G'day folks,
Great to see such an interesting discussion on screw threads, here at ODK!
Agreed, that the Brits are notable for their plethora of 'standardised' screw thread types.
Let's not even mention the 'defined non-standard' threads used by the like of the Royal Enfield Arsenal - they even had their own 'Enfield standard inch' dimension...
A few that I haven't seen mentioned so far, are BSB [British Standard Brass - all 26TPI, but a different thread form to BSC], BSP [British Standard Pipe] and BA [British Association]; there were more...
However, I do feel that I would be remiss here, if I failed to point out that Sir Joseph Whitworth was the world pioneer in introducing the very first series of standardised, scientifically designed screw threads, back in 1841.
A concise rundown, pinched from
http://www.mossmotoring.com/whitworth-system/His idea was simple:
Each diameter of bolt or screw will have its own number of threads per inch (TPI)
The angle between the side of one thread and the adjacent thread should be 55°.
Both the crest and root of each thread should be rounded.
The relationship of the pitch to the radius of the rounded portion of the thread is defined by a ratio of l/6th; in other words, the radius r = (1/6) x (pitch).
Finally there was a system. If adopted, that would allow the fasteners used on one type of machine to be replaced with another “standard” fastener. The logic was hard to beat, and England adopted the system to the extent that by 1881 it was the effectively the British standard.
The Whitworth System was used as proposed for bolts and screws from 1/8″ to 4 1/4″ in shank diameter up to 1908, when an additional thread form was proposed—British Standard Fine (BSF). Presented by the British Engineering Standards Association, BSF was identical to the original Whitworth form except that the pitch was finer—meaning more threads per inch. Now a bolt with a diameter of 1/4 inch could have either 20 threads per inch (BSW) or 26 (BSF). The advantage of the finer thread pitch is two fold. A fine thread bolt is about 10% stronger than a coarse thread bolt of the same size and material. Fine threaded fasteners also have greater resistance to vibration. Those of you who have worked on cars with Whitworth hardware will have noticed that almost all the hardware is BSF for these reasons. Why use any coarse threaded bolts at all? Coarse thread fasteners are well suited for use in tapped holes in material softer than the bolt (such as studs in aluminium cylinder heads), and they are easier to assemble. It’s almost impossible to cross thread a coarse threaded fastener by hand.
The US NC and NF thread standards are of course a 'second generation' screw thread system, as are the ISO Metric thread series,
and the Japanese JIS Metric thread series.
PS: A good bio of SirJW;
https://interestingengineering.com/sir-joseph-whitworth-master-of-screws
