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What was the thinking behind BA threads

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PekkaNF:
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.

Sea.dog:
"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

PekkaNF:
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

djc:

--- Quote from: Joules on August 04, 2019, 06:11:43 PM ---Really looks like something dreamed up to foster Anglo French relations at the expense of logic.....
--- End quote ---

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.

Muzzerboy:
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....

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