American threads

[JohnW2]

To come back to this, the electrical threads I was referring to were for things with a hole like conduits, with minimum OD (for PG7) 12.5mm.

Fair enough.

I assume 3.5mm dia is for things like threaded studs for spade connections.

I don't know about that, but what I was talking about are the M3.5 machine screws used to attach the front plates of virtually all standard UK electrical accessories, hence with countless examples in every UK building (hence very 'ubiquitous'!).

Do you have a spec for them? I don't think it's the same as a metric bolt, as 3.5mm is not a standard diameter. Bolt standards are M1.6, M2, M2.5, M3, M4, M5, M6, M8 ...etc.

I don't have a spec. Maybe not a 'standard diameter,' and certainly only commonly seen in the electrical context, but they do exist more generally, as in ...

Kind Regards, John

 

[fixitflav]

Fair enough.
I don't know about that, but what I was talking about are the M3.5 machine screws used to attach the front plates of virtually all standard UK electrical accessories, hence with countless examples in every UK building (hence very 'ubiquitous'!).
I don't have a spec. Maybe not a 'standard diameter,' and certainly only commonly seen in the electrical context, but they do exist more generally, as in ...


View attachment 263375

Kind Regards, John

OK thanks, interesting. There's nothing about electrical on those screws, so looks to me like they're ordinary metric, and M3.5 is non-standard. From the web, for all sizes, the pitch agrees. As far as I can tell, DIN 84 is the spec for the cheese head.

 

[JohnW2]

OK thanks, interesting. There's nothing about electrical on those screws, so looks to me like they're ordinary metric, and M3.5 is non-standard. From the web, for all sizes, the pitch agrees.

You're welcome. However, what I illustrated was far from alone. A search on Google or eBay etc. for "M3.5" gets many thousands of hits, but something like 95% of them explicitly relate to "electrical screws". However, the other 5% or whatever is still quite a substantial number, and they look like the one I showed - seeming just a 'non-electrical' M3.5 bolt/screw, even if that is a 'non-standard' size. The 'electrical' ones certainly don't have cheese heads.

As far as I can tell, DIN 84 is the spec for the cheese head.

Yes, since you had asked about a spec, I Googled that, and came to the same conclusion as you.

I didn't mention before, but I've always assumed that the M3.5 'electrical screw' came about because it is very close to the 4BA ones which preceded them.

Kind regards, John

 

[fixitflav]

You're welcome. However, what I illustrated was far from alone. A search on Google or eBay etc. for "M3.5" gets many thousands of hits, but something like 95% of them explicitly relate to "electrical screws". However, the other 5% or whatever is still quite a substantial number, and they look like the one I showed - seeming just a 'non-electrical' M3.5 bolt/screw, even if that is a 'non-standard' size. The 'electrical' ones certainly don't have cheese heads.
Yes, since you had asked about a spec, I Googled that, and came to the same conclusion as you.

I didn't mention before, but I've always assumed that the M3.5 'electrical screw' came about because it is very close to the 4BA ones which preceded them.

Kind regards, John

OK it looks like there are 2 types of electrical thread, PG or metric for cable glands etc, and the ones you've identified. Though I can't see what's electrical about the latter, they're just bolts the way I see it! But I think we've kicked this to death, thanks for the discussion.

 

[JohnW2]

OK it looks like there are 2 types of electrical thread, PG or metric for cable glands etc, and the ones you've identified. Though I can't see what's electrical about the latter, they're just bolts the way I see it!

Of course. The only thing 'electrical' about them is the fact that they are in such widespread use in electrical accessories - a very quick bit of messy mental arithmetic suggests that there are probably more (perhaps a lot more) than a billion of them in service in the UK. Beyond that, they are simply M3.5 machine screws - but I don't think I've ever seen an M3.5 nut.

As I wrote before, I think they were probably adopted (perhaps 'invented', given that they were not a 'standard size') because of their very close similarity to the 4BA screws that had previously been used (for decades) for the same purpose. In fact, one can get an M3.5 screw into a 4BA nut, or a 4BA screw into an M3.5 tapped hole/insert (I've never seen an M3.5 nut) for at least a couple of turns before it jams up because of a very slight difference in thread pitch.

KInd Regards, John

 

[JohnW2]

... - but I don't think I've ever seen an M3.5 nut.

I clearly had not looked very hard ...

Kind Regards, John

 

[SUNRAY]

Beyond that, they are simply M3.5 machine screws - but I don't think I've ever seen an M3.5 nut.

As I wrote before, I think they were probably adopted (perhaps 'invented', given that they were not a 'standard size') because of their very close similarity to the 4BA screws that had previously been used (for decades) for the same purpose. In fact, one can get an M3.5 screw into a 4BA nut, or a 4BA screw into an M3.5 tapped hole/insert (I've never seen an M3.5 nut) for at least a couple of turns before it jams up because of a very slight difference in thread pitch.

