Author Topic: What was the thinking behind BA threads  (Read 1098 times)

Offline Joules

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What was the thinking behind BA threads
« on: August 04, 2019, 04:59:41 PM »
I wonder what the origins and thinking behind the creation of BA was. Not a thread you can generally cut on a lathe. Seems to be a mixture of imperial and metric values.  Can anyone point me towards information on this, purely curiosity. I just had to buy a set of 3BA taps and a die.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline philf

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Re: What was the thinking behind BA threads
« Reply #1 on: August 04, 2019, 05:43:06 PM »
Hi Joules,

According to my bible on the subject (Machinery's Screw Thread Book) BA threads were first proposed by the British Association in 1884. They were based on the Thury thread!

The Thury thread appears under horological threads and, with the exception of three very small threads, appear to be identical. There's no explanation of how Professor Thury came up with his threads.

Phil Fern
Location: Marple, Cheshire

Offline Joules

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Re: What was the thinking behind BA threads
« Reply #2 on: August 04, 2019, 05:56:01 PM »
Cheers Phil, I found this since raising my question.

https://www.sizes.com/library/technology/thread_BA1.htm

It goes quite a way towards explanation.  I was puzzled by the vintage part I am restoring being French and expecting the thread to be metric and it turned out to be BA.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline philf

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Re: What was the thinking behind BA threads
« Reply #3 on: August 04, 2019, 06:07:28 PM »
It's odd that the system is based on a 6mm thread with 1mm pitch yet 25.6 TPI ( and not 25.4 TPI)!

I wondered how the diameters were arrived at - each being 9/10 of the previous size.

Phil Fern
Location: Marple, Cheshire

Offline Joules

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Re: What was the thinking behind BA threads
« Reply #4 on: August 04, 2019, 06:11:43 PM »
LOL, donít worry I am still scratching my head over it.   Really looks like something dreamed up to foster Anglo French relations at the expense of logic.....   Hmmmm who would have thought such a thing possible.


Just a thought, when was inch to metric ratified.  I know that 25.4mm wasnít the first value used for the inch ?
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline PekkaNF

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Re: What was the thinking behind BA threads
« Reply #5 on: August 05, 2019, 05:48:57 AM »
Inch has been changing over times and places....I tired to find direct link, but could not.

There is an interesting table on this document on page 7 -8
https://emtoolbox.nist.gov/Publications/NISTMonograph180.pdf

The English system of units was based on a yard bar, another artifact standard [6].
These artifact standards were used for over 150 years. The problem with an artifact standard for
length is that nearly all materials are slightly unstable and change length with time. For
example, by repeated measurements it was found that the British yard standard was slightly
unstable. The consequence of this instability was that the British inch ( 1/36 yard) shrank [7], as
shown in table 1.1.
Table 1.1
1895 - 25.399978 mm
1922 - 25.399956 mm
1932 - 25.399950 mm
1947 - 25.399931 mm

1.2 The Inch
In 1866, the United Stated Surveyor General decided to base all geodetic measurements on an
inch defined from the international meter. This inch was defined such that there were exactly
39.37 inches in the meter. England continued to use the yard bar to define the inch. These
different inches continued to coexist for nearly 100 years until quality control problems during
World War II showed that the various inches in use were too different for completely
interchangeable parts from the English speaking nations. Meetings were held in the 1950's and
in 1959 the directors of the national metrology laboratories of the United States, Canada,
England, Australia and South Africa agreed to define the inch as 25.4 millimeters, exactly [9].
This definition was a compromise; the English inch being somewhat longer, and the U.S. inch
smaller. The old U.S. inch is still in use for commercial surveying of land in the form of the
"surveyor's foot," which is 12 old U.S. inches.




Offline Sea.dog

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Re: What was the thinking behind BA threads
« Reply #6 on: August 05, 2019, 01:21:40 PM »
"This definition was a compromise; the English inch being somewhat longer, and the U.S. inch
smaller."

The English inch at 25.399978 would appear to be minimally smaller than the adpoted standard - 25.400051

Offline PekkaNF

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Re: What was the thinking behind BA threads
« Reply #7 on: August 05, 2019, 04:04:24 PM »
Can't remember if it was "Foundations of mechanical accuracy" that described mishaps that happened to different yard "sticks" that were used as physical standards at different times. Those were the times when for conversion you need to know who defined what, where and when - and how big your oppenent is.

