On Mon, Aug 14, 2023 at 06:40 AM, Peter Brooks wrote:
- is there a standard formula for the size (width and thickness) of bolt hex heads? ?(I¡¯m making some M14x1 bolts).
When Mr Whitworth devised his system of screw threads he made the hexagon twice the size of the thread
across the points - not across the flats.?This means there is a factor of ¡Ì3 involved, so the AF size is never going to be rational in any system of measurement. This is why Whitworth size spanners never fit anything else, and I believe it is why the French and Italians (at least) refer to an adjustable spanner as an English spanner - it's the only thing that fits those daft British bolts! To add to the confusion British Standard Fine (BSF) bolts use the next size down hexagon. During the war this was then applied to Whitworth (BSW) bolts as well because it saved metal. The Whitworth formula for the size of a bolt head is still being taught in British drafting classes, and it is almost always wrong.
Metric (SI) bolts do not use aa formula for the hex size. An M1 nut is 2.5 mm AF, M100 is 145 mm, so as the thread size goes up the nut is proportionately smaller. In general practice (my experience) M14s are 22 mm AF, but the latest ISO standards say 21 mm. I suggest you use 22 mm, then your standard 7/8 AF spanner will be a good fit.
The metric nut thickness is roughly 0.8 of the thread size, but the numbers are rounded off to something a bit sensible. Bolt heads are 2/3 of the nominal size, again rounded off (M14 is 11 mm thick). So some of the metric dimensions follow a formula, but plenty don't.
Beyond that there seems to be no limit to the imagination of engineers when devising "rational" fastener families.
