Whitworth bolt's head size

[39Jet]

At school my Physic's teacher told us that there was a mathematical relationship between the shank diameter and the head size of a Whitworth bolt. My very hazy memory is that square roots were involved. A search on the interweb doesn't produce anything relevant. Is there any relationship between the two (apart from - as one increases so does the other)?

[Austin Carr]

The same as you, I can't find anything on Whitworth but BA does have a relationship:-

They are unusual in that they were probably the most "scientific" design of screw, starting with 0BA at 6.0 mm diameter and 1.0 mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size. They then rounded to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch

Source:-

https://en.wikipedia.org/wiki/British_As...ew_threads

[Tony Betts]

I wouldn't worry about it.

Many or even most fastener/ nut and bolt suppliers, now supply whit and bsf bolts with metric heads.

So just because you are buying a whitworth bolt, doent meen your old spanners will fit.

Tony.

[Cliff Ringrose]

I have just located a Christmas present from 1951! "Drawings and Diagrams for the Metalwork Class". On page 12 are the instructions for drawing a Whitworth bolt & nut. The book is too thick for my scaner, so I will try and describe the formula.
Dimension across flats = 1.5 + .125 Diameter of bolt. Thickness of bolt head = .88 Dia. of bolt. Thickness of nut = Dia. of bolt.
The dimension across the Points of the bolt head is 2 X Dia. of bolt, the inference being that the origional formula was worked out to allow standard round stock to be used, i.e. make 1/2" bolts from 1" stock, 1/4" from 1/2", etc.. I presume this is for the origional large headed bolts, not the later ones that share spanner size with the next size down B.S.F.

[Zetomagneto]

Really! I would be returning any fasteners with wrong size hexagon.
Tony you must be talking cheap Chinese!

[Colin Morgan]

Looks like the variation in head size and bolt size are a bit non-linear to compensate on the smaller sizes - so perhaps, from the numbers, the formula is similar to the above, but with one change:

Head size = 1.5 x bolt dia + a constant 1/6" (~0.16", 4.2mm), which shows up most on the smaller sizes. 

For example, the bolt size of 1/8" (0.125", 3.175mm) has head size 0.34" (8.64mm) AF (which can be made up of 0.1875" + 0.15") and for the bolt size of 1" (25.4mm) is the head size is 1.67" AF so is 1.50" plus 0.17".

Before calculators and spreadsheets this must have been lengthy to work out - especially in fractions of an inch - with inevitable rounding errors.

[Steve kay]

Head size certainly does not always mean cheap Chinese. During WW2 there was a policy of having Whitworth a/f measurements the same as a similar dia BS, the production of very many thousands of tool rolls to accompany transport and weapons etc benefitting from a reduction in the number of spanners needed. Those of us who have built or rebuilt bicycles will have long since accepted that fastenings are often not what they appear to be at first glance. French machines usually being Metric, but Italian bikes into the 80s capable of using Imperial. British machines depending on the manufacturer, Nottingham Ralieghs broadly Imperial but a widespread use of 16 tpi on various diameters, whereas a competition Raleigh from Worksop used differing nuts and bolts. As to the choice of mirror threads on bottom brackets, and Sunbeam's natural use of sizes and threads that Wolverhampton naturally considered superior and thus different, the cycle workshop is full of different tools and a thousand jam jars containing various nuts and bolts. The adjustable spanner does not invariably indicate laziness and lack of moral fibre, it is sometimes the only way to shift an utterly obscure size fastening. As Holmes once remarked to Watson "That man owns a 2CV" "Why, how do you know that Holmes?" "I note an M7 spanner in his tool box."

And many thanks to those who managed to restore the forum to working order!

[Charles P]

 As Holmes once remarked to Watson "That man owns a 2CV" "Why, how do you know that Holmes?" "I note an M7 spanner in his tool box."

Holmes always was a smart-arse, making guesses based on incomplete evidence.

He could have owned a Bugatti.

Charles

[Paul N-M]

Very interesting. I love this forum.

Paul N-M

[Stuart Giles]

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.