KInd Regards, John

I don't think so... When I was a youngster, well under 10 yo so early 60's I was given some 'Stabil' - a German toy very much like Meccano. Those threads were M3.5, I clearly remember Dad pinching some of them to fit some of the new electrical back boxes with 'funny threads'. I don't know when the boxes went metric but was buying imperial boxes in the 70's.

As to fitting to others nut, again I'd say no, one way is bigger and usually strips one or both threads and the other way may not grip.

 

[JohnW2]

I don't think so... When I was a youngster, well under 10 yo so early 60's I was given some 'Stabil' - a German toy very much like Meccano. Those threads were M3.5, I clearly remember Dad pinching some of them to fit some of the new electrical back boxes with 'funny threads'.

I'm not sure what you are disagreeing with, or why. I was merely suggesting that when UK electrical accessories 'went metric' they chose M3.5 (rather than the 'standard' M3 or M4) as the 'funny thread' because it was very close to the 4BA that had previously been used. I suspect that enabled them to continue using the same mouldings, with tapping the insert with a slightly different thread being the only change they had to make.

As to fitting to others nut, again I'd say no, one way is bigger and usually strips one or both threads and the other way may not grip.

Well, your "saying no" is not consistent with the confirmatory experiments I've just undertaken.

For a start, the size difference is only very slight. I'm just measured a few examples of both (with a micrometer) and the average major diameter of M3.5 'electrical' screws seems to be a bit over 3.42 mm, compared with about 3.43 mm for 4BA screws - so less than 10 microns difference. I've also confirmed that one can get an M3.5 screw into a 4BA nut, or a 4BA screw into a M3.5 box insert, for a turn or two with minimal force before it even becomes 'tight' (let alone anything stripping).

I've also just confirmed that I can very easily (hardly any force) run a 4BA tap through an M3.5 threaded hole, or an M3.5 tap through a 4BA not and, having done that, can then easily insert, and satisfactorily tighten, the 'new-size' screw into the 're-tapped' hole. I would never advocate that anyone 're-tapped' a threaded hole with a slightly different thread from the original, but the fact that it can easily be done illustrates my point that the two threads are extremely similar!

Kind Regards, John

 

[SUNRAY]

I'm not sure what you are disagreeing with, or why. I was merely suggesting that when UK electrical accessories 'went metric' they chose M3.5 (rather than the 'standard' M3 or M4) as the 'funny thread' because it was very close to the 4BA that had previously been used. I suspect that enabled them to continue using the same mouldings, with tapping the insert with a slightly different thread being the only change they had to make.

Kind Regards, John

I was disagreeing with M3.5 being 'non standard' as I had experience of them some 10 years prior to finding them used for electrical accessories. That said I don't have a date for them being adopted for such.

My experience of mixing the threads seems to be very different to your experiment. I don't have any specific examples to offer.
I'm hoping to do some sorting out soon, weather permitting, If I come across some old bakelite BB's with brass inserts I'll have a play.

 

[JohnW2]

I was disagreeing with M3.5 being 'non standard' as I had experience of them some 10 years prior to finding them used for electrical accessories.

Oh, I see - that wasn't apparent since you quoted most of my post. Anyway, you'd have to take that one up with fixitflav, since he/she was the one who said it was 'non-standard' - but I suppose it really depends on what one means/understands by 'standard' and 'non-standard'.

Perhaps it's a bit like BS1362 fuses, with only 3A and 13A being 'preferred' sizes (with specific marking colours assigned to them by BS1362) and all other ratings 'non-preferred' (and having to have black markings)?

My experience of mixing the threads seems to be very different to your experiment. I don't have any specific examples to offer. I'm hoping to do some sorting out soon, weather permitting, If I come across some old bakelite BB's with brass inserts I'll have a play.

I can only report my own findings, but they did all relate to modern boxes.

I'm certainly not suggesting that one can use an M3.5 screw with a 4BA-tapped insert, or vice versa, since one usually can't. As I said, one can only get a turn or two, and to attempt to go further than that would require appreciable force, and could well result in thread(s) being stripped, the screw shearing or the insert being ripped out of the box.

However, I have on many occasions run an M3.5 tap into the 4BA insert (or lug) of an old backbox, and that has always enabled an M3.5 screw to be used (and tightened satisfactorily) with no problems.

Kind Regards, John

 

[plugwash]

Wikipedia claims that M3.5 is a standard size of metric thread, though it's certainly one of the less common ones. It claims it has a thread pitch of 0.6mm. The minor diameter is apparently the major diameter minus approximately 1.08 times the pitch or about 2.85mm.

