English or metric. How to tell?

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Originally Posted by Bruce_in_Oz

The other very British" thing about BA threads is the size relationship:

To get the dimensions of a size say, 1 BA, multiply the previous larger size (Zero) buy 0.9. Ditto all the way down.

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Bruce, that's not actually correct.

1BA is 88.333% of 0BA diameter, 2BA is 88.679% of 1BA diameter, etc - and it doesn't follow in a linear scale on pitch either.

BA is a weird thread, as despite the 'British Association' name, it is actually a Swiss thread, and metric thread form.

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Originally Posted by Flying10uk

Yes, but in practice the two threads will fit together fine, without needing to be forced together. When I last measured the external diameter of a 0 BA thread it was 6mm diameter.

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M6 coarse and 0BA are indeed 6mm dia x 1mm pitch.

HOWEVER, interchange them at your peril, they couldn't be more different.

M6 coarse is 60 degree thread form, 0BA is 47.5 degree thread form.

Interchanging them is NOT a good idea!

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Originally Posted by Salt Flat

duplicate info posted 1 minute after BARs post

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...I don't think it's your timing...he is very fast :)

David.

I am familiar with the differences in form. BA was derived from those Metric Thury watchmakers threads, the form and included angle of which are unknown to me, but can probably be found on the wall-charts in half the alpine workshops in Switzerland.

Weird (like "Enfield" threads) works, but it can be a pain.

Here's a BA comparison chart:

BS 93 British Association (B.A.) Screw Threads Data Charts

My first car was a 1950 Vauxhall Wyvern. Made for the "Commonwealth" market, nevertheless, it contained a LOT of US "unified" threads. The catch was that a lot of the bolt-heads and nuts associated with these UN threads were WHITWORTH, so that "imperial" mechanics could work on them with their usual wrenches.

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Originally Posted by David TS

M6 coarse is 60 degree thread form, 0BA is 47.5 degree thread form.

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All depends on how much torque is put on the nut/thread.

If memory serves me correctly 2BA will fit 3/16 B.S.F.

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A story that I heard was that BSW dropped a spanner size on all it's threads, making it the same as B.S.F. spanner sizes, during WW2 in order to save steel. How true and how much steel was saved I don't know but it does give one explanation as to why one encounters 2 sizes of B.S.W. nuts/bolts.

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Originally Posted by Flying10uk

All depends on how much torque is put on the nut/thread.

If memory serves me correctly 2BA will fit 3/16 B.S.F.

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It isn't a matter of how much torque is put on it, it is just bad engineering practice, and an easy way to ruin a thread, which wouldn't be a good outcome on an internal thread.

And mixing BA and BSF are again bad practice, 47.5 degree thread form vs 55 degree thread form.......

David I don't know how much practical experience of thread and screw cutting you actually have.

Perhaps you have access to a vast stock of obsolete screws and nuts of every imaginable size and every conceivable thread, not all of us do.

Therefore, if I don't have the exact size required to do a particular job I may look for another way round the problem. That my mean making a screw on my vintage lathe by cutting the thread or it may mean using something else that is similar and will do the job.

I don't make a habit of tightening up screws/bolts so tight that the thread strips.

2 threads to be careful of are 1/2" BSW and 1/2 UNC because there is 1 t.p.i. difference between the 2 and try mixing them and you'll strip the threads, even though the threads are close enough to look the same visually.

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Originally Posted by Flying10uk

David I don't know how much practical experience of thread and screw cutting you actually have.

Perhaps you have access to a vast stock of obsolete screws and nuts of every imaginable size and every conceivable thread, not all of us do.

Therefore, if I don't have the exact size required to do a particular job I may look for another way round the problem. That my mean making a screw on my vintage lathe by cutting the thread or it may mean using something else that is similar and will do the job.

I don't make a habit of tightening up screws/bolts so tight that the thread strips.

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My experience? My dad was a toolmaker and watch repairer and taught me much, and I started my electronics and engineering apprenticeship with Marconi way back in 1969. After school I worked in a number of production machine shops, and small auto repair workshops. Wind forward to today, I have just built a new 400 sq ft workshop, which houses two Colchester and one Harrison lathe (all screw cutting), three milling machines, and various other sundry machine tools. I have also accumulated a large stock of BA, UNF, metric, Whit, BSF and cycle fasteners over the years.

I don't say the above to brag, but rather to simply answer your question.

So whilst I do have access to a number of fasteners, or the ability to make them, one thing I will not do is fit mismatching fasteners together, my head will just not allow me to do it! ;)

Interesting how a simple question morphed into a detailed discussion of pitches, threads, acronyms, diameters and similar big words related to screws, but tending to the arcane if not more mysterious words.

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Originally Posted by Daan Kemp

Interesting how a simple question morphed

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Yep...