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BA Threads or Metric Threads?

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nick Clark11/07/2009 21:18:54
5 forum posts
Im new to model engineering and so far I have only dealt with Metric taps and Dies for thread cutting.
Can anyone tell me why model engineers seem to use BA threads as opposed to Metric equivilents.
Are BA threads popular in model engineering as someone told me that BA is obselete now as it was phased out in 1966?
I dont want to purchase taps and dies if its not really something that people use anymore in Model engineering?.
If anyone can enlighten me I would be gratefull.
Geoff Theasby12/07/2009 04:51:09
533 forum posts
14 photos
BA threads aren't obsolete, neither are Imperial threads, or ME threads, come to that.   As in full-sized practice, they remain in use for repairs and renovations, and people like Stuart still use them even in new kits.
The recommendation is that we use Metric for anything newly designed.   However, we shall be able to buy BA (which is metric based) and Imperial for years to come.   Imperial sized screws can still be bought from industrial suppliers, even Whitworth threads.
I, too, am new to model engineering, although having worked in industry for many years, and my first lathe is a Unimat 3, built to metric standards, so I have concentrated on metric taps and dies.
JasonB12/07/2009 07:33:38
12663 forum posts
1147 photos
As Said BA is infact Metric based, there are several reasons for using it
There is a larger range of sizes available eg the increments are smaller, BA nuts look correct on a period model as the mass produced metric ones are too thin and chamfered both sides.
Also BA sizes suit imperial rod, 5BA will thread 1/8" and 7BA will thread 3/32" so if you opt for metric tap & Dies you will then have to alter what material sizes are used and source metric as most of the ME suppliers still stock imperial.
The best thing to do is buy what taps and dies you require as & when you need them for the model you are working on rather than buying sets which usually are of inferior quality.
Circlip12/07/2009 17:32:27
863 forum posts
Another small but subtewl point in favour of BA in  certain applications is the fact that small increments in adjustment are possible, eg. Valve timing etc.
  I use metrics for the main structural fixings, but the range of sizes doesn't translate freely for all.
  Oh aye, nowt wrong in cutting metric threads on imperial hex bar stock if you're so inclined.
   Regards  Ian.
Steve Larner12/07/2009 22:17:14
2 forum posts
Posted by Geoff Theasby on 12/07/2009 04:51:09:
BA threads aren't obsolete, neither are Imperial threads, or ME threads, come to that.   As in full-sized practice, they remain in use for repairs and renovations, and people like Stuart still use them even in new kits.

Doesn't alter the fact that they were declared obsolete in 1966
Steve Larner
Jim K13/07/2009 04:36:21
44 forum posts
They might have been declared obsolute by BSI but in model engineering and other engineering activities they are still very much used.
I suppose it is up to the builder if he wants to aquire BA to build as per the original drawing then ok if he wants to convert to Metric or other then so be it also.
In experience any model engineer I have met who builds many models or is in the process all have most thread forms in their collection of tools. If someone is just going to do a one off then it would be highly unlikely that he would collect all forms.
Another point broached in and earlier post is what is available in model engineer supply stores. You will find that it is all BA nuts and bolts that have been scaled to suit the model engineer. i have not yet come across metric that has had the nuts for instance scaled. this does not mean you cant scale down your own and make them yourself if you want to go metric,but it is a lot of work if you have many sizes and numbers of fixings.
If you are trying to cut down on the amount of tools that you buy then why not find a local model engineering club and join it, you will find that they have usually got a full stock of taps & dies handed down from previous members and you can usually borrow from the club.
David Clark 113/07/2009 09:13:21
3357 forum posts
112 photos
10 articles
Hi There
BA are not obsolete.
The standard has been recently updated.
To standardise on one set of threads seems a bit daft.
Use what is most apropriate.
regards David
nick Clark15/07/2009 21:04:35
5 forum posts
Thanks for all the info its a great site,
When you say BA threads are metric based what does that mean?
And the last question or point is regarding Jims Post he says
"You will find that it is all BA nuts and bolts that have been scaled to suit the model engineer"
Does this mean there are all different sizes of nuts for say a nut of 6BA.
What Im trying to say is if I bought a 6BA brass nut from one supplier and then later got another supplier will they all be the same or scaled differently from each supplier?
Perhaps im getting confused?
Thanks for all the replies so far .
Jim K16/07/2009 04:47:21
44 forum posts
BA threads are not metric based they are imperial in all dimensions and the thread form does not even match metric.
For the dimensions of the bolts the manufacturers of the fixings have reduced the dimensions accross the flats on the hex of the fixing and nut to make them look more dimensionally correct, I am not sure if they are reduced to scale to any particular model scale maybe they have taken an average of 3-1/2", 5",7-1/4" guage maybe some of the other readers can add to this.
To answer your query BA has had the cross flat dimension reduced by the model engineering fixing manufacturers and probibly match the ME dimensions or are close to the dimensions. So yes if you go to the normal hardwear store and by BA nuts and bolts then buy more from a Model Engineering supplier you will probibly find them different, but the dimensions from each model engineering supplier should be the same.

