Imp to Metric thread comparisons

[silverfox]

Is there info anywhere that gives metric equivalents for the BA and ME ( 26/32/40 tpi) thread sizes?

[andyhigham]

BA thread IS metric

[uuu]

I think you need this earlier thread: metric-thread-cross-reference

Wilf

[gwr14xx]

Andy, BA threads ARE NOT metric - for a start, the thread angle is 47½ degrees not 60 degrees.

Regards,
Eddie.

[andyhigham]

The thread angle has nothing to do with it. UNF/UNC/BSC threads all have 60 deg angle.
The BA thread series is based on 0BA being 6mm dia x 1mm pitch

[Jo]

Back in the 1970's or 80's I seem to recall a number of notable ME Editors reviewing changing over to Metric threads and decided against it.

You can of course replace any thread with one of similar size so long as you work out the consequences. One of going from BA to Metric is the Nut A/F's are bigger, so you may not get your spanners in to tighten them and while there are fine Metric threads that you could replace the fine TPI threads use in model making you will find that not only are the taps and dies more expensive but you will have to make all your own fasteners from scratch as they are not available off the shelf. You also will have to make all your own boiler fittings as they are supplied with the imperial threads.

Jo

[silverfox]

Thanks Details passed on

[uuu]

The BA committee tied itself in knots:

"The committee is not insensible to the simplicity of the metrical system and to its possible universality...

...the Committee has felt it impossible to suggest a change which has little chance of adoption...

...it was determined that the unit of length taken should be the ‘mil,’ (thousandth of an inch) and that the decimal system should be adopted for expressing dimensions...

...an examination of the numbers at once shows that they are, for the most part, awkward fractions of a millimetre, and the metric system of measurement thus enjoys no advantage in this respect over that based on the inch."

Then - at the end, does a complete reversal and says:

"That the series of diameters for screws from 1/100th in. to ¼ in. be that given in millimetres..

,,,English manufacturers of screws, screw-plates, &c., adopt the exact pitches given in millimetres"

U-turning is clearly the British way.

Wilf

[44767]

I have posted these tables that I did elsewhere on the forum and although they don't refer to BA equivalents, they are the metric sizes to use to represent a scale full size bolt. These fastenings are available. They have standard metric threads, so the taps are relatively cheap, but with more scale head sizes for use in working models. There is a huge range of them in various materials, finishes, head types and lengths. The sizes goes down to M0.6.

Polly Models stocks some of the range but the manufacturer/supplier is: here . Just get Google to translate the page as it's in German. This supplier also does a beautiful set of scale spanners to tighten these.

Just a note on the BA fastenings that are on the market today: BA nuts and bolts have across flats dimensions that reduce in size for each BA size much like the pitch of the thread but it is very unlikely that the manufacturers of "BA" hex head bolts and nuts will be able to source the correct size of hex bar for manufacturing these to the standard. I am sure they will be using metric hex bar so I think it's time you let BA go and use these metric scale bolts!













Cheers,Mike

[simplyloco]

Here we go again! Anyone like to predict how many pages this time?
John

[jasonb]

Contry to comments above you can by small hex metric fasteners that will not only allow your spanners to fit but they are nicely proportioned to look more like old Whitworth sizes, only downside is thay cost a bit more than mass produced ISO ones but the same can be said for machined BA fixings too. Continental Europe will provide plenty of metric threaded boiler fittings etc so no problem their either.

As For taps and dies costing more in the metric fine series some of out suppliers sell them at very reasonable costs, I would not call £4 for a M6x0.75 tap expensive particularly in HSS Co and as they are available in spiral point or spiral flute you only need purchase one rather than the usual set of three.

For ME series equivalents diameter you should be able to work out yourself. 26tpi would be 1.0mm pitch, 32tpi would be 0.75mm pitch both of which are quite close, 40tpi not such a good standard match but 0.5mm pitch is OK in the smaller sizes wher taps are easier, once you get a bit larger then you can single point screwcut what you want

I made up this little chart for when I'm making US sourced engines, just follow the colour along for nearest UNC/BA/Metric

[andyhigham]

Good choice of tooling here www.tracytools.com/taps-and-dies/metric-taps-dies

[Roger]

Aug 24, 2020 11:30:53 GMT andyhigham said:

The thread angle has nothing to do with it. UNF/UNC/BSC threads all have 60 deg angle.
The BA thread series is based on 0BA being 6mm dia x 1mm pitch

... and all the rest are not round metric sizes, so it's nothing like Metric threads really.

[jasonb]

Aug 24, 2020 15:49:18 GMT Roger said:

Aug 24, 2020 11:30:53 GMT andyhigham said:

The thread angle has nothing to do with it. UNF/UNC/BSC threads all have 60 deg angle.
The BA thread series is based on 0BA being 6mm dia x 1mm pitch

... and all the rest are not round metric sizes, so it's nothing like Metric threads really.

You could say the same for UNC/UNF in the smaller sizes, they don't follow stock sizes with the exception of #5 but everyone would say they are imperial

[oldnorton]

I have gradually come round to the sense for a newly starting model engineer to try and work solely with metric fasteners.

The problem is the rather big jumps between M1.6, M2, M2.5, M3 and M4. The system needs a M2.8 and maybe a few more. 6BA and 7BA are very useful in 5" locos.

Also, there are several suppliers of BA in the UK, but getting small head metric I think involves dealing with Germany?

[coniston]

Aug 24, 2020 12:23:38 GMT simplyloco said:

Here we go again! Anyone like to predict how many pages this time?
John

I guess it will be either 10 or 12 depending on your own view point?
Chris D

[Roger]

Aug 24, 2020 19:36:54 GMT oldnorton said:

I have gradually come round to the sense for a newly starting model engineer to try and work solely with metric fasteners.

The problem is the rather big jumps between M1.6, M2, M2.5, M3 and M4. The system needs a M2.8 and maybe a few more. 6BA and 7BA are very useful in 5" locos.

Also, there are several suppliers of BA in the UK, but getting small head metric I think involves dealing with Germany?

I'm not sure the jumps are that big until you get to M3. A bigger problem is that you can't get M1 or smaller in Steel, only Brass which is too weak to be much use in my opinion.

M3.5 coarse is a standard, but isn't available in ME yet as far as I know. There's no reason why it couldn't be though.

It's interesting to note that the bolts from Knupfer use 2mm AF for both M1.2 and M1.4, which rather defeats the point in having different sizes for the most part.

[Midland]

Roger
We have done this and personally I do not want to use a coarse thread when we are fine sophisticated engineers (well most of us)! It is so distasteful to be coarse. But do keep the dicussion up and running, I will simply drink wine in an imperial glass and wish you good health!!
Cheers David

[Roger]

Aug 24, 2020 21:23:25 GMT Midland said:

Roger
We have done this and personally I do not want to use a coarse thread when we are fine sophisticated engineers (well most of us)! It is so distasteful to be coarse. But do keep the dicussion up and running, I will simply drink wine in an imperial glass and wish you good health!!
Cheers David

Hi David,
I apologise, and suggest you use Metric Fine Threads more fitting to your level of sophistication.

[jasonb]

You don't just have to go to Germany, Polly keep a limited range of the metric screws from the same supplier and if you can't translate the above mentioned supplier then GHW have an English version of their site.

ghw-modellbau.de/index.php?cPath=4_5

I suspect the common 2mm hex is due to available stock sizes and nott being able to get them below 1mm in steel should not put anyone off as BA is not readily available below 12BA form the usual suspects.