enginehistory.org

[kmccutcheon]

A reader sent the following information and question:
The BA size numbers are actually functional in generating the pitch of the fasteners; and the diameters are derived from the pitches.

(1) The basic sizes in millimetres for pitch and major diameter are rounded off to the second significant figure after calculation from the formulae given below.

(2) The pitch, in millimetres, are calculated from the formula

p = (0.9)**n
where ‘n’ is the number designating the size of the thread. (please forgive my FORTRAN notational convention of raising 0.9 to the nth power. KDM)

Thus, for BA #0, (0.9)**0 = 1, and the pitch is 1.0mm

(3) The basic major diameters are calculated from the formula

D = 6 p**(6/5)

And the diameter is 6 x (1.0)** 6/5 = 6mm.

So, as you see, the "BA number" is a functional generator of the pitch and diameter.

All that aside, the question which has haunted me for many years is this:

WHAT IS THE FORMULA WHICH DETERMINES THE DIMENSIONS OF HEXAGONAL BOLT HEADS AND NUTS IN THE BSW / BSF SYSTEM?
_________________
Kimble D. McCutcheon

[wallan]

There is no standard formula to generate the dimensions of hexagonal Whitworth fasteners. Sizes are specified by BSI. (check Kempes) The following is quoted on a Jaguar car buffs site:

http://www.jag-lovers.org/xk-lovers/library/whitworth_system.html

"After this article was published in the Classic Jaguar Association newsletter, another member from the U.K. responded by letter and stated that the hex sizes were originally governed by the commercially available steel hex bar stock sizes, in the days before automated screw machines, when nuts and bolts were cut from hex bar stock."

I seem to remember reading this in either one of Rolt's books, or in a Newcomen Society report. (it should also be stated that some of the early nuts would have been machined from round stock, with an integral washer, so round bar stock sizes would have been involved in calculations)

One other problem added to the situation designed to confuse non-Brits is that the original Whitworth heads (AF) were too large relative to the actual bolt shank, (that's why spanners/wrenches are the length/size they are: so you can't apply too much torque) and that in the first part of the 20th Century, the head sizes were reduced to the size one below. (British Standards specify that the AF measurement of any bolt not be greater than 1.75 that of the shank: 1/4 inch Whitworth is almost bang on the button) This causes even more problems when you need replacement Whitworth fasteners, for machinery over a 100 years old. It's also the reason why some old spanners/wrenches have 2 Whitworth numbers on them.

Why do you think we are all turning metric?