BA Threads

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Being born in Coventry (well physically Leamington Spa due to it being Wartime!) and my father being a Capstan Setter/Operator, the name of the company was very well known to me when growing up.

Anyone in the midlands area knew that if you wanted to be a toolmaker then getting an apprenticeship at CG&T was a 'top prize' - and I considered myself very fortunate. Once I'd been accepted I still had to wait nearly 6 months for a place to become available - and they paid me! - 17/6 per week - (87½p) o_O to start.
 
This all seems very strange.
Why was this undersizing adopted?

I'll agree that there were some strange decisions made but it was early days.

Yojevol said:
Why is it apparently associated with the BA number which is arbitrary?

Why was the numbering system chosen? I imagine the decided that 6mm - 0.25mm would be the range having 26 sizes, 0BA - 25BA
I don't think the use of numbers as a reference is arbitrary for small diameters - it's so much easier to use a single character as a designation - Can you imagine having to write (say)

2.8 x 0.53 BA rather than 6BA ?

and that's without specifying 'mm'...
and since a 'geometric' progression had been determined, the Nº series made even more sense.
Yojevol said:
Why was the major diameter of the internal thread chosen as the base? It's the least accessible of the 4 diameters (internal/external, major/minor)
I haven't previously seen any reference to the 'Internal' OD being used as the base
 
@J-G Ref, the numbering system. I wasn't commenting on the numbering system itself, but apparent relationship of the thread diameter 'correction' to it, which, IMO (no H), seems arbitrary.
Brian
 
Ah -- That makes sense :)

I think the Basic dimensions of the BA 'Standard' was (is?) the most mathematically determined, and yes, the choice of 0.9 as the ratio does seem somewhat arbitrary but I suspect that it was determined very specifically after many hours of calculation.

I haven't considered the US Numbered threads below ¼" but I suspect that may be a similar 'can of worms' :eek:

I must admit that when I posted the original question I didn't anticipate such an interesting discussion !
 
I have a small TCP tin with BA taps in it, down to about 8BA. Used them when i was in the toolroom, now i cant see them without glasses and as for the threads and drills needed to tap.......forget it.:oops:
 
I have Taps & Dies down to 12BA - though I seldom use less than 10BA - but when I was an apprentice, I spent some time in the 'Plating' shop where I had to 'wire up' ready to chrome plate the inside threaded portion of 20BA screw ring gauges - the bore on that being ~0.34mm !
 

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