of 127 teeth"

Now of course we can cut an exact 1mm pitch thread with a 127 tooth gear since the inch is defined as 25.4mm
There's an interesting article HERE on the varying definition of the the "Inch" with the passage of time. (I have posted that one before, but it's still worth a read.)
http://metricationmatters.com/docs/WhichInch.pdf

 

Bill

 

Edited By peak4 on 31/01/2021 20:04:12

Anthony Knights31/01/2021 22:43:05
486 forum posts
206 photos

Thank you peak4 for an interesting read. So we copied the 47.5 degrees and all the fault of the Swiss.

duncan webster31/01/2021 22:54:27
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If you take the arc tangent of 0.5 you get 26.565 degrees. Double it is 53.13 which is very near Mr Whitworth 's 55. I don' think thus is coincidence, it means you can feed down the flank of a thread without angling the top slide.

Whoever decided on standard metric thread pitch had clearly never driven a centre lathe. You need a fair number of wheels for the thread indicator instead of just one with tpi. They could equally have had threads per 25mm, or even threads per metre if they wNted to be pedantic.

CHAS LIPSCOMBE31/01/2021 23:26:06
15 forum posts

I read and enjoyed the link provided by Peak 4, thanks Bill. Has anyone heard of the Enfield inch for rifle etc manufacture? I was told that at one time, a contractor made some parts to Enfield drawings which did not fit. The reason was that the drawings used the Enfield inch which was different to anyone else's inch

Chas

Michael Gilligan31/01/2021 23:46:24
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Posted by Anthony Knights on 31/01/2021 22:43:05:

Thank you peak4 for an interesting read. So we copied the 47.5 degrees and all the fault of the Swiss.

.

Well, nearly : I haven’t located my book yet, but Wikipedia covers that point quite nicely:

[quote]


The Thury thread form had the crests rounded at 1/6p and the roots rounded at 1/5p so the thread angle was close to 47.5° but not exactly. This was simplified in the BA thread definition by defining the thread angle to be 47.5° exactly and the thread form to be symmetrical with a depth of 3/5p.

[/quote]

So, not so much ‘copied’ as ‘simplified’ ...
[ thereby sacrificing mathematical purity on the altar of manufacturing convenience ]

MichaelG.

peak401/02/2021 00:05:07
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1373 forum posts
157 photos

Here's a tale which I don't fully remember, recanted to me by a friend many years ago.
Norman and another chap were measuring some bike bits during a re-build, but something was going awry with the measurements>
Can't remember who it was now, but the other chap brought his own vernier calliper along, which showed a different size to Norman's.
It seemed odd as both items were Sheffield made.
On further investigation of part numbers, and contact with Moore & Wright, it transpired that one set were made for a continental export market, pre-war some time, and their inches were a different length to ours.

Bill

Michael Gilligan01/02/2021 00:10:52
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17641 forum posts
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I just happen to have this to-hand from a recent eMail:

1959 : The Canadian inch of exactly 25.4 millimetres was accepted by English speaking nations as the international metric inch

http://metricationmatters.com/docs/WhichInch.pdf

inch: **LINK**

MichaelG.

Edited By Michael Gilligan on 01/02/2021 00:12:26

duncan webster01/02/2021 00:47:07
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3130 forum posts
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Posted by peak4 on 01/02/2021 00:05:07:

Here's a tale which I don't fully remember, recanted to me by a friend many years ago.
Norman and another chap were measuring some bike bits during a re-build, but something was going awry with the measurements>
Can't remember who it was now, but the other chap brought his own vernier calliper along, which showed a different size to Norman's.
It seemed odd as both items were Sheffield made.
On further investigation of part numbers, and contact with Moore & Wright, it transpired that one set were made for a continental export market, pre-war some time, and their inches were a different length to ours.

Bill

That happened to me when I worked on a narrow gauge railway. The Chief Engineer had a sister who lived in Denmark. When he was visiting he bought a load of tape measures on the market. Then we started with mystery problems which he put down to me being useless until we realised that Danish inches are not the same as UK inches, so if I measured u with a UK tape, then cut te bits with a Danish tape we had problems. Problem went away when we dumped all the Danish ones

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