BA Threads

[J-G]

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) to start.

 

[J-G]

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.

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.

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

 

[Sandyn]

they paid me! - 17/6 per week - (87½p) to start.

amazing

 

[Yojevol]

@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

 

[J-G]

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'

I must admit that when I posted the original question I didn't anticipate such an interesting discussion !

 

[hawkeyefxr]

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.

 

[J-G]

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 !