I never seen an unified table that allows conversion of historic units to modern standard units by year and place...these two comes pretty close as an entertainment (when I need old Swedish units or such oddity)
https://www.convert-me.com/en/convert/length/inch.html?u=inch&v=1
https://www.convert-me.com/en/convert/history_length/?u=millimeter&v=25.4


Offline djc

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Re: What was the thinking behind BA threads
« Reply #8 on: August 09, 2019, 03:15:21 PM »
Really looks like something dreamed up to foster Anglo French relations at the expense of logic.....

I would beg to differ with you on this point. Of all the thread systems, BA is the most logical, based as it is on two simple mathematical formulas relating thread number, diameter and pitch.

It is also a geometric progression, which is generally a good thing in engineering (c.f. the speed ratios in an geared lathe or mill).

Pick any of the other systems and see if you can derive similar relationships. You cannot. Look at how Whitworth developed. Old Joe went traipsing around a load of workshops and picked what seemed to be most commonly used. How is that more logical?

That is also why it is popular in model engineering: because it scales well. Standard metric or imperial, for instance, based loosely as they are on an arithmetic progression, do not do so, so fasteners often look either too big or too small.

Offline Muzzerboy

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Re: What was the thinking behind BA threads
« Reply #9 on: August 10, 2019, 12:53:30 PM »
Apart from being a geometrical progression, there's nothing massively logical about BA. It may "look" good in all sizes but most BA pitches are impossible to make on a lathe without a bucket of changewheels, albeit they are mostly too small to cut on a lathe. Most other thread systems retain some proportionality but make discrete steps as they increase in size, to allow practical dimensions for pitch etc. It's ironic that BA threads often seem to be used to reproduce real world threads such as Whitworth at model scales....

Offline Will_D

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Re: What was the thinking behind BA threads
« Reply #10 on: August 10, 2019, 03:55:19 PM »
but most BA pitches are impossible to make on a lathe without a bucket of changewheels,
But why would you want to screw cut a 6 ba thread? You use dies!

Dies when adjusted correctly cut a perfectly formed thread (including the roundy tops and bottoms) by a process of shear and plastic deformation.

Screw cutting on a lathe really comes into it own when you haven't the physical grip on the work piece or the die to resist the torque.

Just look at a pipe fitter cutting 2" x 11 tpi pipe threads - it certainly leaves its mark on the pipe from the pipe vice

Or when you want to cut a unique thread like 40 mm by 1.5 or 1" by 50!!

Simple screw cutting involves pushing a pointed tool (with no accurate tip radius) into the work, the crests of the thread are left raggy and are usually rounded of with afile!
Engineer and Chemist to the NHC.ie
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Offline djc

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Re: What was the thinking behind BA threads
« Reply #11 on: August 11, 2019, 05:14:42 AM »
But why would you want to screw cut a 6 ba thread? You use dies!

OK, I will bite...

10 How was the 6BA die made?
20 With a 6BA tap.
30 How was the 6BA tap made?
40 With a 6BA die.
50 Goto 10

Screwcutting for BA could be made very logical if you approach it with the right mindset. Start with a 1mm pitch leadscrew. Gives you 0BA. Gear it 9/10 for 1BA. Gear it 9/10 x 9/10 for 2BA. You only need two sizes of change gear (and a very big banjo!).


Offline timby

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Re: What was the thinking behind BA threads
« Reply #12 on: August 11, 2019, 06:44:56 AM »
But why would you want to screw cut a 6 ba thread? You use dies!

OK, I will bite...

10 How was the 6BA die made?
20 With a 6BA tap.
30 How was the 6BA tap made?
40 With a 6BA die.
50 Goto 10

Not if you have one of these,   



« Last Edit: August 11, 2019, 03:59:39 PM by timby »

Offline djc

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Re: What was the thinking behind BA threads
« Reply #13 on: August 12, 2019, 03:01:24 PM »
Not if you have one of these

Looked a bit big for 6BA :-)

And what, pray tell, do you think was moving the workhead when it was grinding the threads?

A ballscrew, which is a fancy leadscrew, with electric cogs for pitch. So it was screwcut.

You need to find an example that is not screwcutting. Maybe that lost episode of the A Team where they were locked in a barn and they filed a 6BA thread on a pitchfork handle.

Offline DavidA

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Re: What was the thinking behind BA threads
« Reply #14 on: September 04, 2019, 04:31:34 PM »
One could say, tongue in cheek, that it was a far - sighted plot to make money out of model makers.

'I say, that man is using Metric screws on his British loco models. Fella should be horse whipped'.

Dave.