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What was the thinking behind BA threads

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Joules:
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.

philf:
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.

Joules:
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.

philf:
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.

Joules:
LOL, don’t 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 wasn’t the first value used for the inch ?

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