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Almost 4BA but not quite

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Peter Cook 607/05/2022 18:48:43
282 forum posts
80 photos

I have a vintage screw head polisher stamped G Boley, which has a set of 5mm OD small (sub millimetre) collets. I was thinking about making an adapter to use them in my 6mm watchmakers lathe. To do so I would need to make a drawbar.

The collets are internally threaded, and the screw polisher has a drawbar for them. However I can't work out what the thread is on the drawbar.

The thread OD on the drawbar is 3.32mm ( 0.130" ). As close as I can measure the pitch is exactly the same as 4BA (0.66mm 38.48 tpi). A 38tpi guage shows as slightly too small a pitch, a 4BA screw fits more closely ( I don't use BA normally so don't have a BA thread guage). Thread form looks like the BA form by eye. A 4BA nut threads easily onto the drawbar, but a 4BA bolt will not screw into the collets (its 3.46mm OD).

None of my reference books have a thread with these characteristics. 4BA is the closest. It could be a Boley special I suppose. I wondered if anyone had come across anything?

If I go ahead I will probably get a 4BA die, and thread a slightly undersize rod with the die closed down tight.


Edit to remove errant smiley)

Edited By Peter Cook 6 on 07/05/2022 18:49:48

Michael Gilligan07/05/2022 19:04:31
20200 forum posts
1053 photos

This is a very useful compilation, Peter




… and I suspect that the thread might be No. 4 THURY

Edited By Michael Gilligan on 07/05/2022 19:07:55

Clive Foster07/05/2022 19:19:00
3137 forum posts
109 photos

My version of Andy Pughs list gives no 27 Siemens & Halske as being 0.133" / 3.37 mm diameter with a pitch of 38.5 tpi / 0.66 mm.


The Siemens & Halske thread was used in telephones around 1900. It has the dubious distinction fo having several thread angles between 50° and 68° depending on size.

Found a single table listing here :-


Michaels link is to an earlier version of Andys list than mine. Its been edited and extended over the years buy Andy and other folk. Probably the most comprehensive multi type list available.

Edited By Clive Foster on 07/05/2022 19:22:53

Mark Rand07/05/2022 19:53:43
1275 forum posts
28 photos

Number 4 Thury 0.1425" 3.62mm OD 38.7tpi 0.656mm Pitch


Peter Cook 607/05/2022 21:49:11
282 forum posts
80 photos

Thanks, absent a source of cheap Thury No4 or Siemens & Halske No 27 die, my 4BA on an undersize rod looks like a probable!

duncan webster07/05/2022 22:17:20
3990 forum posts
65 photos

No doubt someone with a fascination for working out change gears will be along with a suitable setup, not me, life's too short!

However, I'm confused, you say you have a drawbar which fits the collets "A 4BA nut threads easily onto the drawbar", so what do you need to make?

Michael Gilligan07/05/2022 22:19:46
20200 forum posts
1053 photos


May I suggest you read the first paragraph of the opening post ?


Peter Cook 612/05/2022 16:28:17
282 forum posts
80 photos

Just to report back on a failure. The S&H 27 thread looked the most likely (the OD on Thury No 4 was too big).

I tried turning a bit of 4mm rod down to 3.30mm diameter, and threaded it with a 4BA die cranked as small as I could get it. Partial success.

Half the collets threaded onto the resulting rod OK, the other half only went on about six threads then stuck.

I suspect the problem is I can't close the die far enough to get the minor diameter small enough ( although none of the thread references I can find give a figure).

The collets that threaded on are a close sliding fit on a drill shank that measures 2.75mm. The ones that won't will not accept that drill - but will accept one that measures 2.70mm. 4BA nominal minor diameter is 2.8+, so with the die closed I was probably getting a minor diameter of something like 2.75. OK for worn collets not small enough for the less worn ones.

So I guess if I want the drawbar it's going to have to be singe point threaded on the lathe!!

Brian Wood12/05/2022 17:36:40
2567 forum posts
39 photos


Before you go that far, it will be tricky anyway, try filing the O/D of the threaded rod you have made with a fine file just to take the sharp tip of the thread.

If you still feel you need to cut the thread, do you have gearing tables for your lathe? I can give you set ups for Myford, gearbox and non gearbox, Sieg mini lathes with 16 tpi leadscrew, 1.5 mm leadscrew and 2 mm pitch leadscrew if one of those is helpful to you



Peter Cook 612/05/2022 18:19:22
282 forum posts
80 photos

Thanks for the offer Brian. I did skim a bit off the tops of the threads (about half way down actually) to check it wasn't the OD. Same result - so It's definitely an issue of getting sufficient thread depth.

I have a Taig lathe, which doesn't have a screw cutting leadscrew - I have made a copier based thread cutting add-on, but not used it in anger yet. So I now need a length of 4BA threaded rod ( I may make some with the die I have) and use a 4BA nut as a follower and some workshop time.

This project has now slipped down the round tuit list behind a recalcitrant Brocot perpetual calendar which is stopping the associated clock.

Bazyle12/05/2022 19:37:59
6325 forum posts
222 photos

If the die is a good quality one that actually cuts rather than a modern cheap junk one that sort of squashes the material into submission you can use it a a thread chaser by opening it out and interposing some plastic shim under one set of 'teeth' to push the bar over or some rod into the holes to push the rod off-centre.

Peter Cook 612/05/2022 20:01:18
282 forum posts
80 photos

Thanks Bazyle, I have not come across that technique. The die was cutting reasonably well so I will try that.

gerry madden13/05/2022 14:28:27
252 forum posts
132 photos

Michael G - thanks for posting the link to the comprehensive list of threads. I wanted to condense it and then print it but have a small problem. When I copy/paste into a spreadsheet the first column gets converted into a mixture of dates and other nonsense. If I then try to reformat that column to 'general' or something else, the content gets increasingly nonsensical. What am I doing wrong or what should I change to ensure that first column remains intact after pasting ?


Clive Foster13/05/2022 15:33:53
3137 forum posts
109 photos


PM me your E-Mail and I'll send you an Excel spreadsheet version.


noel shelley13/05/2022 17:01:55
1353 forum posts
21 photos

At risk of spoiling a die, using a 1mm cutting disc open up the split a bit so it will go smaller - the worst that can happen is the die splits in 2. Worth a try ? Noel.

Michael Gilligan13/05/2022 17:21:37
20200 forum posts
1053 photos
Posted by gerry madden on 13/05/2022 14:28:27:

Michael G - thanks for posting the link […]


Apologies for the delayed response … we have been out all afternoon.

Looks like you’re sorted now, thanks to Clive


Peter Cook 613/05/2022 18:02:52
282 forum posts
80 photos
Posted by noel shelley on 13/05/2022 17:01:55:

At risk of spoiling a die, using a 1mm cutting disc open up the split a bit so it will go smaller - the worst that can happen is the die splits in 2. Worth a try ? Noel.

Thanks for the idea, but the limiting factor is not the width of the gap - its the amount of torque I can apply to the 3/32 allen key that closes it up in the tailstock die holder! However you have triggered my brain into thinking about making the basic thread, then swapping to a more basic holder in which I can apply more closing force.

I suspect the die will snap before I get the minor diameter small enough, so I will probably try Bazyle's idea first - then move to brute force!

bernard towers13/05/2022 21:40:58
619 forum posts
109 photos

good idea Bazyle a bit like spill reaming only with threads.

peak413/05/2022 23:19:52
1714 forum posts
183 photos

Peter, where do you live?
There might be someone local with a Herbert/Coventry die head and some 4BA chasers.
Very easy to cut under-size concentric threads with one.


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