American threads

[JohnW2]

I think the nominal major diameter is a maximum for an external thread and a minimum for an internal thread. So the real-world major diameter of an external thread will be slightly smaller.

Fair enough. As you imply, that would be consistent with the (themselves seeming pretty consistent between screws) measurements I made last night.

Kind Regards, John

 

[SimonH2]

Anyway, returning to the "AF" issue, I've just had a look at some of my father's spanners (probably 70-75 years old). Many have sizes cast or engraved on them described as "AF". For example, the left and middle ones in the photo below are marked "5/8 AF" and "9/16 AF" respectively, clearly far to small to fit a nut or head of a bolt with 5/8" or 9/16" threads of any type - and, indeed, have measured openings 'across flats' of approximately 5/8" and "9/16" respectively. On the other hand, the one on the right also has an opening of approximately 9/16" ('across flats'), but is marked "5/16 W", hence presumably designed to fit a nut or head of a bolt with a 5/16" Whitworth thread.

So, at least when it appeared as a marking on spanners, "AF" definitely meant "Across Flats", and had absolutely nothing to do with the thread of the fastener (whether Fine, Coarse, American, British or anything else)!

My experience is the same - I have some Whitworth spanners, and it's ... interesting as dad used to carry on using some of those nuts and bolts he'd had lying around for ... a long time. I used to remember exactly which ring spanner needed for the bolts used to hold the vice onto the bench.

The idea of sizing a bolt or fastener due to the size of it's head is definitely an odd one!.

Does anyone do that ? Yes there are (as described) Whitworth spanners - but that's merely a matter of marking the spanner according to the thread of the nuts/bolts the spanner fits.

When I am working on something (car, washing machine or whatever) when I first have a need for a spanner for a particular nut, screw or bolt, I will very often not even have sight of, let lone knowledge of the type and size of, the associated thread - but what I can see, feel or even measure, is the required 'across flats' dimension of a spanner that will fit it - and that is the basis on which I necessarily have to select my 'first try' spanner!

Indeed.
And in the same way that used to fascinate me as a young child without that experience, these days I can mostly just pick up the right size spanner or socket without having to think much. That does work best with stuff I'm familiar with, and less with stuff where I'm attacking it for the first time.
What is interesting is where non-standard heads are used - which vehicle manufacturers seem to like doing. 15mm AF seems to be a common variation - a size that is commonly not in smaller spanner/socket sets - and it's annoying when you need a 17mm socket for two bolts, and a 15mm for the third (thank you Land Rover ), or you need a 15mm 12 point socket when your only 15mm socket is 6 point

 

[JohnW2]

Indeed.... And in the same way that used to fascinate me as a young child without that experience, these days I can mostly just pick up the right size spanner or socket without having to think much.

Hmmm - all credit to you, then, but, even after decades of 'experience', I would still not put too much money on my picking up the right spanner first time

Kind Regards, John

 

[SimonH2]

Most stuff uses a small set of sizes. M6 takes a 10mm spanner, M8 takes a 13mm spanner, M10 takes a 17mm spanner, M12 takes a 19mm spanner - and that's most of the common (at least, for the sort of stuff I do) fasteners catered for. But then, while Land Rover have been metric for decades, there's still some imperial on them. The bolts for the prop shaft flanges are UNF and need a 9/16" AF spanner/socket - and I have a special socket with a slim neck for them. And the venerable Rover V8 engine remained imperial threads right to the end - the 4.6l block was dimensionally identical, with the same threads, as the original 3.5l block.
More of a pain is working out while of multiple lengths of bolts go in which holes when putting stuff back together. Apple are masters at that, on one of my laptops there's a cover held on by three screws - 2mm, 2.5mm and 3mm in length (or something like that)

Where I do struggle is picking the right size hex key (a.k.a.Allen key). Partly because I don't use them as much, but I usually get a size or two bigger than fits.

 

[Mike13]

BA threads have an IMPERIAL diameter and a METRIC thread pitch, there is also a French bolt standard that I cant remember the name of that has Metric diameter and imperial thread pitch.

I thought that Whitworth was introduced in one of the wars to reduce the amount of metal in the bolt and head for a given bolt diameter?

 

[SUNRAY]

BA threads have an IMPERIAL diameter and a METRIC thread pitch, there is also a French bolt standard that I cant remember the name of that has Metric diameter and imperial thread pitch.

I thought that Whitworth was introduced in one of the wars to reduce the amount of metal in the bolt and head for a given bolt diameter?

My understanding is BA sizes are fully metric.

In Germany they were used before the 'M' series and known as 'Swiss' where I believe they were standardised for the clock industry.
IIRC they started with 1BA at 6mm diameter x 1mm pitch, then pitch is multiplied by a fixed number (something like 0.8 or 0.9) for each step and the diameter is calculated by raising the pitch to the power of 1.2.

Mr Whitworth was an engineer and AIUI he designed the thread to take heavy loading without requiring too much force to tighten, or something along those lines.

EDIT: I should have looked it up first.
0BA is 6mm diameter x 1mm pitch, pitch is multiplied by 0.9 for each number increment and the diameter (6 x pitch) raised to the power of 1.2

 

[JohnW2]

BA threads have an IMPERIAL diameter and a METRIC thread pitch ...

As SUNRAY has said.... If one is to believe Mr Wikipedia, the only thing Imperial about the diameter of a BA thread is that the metric-based diameter was converted to Imperial...

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.

Kind Regards, John

 

[SUNRAY]

As SUNRAY has said.... If one is to believe Mr Wikipedia, the only thing Imperial about the diameter of a BA thread is that the metric-based diameter was converted to Imperial...


Kind Regards, John

I nearly managed to edit my post first.

 

[SimonH2]

It may be instructive to go and read the Wikipedia article on Whitworth Threads.
And I'm minded to think of this XKCD cartoon