This might help you Colin has covered each thread form in detail
The model Engineer's handbook by Tubal Cain covers just about everything and may even have a table showing the dimensions of fixings used by ME
JasonB16/07/2009 07:34:41
12663 forum posts
1147 photos
Ther are standard size hex for all BA threads but it is possible to by "one size smaller" heads as well eg
4BA hex is 0.248" but you could also get them with a 5BA hex which is 0.220"
A standard 4BA from any supplier should be the same size.
It is also METRIC BASED. 0BA is 6.0mm OD (0.236") and the next size down is something like 90% of that, then 90% of that etc. Not got time to go into it now.
Jim K16/07/2009 07:55:55
44 forum posts
My Apologies On further research i see the tie up with the metric system.
Norman Barber16/07/2009 11:50:59
13 forum posts
In 1980 Model & Allied Publications Ltd. set up a working party to study the implications of a future change from BA and ME to metric threads on the practice of model engineering.  The report was finalised in 1981.  It was considered by the British Standards Institution and whilst it could not be issued as a British Standard it was published by BSI under the title "Guidance on Metric Threads and Fasteners for use by Model Engineers".  I do not have a copy of this document but the data is sumarised in The Model Engineers Handbook by Tubal Cain.  The existance of this document does not seem to be very widely known and it certainly does not seem to have had much influence on either model engineers or the trade.  I personaly consider BA fasteners far more suitable for model work than the small metric fasteners, the pitch of the thread being smaller for a given diameter and the hexagon heads much smaller, particularly if the "one size smaller" hexagons are used.
For what it is worth the BA series of threads is based on the relationships pitch P=0.9 to the power N where N is the BA number and the diameter of the thread D=6 x P to the power 1.2. (Sorry - I can't type the powers in a mathematicaly correct manner!!)
JasonB16/07/2009 17:09:20
12663 forum posts
1147 photos
Thanks Norman I knew a 9 came into it somewhere but it was a bit early in the morning.
A year or so ago ME covered the origins of BA threads in teh Letters to a Grandson Series in quite easy to folow terms
Also further to nicks query about hex sizes, Standard size BA hex is 1.75 times the major diameter.
mgj16/07/2009 19:01:24
1008 forum posts
14 photos
Well one thing is for sure - metric is far more freely available and a great deal cheaper. I go down to my local fastener people and come back with all the nuts setcrews etc, and I have extreme difficulty spending more than a couple of quid for 20 of this and 20 of that. and I can get them easily in stainless, button head socket caps (pretending to be rivets) etc etc. 
So for me, the only time I'd deviate from using metric would be for something specific - putting a nut onto a shaft of specific imperial size, something like that. If I need particulary fine threads, then the metric fine series seems to do.I have a complete set of BA taps dies spanners and sockets, but they are rarely used nowadays. 
If you bust a tap, you can get a new (cheap) metric one from Halfords or Screwfix - even on a Sunday if need be. BA are a little less convenient!
JasonB16/07/2009 19:16:48
12663 forum posts
1147 photos
Why use metric button heads which are a bit on the flat side and need filling when you can get threaded rivits or unslotted round head screws in BA
I think it all comes down to what you are happiest using and whats available locally, for example I have been working on an American design for a V-twin IC engine that uses their small series of threads. So when the drawing called for 8-32 UNC I used an M4 thread on 5/32" rod
Stephen Wallin16/07/2009 20:14:59
1 forum posts
I am a spot suprised that BA are widely proposed for engineering models, as the ME series, based on Whitworth thread forms, is usually more accurate than BA. Most metric pitches are a bit too course for the diameter for model use, even with the fine pitch series.

It was mentioned that BA are Imperial, but that not quite right as the base used is metric, and of Victorian origin. 

The published tables may be quoted in Imperial sizes, usual thousanths, for the smaller sizes, but the No0 was deliberately started at a metric size.

I worked in instrument making, and BA is still widely used, and a very useful system for smaller sizes especially below 12BA and on to 20BA, (a bit smaller than metric .5mm).

They were instrument threads, giving a fine pitch for secure nuts and fastenings, and are not a model of an actual Whitworth etc, which have a different faced angle etc, and for each size have fewer TPI. 

There is nothing against substituting metric for Imperial etc., as appropriate, but where appearnce matters ME works.

JasonB16/07/2009 20:32:07
12663 forum posts
1147 photos
The only problems with ME are that they are not available below 1/8" . But the are the usual thread for steam fittings, pipe joints etc.

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