Metric thread cross reference

[Roger]

I know I'm almost alone in my dislike of imperial threads, but for those few who are interested in using Metric equivalents, here's a cross reference chart I've put together for my own use.
There are many Metric Fine Threads that nicely cover the range used by Model Engineers and the BA sizes can easily be substituted by Metric Coarse threads.
Roger

Attachments:

Threads.xls (17 KB)

[andyhigham]

BA is actually a metric thread

[vulcanbomber]

Dec 31, 2013 19:31:40 GMT andyhigham said:

BA is actually a metric thread

No its not, BA threads are IIRC 47.5 degrees, metric is 60.

[GWR 101]

Yes bearing in mind that the thread angle is 47.5deg. as opposed to 60deg. and the root radii is approx. 3 times greater than metric as a factor of the pitch.

Thanks Striplar have printed off table and attached to notice board in w/shop, saves having to consult two sets of tables. Happy and constructive New Year to all. Paul

[andyhigham]

The thread angle has nothing to do with it.
The BA thread is based on 0 BA being 6mm dia x 1mm pitch.
Each increasing BA number has a pitch 0.9 x the next lower number.
The diameter is 6 x the pitch

How is this not metric?

[Roger]

Dec 31, 2013 20:15:46 GMT andyhigham said:

The thread angle has nothing to do with it.
The BA thread is based on 0 BA being 6mm dia x 1mm pitch.
Each increasing BA number has a pitch 0.9 x the next lower number.
The diameter is 6 x the pitch

How is this not metric?

It may have it's roots in metric units but it's not a Metric thread in the modern sense. It hails back to a time when thread standards were created locally and without regard to the wider world. You won't find BA sizes adopted in other Metricated countries. Some of us want to firmly put these obsolete sizes in the past and move on. The Zeus book is littered with obsolete threads and a cull is more than overdue in my opinion. These things don't happen overnight, but our young people will firmly turn their back on them when it's their world not ours.

[uuu]

Part of the problem is that we're modelling where appearance is important, not designing for function. The BA approach gives a beautifully graded range of sizes, that fits this purpose so well.

[Roger]

Dec 31, 2013 20:44:28 GMT uuu said:

Part of the problem is that we're modelling where appearance is important, not designing for function. The BA approach gives a beautifully graded range of sizes, that fits this purpose so well.

But if you look at the chart, it's very close to Metric coarse threads so there's no penalty in using them. 

[uuu]

But the thread is not the bit you see. It's the screw heads and nuts.

[GWR 101]

Not sure if this solves the issue or just puts more coal on the fire. en.wikipedia.org/wiki/British_Association_screw_threads

Happy New Year to one and all. Paul

[bhk]

I had always thought why does the model engineering world still use BA threads. Then I tried to buy nuts and bolts below M5 and found it almost impossible to get them in any other form other than machine screws.

[alanstepney]

BA was originally based upon the Swiss Thury thread.
The British Association then defined it in Imperial dimensions, and that became the British Standard.

(Compare Thury with BA threads and you can see that most are the same.)

Model Engineer magazine did publish a list of suggested Metric threads that compared closely with BA.
(Its on my website.)

[Roger]

Dec 31, 2013 23:04:29 GMT bhk said:

I had always thought why does the model engineering world still use BA threads. Then I tried to buy nuts and bolts below M5 and found it almost impossible to get them in any other form other than machine screws.

This is part of the problem I'm afraid, it's about time companies like Reeves started to supply hex headed screws in Metric sizes. Perhaps then we might see some progress.

[uuu]

We have got a problem with suppliers here. At the midlands exhibition, I was trying to buy 4BA hex head screws, and had the devil's own job finding them. Most being sold were 5BA heads, I didn't get what I wanted. And the head sizes aren't exactly right either way, perhaps they're just made out of any old bit of hex they could get hold of.

Although I'm a fan of BA from an appearance point of view, consistency of supply is as much of an issue. Roger, if you're going to make your Speedy all metric, good luck with the boiler fittings!

[Roger]

Jan 1, 2014 9:57:31 GMT uuu said:

We have got a problem with suppliers here. At the midlands exhibition, I was trying to buy 4BA hex head screws, and had the devil's own job finding them. Most being sold were 5BA heads, I didn't get what I wanted. And the head sizes aren't exactly right either way, perhaps they're just made out of any old bit of hex they could get hold of.

Although I'm a fan of BA from an appearance point of view, consistency of supply is as much of an issue. Striplar, if you're going to make your Speedy all metric, good luck with the boiler fittings!

I can see having to make the screws that I need. That's all very well for the larger sizes but something of a challenge in the smaller ones. Maybe I'll get a batch made instead.
My 'Speedy' is a hybrid in as much as the dimensions are all metric on my model and drawings but they are just converted accurately from imperial ones. I'm using metric silver steel sizes though and all metric threads.
I don't really know what the issues will be with the boiler fittings, but I can't see any show stopping issues when you look at the range of threads available. I think there's a common misconception that Metric Threads aren't available in as fine sizes as Model Engineers ones, and that's simply not the case

[alanstepney]

The problem the suppliers have is finding hex bar with the right head dimensions for BA screws / nuts.
Hence they use the nearest standard size, which is, unfortunatley, metric.

(Could probably get it to any size, given a large enough order.)

[masterdrain]

Hex head sizes down to M2.5 from www.westfieldfasteners.co.uk/ScrewBolt_M.html. All in A2 SS, so a bit soft.

[isc]

The main problem with small size hex head bolts is demand, there isn't any in industry, and even if all Model Engineers bought a hundred or so each, they would barely scratch the surface of the market. First someone has to produce the hex rod in the right size. isc

[Roger]

Jan 1, 2014 13:15:03 GMT isc said:

The main problem with small size hex head bolts is demand, there isn't any in industry, and even if all Model Engineers bought a hundred or so each, they would barely scratch the surface of the market. First someone has to produce the hex rod in the right size. isc

This is true, but that doesn't stop Reeves and the like from doing exactly that for imperial sizes. I don't think you'll find hex headed imperial sizes like they sell anywhere in the full size engineering world.

[Roger]

Jan 1, 2014 13:04:15 GMT masterdrain said:

Hex head sizes down to M2.5 from www.westfieldfasteners.co.uk/ScrewBolt_M.html. All in A2 SS, so a bit soft.

Now that's handy. I wonder if anyone makes them in other than stainless steel, I don't think those would look right on a model unless they were painted over of course.

Here's another supplier of the same but at much cheaper prices by the look of it.
www.pts-uk.com/Products/Hexagons_Hexagon_Head_Set_Screws