5BA Threads

I'm sure this should be an easy question to answer. I understand that the nominal diameter of a 5BA thread is 0.126" (3.2mm). Is the actual external thread diameter (e.g. of the average 5BA bolt) less than this and, if it is, why is this?

Unfortunately I don't have any to measure, though I do have a 5BA tap and split die, which of course are impossible, or at least not easy, to measure. I suppose I could tap a 5BA hole and adjust the split die to cut an external thread to fit it, and then measure the resulting external thread??

If there are any instrument bowmakers reading this, I have been told that the Hill workshops used a modified 5BA thread for their violin, viola (and possibly cello) bow screws. However, my limited experiments point to it being a straight (unmodified) 5BA.

Any information very much appreciated,

Mike

Edited By Mike Donnerstag on 24/09/2020 19:34:01

Edited By Mike Donnerstag on 24/09/2020 19:34:23

This should help.

https://www.ring-plug-thread-gages.com/PDChart/BA-thread-data.html

I've just measured several 5 BA fasteners purchased via the ME trade. They measure between 0.122" and 0.123" od. I would expect commercial tolerances to be slightly below nominal size.

That answers my question - many thanks chaps!

5BA is handy for threading 1/8” rod.

Mike

As I was completing the assemply of a small brass bracket yesterday using a couple of 6BA countersunk hex socket screws it struck me that these must be as rare as hens teeth these days.

I inherited the Lab's old stock of BA fasteners about 20 years ago so they are possibly as much as 50 years old by now, maybe more. I did have the passing thought that maybe I should not be quite so profligate with them but then realised how little the stock had gone down over the last 20 years maybe not.

Someone is now going to astonish me with a supplier no doubt.

regards Martin

As a youngster I was told that BA threads were outmoded and "archaic" and that when I was old I would be able to cast back my memory and remember such a weird pointless thing like old timers then could remember the Boar War.........

But here we are all these years later and BA is still very much alive and indeed, has become more alive during the last ten years or so......In the horological world there are no plans to replace BA and nothing to replace it with as fine metric does not, in practice, turn out to have the same facility or 'feel' in the work.

Yes, Cromwell Tools have them in their catalogue, you might be a bit shocked at how much they cost though.

Yes I broke my 6BA taper tap 60 yrs old rpaced from Cronos Tools Cost £1.40 ,

Regards Brian

I have just received some 5BA cheese head steel screws for a french clock, from EKP Supplies £3.50 for 25 they have steel and brass different heads and lengths.

Dell

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Presumably told by someone who didn’t appreciate the sophistication of the thread-form

Blessed are the ‘Bean-Counters” ... ?

MichaelG.

+1 for EKP screws.

They make many of the screws themselves on Swiss automatic machines, all controlled by cogs and eccentrics etc., and which must be almost antiques by now.

Rod

BA Bolts have one of the biggest ranges, and he makes a lot of his own.

Wow, at £2.75 each for the 6BA hex socket countersunk I'm sitting on a fortune not to mention all the other sizes and varieties.

:O)

Martin

O.K. if we are boasting, I have a few 7ba socket head cap screws, where did they come from? I hear you say, I wish I could remember.

Reeves sell some even BA cap screw sizes at rather less cost.

https://www.hswalsh.com/search-products?search_api_views_fulltext=BA+screws

https://www.cousinsuk.com/product/ba-sizes-assorted

Edited By Bob Stevenson on 27/09/2020 17:05:25

Not countersunk hex socket screws they don't

regards Martin

Having done considerable investigation and experimentation, I found that many threads used for instrument bow screws were in fact modified by over-tightening the die to reduce the major diameter from 3.2mm to 3mm. This was true of the 5BA screws and the 5-40 UNC ‘Inch thread’ screws and means that the standard tap is too large.

Can anyone give any tips on making custom 3mm diameter taps?

Do any bowmakers on here know whether these ‘special’ taps are available anywhere?

Many thanks,

Mike

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This is all rather bewildering, Mike

In trying to ‘catch up’ ... I have just found this page: **LINK**

https://elcoda.com/bow-screw-250938.html?language_code=en

... which seems to present another ‘variation on the theme’

[quote]

Bow Screw with Rolled Inch Thread, Bronze Eyelets, Violin, Viola, Cello
These bow screws are finely-crafted in the style of the great French bow makers.The rolled thread allows the thread portion of the screw to have a larger diameter than the shaft portion.
Violin,Viola - dimensions 74 mm, thread 1,8", shaft diameter 2,7 mm.
Cello - dimensions 80 mm, thread 9,64", shaft diameter 3,0 mm

[/quote]

Can you provide any close-up photos of typical specimens of what you are working with, please ?

MichaelG.

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P.S. There are crucial differences between the BA and Unified thread forms, so I think this merits serious further investigation.