Hi,
Way back when, everybody made their own nuts and bolts. presumably filing the flats to suit themselves, and to fit the spanner they had made (or vice versa)
I believe it was Mr Whitworth who standardised spanner sizes, but using some complicated formula (anybody?) for the width of the spanner jaws, and a somewhat coarse thread, with a "thread angle" (or whatever it's called?) of 55 degrees, and a hexagon A/F (across flats) based on his mother-in-laws shoe size (or something like that) I'm joking, I think.
Sometime later, possibly due the war, and the need to save raw materials, USF and USW were invented, with smaller hexagons. I can't remember if the thread angle was changed to 60 degrees. and the head was reduced, but rationalised, so that a 1/4" Whitworth spanner would fit a 5/16" BSF nuts, too. I don't know if the same applied to other sizes. You needed to be a rocket scientist, to calculate the spanner size needed for each thread size.
Enter WW2 (or was it WW1?), and the Yanks joining (on our side), and bringing their ANF/ANC (or was it UNF/UNC?) with them. They had rationalised things even further, so that the thread angle was now 60 degrees, but I have no idea about thread pitches.
The A/Fs were also rationalised by rounding off to the nearest 1/16". That way, nuts and bolts are identified but their thread sizes, but spanners are identified by their AF. Character building.
Metric followed suite , but in 1mm increments for bolt diameter, and spanner AF ... which are entirelty different. EG ... A 6mm thread diameter nut or bolt will require a 10mm AF spanner (usually).
So no more rolling up to the stores window and asking for a 1/2" AF nut.
I will ignore cycle thread (very fine pitch), but ir will be worth while having both ring and open ended spanners, sizes OBA thru 4BA (even numbers), wrapped in an Irish linen handkerchief, and kept in SWMBO's knicker drawer. You won't want them very often, but when you do, you nwill want to know where they are.
I'm happy to be corrected. I know I've made one incorrect statement, but can't be naffed to correct it.
602
PS... BA is short for British Armstrong ... I think.
"U" as in UNF stands for UNIFIED. But should it be "A" for AMERICAN?
Overtaken by Gibbo ... I think 3/16" is 0.185" (its been a long time ...
), but a couple of long handled ring spanners soon sort out a 5 thou difference.