29-12-2019, 10:33 AM
In the very long ago days of my youth, the Technical drawing "O" level exam seemingly always required the hapless students to draw up some unfeasible non-standard Whitworth form bolt; something like a bolt with 17/32" shank and a prime number of threads. Consequently we were drilled remorselessly by the teachers of how to calculate and draw the bolt head dimensions. Having entered a metricated world at work and college after my schooldays, I can't remember the exact formula, but it definitely involved a relationship between the shank diameter and the length of the flat on the hexagon bolt head.