Wikipedia claims that a 4BA thread has a major diameter of 3.6mm and a thread pitch of 0.66mm. The minor diameter is apparently the major diameter minus 1.2 times the thread pitch. So about 2.8mm.

So, assuming that info is correct, it doesn't surprise me that you are able to re-thread a 4BA to M3.5 or vice-versa and get a usable grip (albiet probably weaker than a freshly threaded hole)

 

[fixitflav]

I didn't mention before, but I've always assumed that the M3.5 'electrical screw' came about because it is ver

Oh, I see - that wasn't apparent since you quoted most of my post. Anyway, you'd have to take that one up with fixitflav, since he/she was the one who said it was 'non-standard' - but I suppose it really depends on what one means/understands by 'standard' and 'non-standard'.

Perhaps it's a bit like BS1362 fuses, with only 3A and 13A being 'preferred' sizes (with specific marking colours assigned to them by BS1362) and all other ratings 'non-preferred' (and having to have black markings)?
I can only report my own findings, but they did all relate to modern boxes.

I'm certainly not suggesting that one can use an M3.5 screw with a 4BA-tapped insert, or vice versa, since one usually can't. As I said, one can only get a turn or two, and to attempt to go further than that would require appreciable force, and could well result in thread(s) being stripped, the screw shearing or the insert being ripped out of the box.

However, I have on many occasions run an M3.5 tap into the 4BA insert (or lug) of an old backbox, and that has always enabled an M3.5 screw to be used (and tightened satisfactorily) with no problems.

Kind Regards, John

OK I stand corrected on M3.5 not being a metric bolt. But it's a relief that it is a metric bolt, not some other spec. M3.5 doesn't appear on lists I've seen, though other non-standards do - M14, M18, M22, M27, M33 to name a few. To misquote Animal Farm, it seems some sizes are more non-standard than others.
BTW I'm a "he"

 

[JohnW2]

Wikipedia claims that M3.5 is a standard size of metric thread, though it's certainly one of the less common ones.

Yes, it does, as one of the R20 series (rather than more common R10 series) of sizes. However, I see that it is only a list of 'preferred' diameter/pitch combinations listed in ISO 261, so maybe any diameter is acceptable provided that the thread conforms mathematically to the pattern described in the Standard?

In any event, we all know that M3.5 screws not only exist but are in very widespread use, at least in the UK (as I wrote, I'd be surprised if there are not at least a billion in service in UK electrical accessories) - so the question of whether or not it is a 'standard size' is perhaps essentially semantic/'bureaucratic'?

.. It claims it has a thread pitch of 0.6mm. The minor diameter is apparently the major diameter minus approximately 1.08 times the pitch or about 2.85mm. Wikipedia claims that a 4BA thread has a major diameter of 3.6mm and a thread pitch of 0.66mm. The minor diameter is apparently the major diameter minus 1.2 times the thread pitch. So about 2.8mm.

It does, albeit those major diameters are slightly greater than the ones I measured last night.

So, assuming that info is correct, it doesn't surprise me that you are able to re-thread a 4BA to M3.5 or vice-versa and get a usable grip (albiet probably weaker than a freshly threaded hole)

Quite so. The proof is obviously in the pudding, and I had absolutely no difficulty in re-tapping (and subsequently satisfactorily using) the threads in both directions. As you say, the result will inevitably be less satisfactory ('less grip') than with a freshly threaded hole (I'm sure my late father would be turning in his grave at the thought of re-threading to a different thread!!) but, at least in my experience, it's perfectly adequate for the faceplate screws of an electrical accessory. I doubt that I have ever had a real-world need to re-thread a M3.5 thread as 4BA (last night was just an experiment), but I have on plenty of occasions satisfactorily re-threaded 4BA as M3.5.

KInd Regards, John

 

[JohnW2]

OK I stand corrected on M3.5 not being a metric bolt. But it's a relief that it is a metric bolt, not some other spec. M3.5 doesn't appear on lists I've seen, though other non-standards do - M14, M18, M22, M27, M33 to name a few.

Fair enough, but as plugwash has just pointed out, the Wikipedia seems to indicate that M3.5 (and, as you suggest, a lot of other far-less-familiar R20-series ones!) does appear in the ('preferred') 'list' in ISO 261.

To misquote Animal Farm, it seems some sizes are more non-standard than others.

Indeed. However, as I've just written, when something is in such widespread use (in electrical accessories) as this is, the question of whether it is 'standard' is almost moot, and certainly not very useful!

BTW I'm a "he"

Fair enough, and if one made that assumption about everyone in this forum (or many others like it), one would rarely be wrong - but I have been wrong on occasions, hence I try to be 'non-assumptional' in my use of pronouns!

Kind Regards, John

 

[plugwash]

albeit those major diameters are slightly greater than the ones I measured last night.

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.