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machining 2 BA thread

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phil burley04/03/2016 13:11:41
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I have 2 lathes one imperial one metric , I would like to screw cut a 2 BA thread . But the pitch doesn't seem to be either . What am I missing here ?

Nobby04/03/2016 13:16:21
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587 forum posts
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HI Phil
Could yo use a die ?
Nobby

Emgee04/03/2016 13:17:13
2409 forum posts
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2BA TPI is 31.35 and metric 0,81mm pitch.

Emgee

Edited By Emgee on 04/03/2016 13:19:26

Lambton04/03/2016 13:22:18
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Phil,

Do as Nobby suggests.

If you use a good quality tail stock die holder you will be OK. There is no point in trying to screw cut any BA threads by "normal" methods. the posh way to cut them is to used a Coventry die head.

Eric

Michael Gilligan04/03/2016 13:50:42
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Posted by phil burley on 04/03/2016 13:11:41:

What am I missing here ?

.

Some fairly specialised change wheelsdevil

On your lathe of choice ... screwcut part-depth, at either 32tpi or 0.8mm pitch

This will give a track for the die to follow ... finish with the die.

MichaelG.

.

Obviously, the acceptable 'part-depth' will vary according to the pitch error and the length of thread.

Roderick Jenkins04/03/2016 13:52:45
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If either of your lathes has changewheels as opposed to a Norton type gear box then you can very likely find a combination of changewheels to give a good enough thread using the program at **LINK** If you have a Myford with a gearbox then replacing the 24 tooth tumbler stud gear with a 20T gear and selecting 26tpi on the gearbox will give you a metric pitch of .814.

HTH,

Rod

Neil Wyatt04/03/2016 13:56:10
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On an imperial lathe 63:57 is a starting point that should allow you to add a further gear pair and get 2BA.

For Metric, fit them the other way round 57 driving 63.

Neil

AndyP04/03/2016 16:14:42
189 forum posts
30 photos

Just to be perverse I put 0.81mm pitch into the little programs I use to work out gear trains for screwcutting on my C1 lathe and a different program for my metric Boxford AUD. In both cases I came up with a gear combination to cut that pitch either exactly for the C1 or with an error of 0.006% for the Boxford so it is possible at least.

I would use a die though.

Andy

Michael Gilligan04/03/2016 16:22:43
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20112 forum posts
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Andy,

Just out of interest ... what is the train for the C1

Thanks

MichaelG.

Ajohnw04/03/2016 16:53:02
3631 forum posts
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My understanding is that BA is a metric specification. Try converting the diameters. 2BA for instance is 4.7mm dia.

A quote from the wiki

The pitch of 0BA is 1 mm and the pitch of each higher numbered thread is obtained by multiplying the pitch of the lower number by 0.9 so K-BA has a pitch of \scriptstyle p=0.9^K.[1]:12 The major diameter is given by \scriptstyle 6p^{1.2}[1]:12 and the hex head size (across the flats) is 1.75 times the major diameter.

All derived no doubt when there wasn't precisely 25.4mm to the inch. It's essentially an instrumentation type thread. It's closer to metric coarse than fine

angry 2 Lotus used 0BA to fix the radius arms to the rear hubs on the Europa. That cause me a lot of grief as other threads will fit and then work loose.

John

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Bazyle04/03/2016 17:36:16
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Goodness you people are slow today. You have a metric lathe and M5 pitch is 0.8mm. Don't try to tell me you are bothered by the .01 pitch error.

Michael Gilligan04/03/2016 17:51:51
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20112 forum posts
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Posted by Bazyle on 04/03/2016 17:36:16:

Don't try to tell me you are bothered by the .01 pitch error.

.

That, Bazyle, would rather depend on the length of the thread.

... As yet unspecified

MichaelG.

Bazyle04/03/2016 18:01:18
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But it's patently not a leadscrew, just a fixing, so the engaged length is not going to be very long. Also it is possibly part of a Stuart Beam engine?

Edited By Bazyle on 04/03/2016 18:04:07

Michael Gilligan04/03/2016 18:04:14
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20112 forum posts
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Posted by Bazyle on 04/03/2016 18:01:18:

But it's patently not a leadscrew, just a fixing, so the engaged length is not going to be very long.

.

Are you clairvoyant, Bazyle?

AndyP04/03/2016 18:06:19
189 forum posts
30 photos

Michael

I always have difficulty understanding other peoples change wheel descriptions so this may be wordy but hopefully clear.

36 (fixed) on the spindle driving a 54 idler driving a 40 which shares it's stud with a 24 which drives a 40 on the 1.5mm pitch leadscrew.

Andy

Michael Gilligan04/03/2016 18:11:58
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20112 forum posts
1044 photos

Thanks, Andy

Not much help on my little Jason lathe [which has 32 (fixed) and a 16tpi leadscrew] but appreciated anyway.

MichaelG.

Ajohnw04/03/2016 18:29:40
3631 forum posts
160 photos

Personally if i didn't have a 2BA die I would replace it with M4.5 coarse. Really these days when people are buying things like taps and dies it's probably best to go for near metric sizes and forget BA. Arc for instance sell a set of the smaller sizes that starts at M1 that will cover the small BA sizes. A few more will cover BA's range.

The only problem with older designs is that some 40 tpi and 32 tpi may be a good idea but metric fine may substitute for these in some cases.

To me BA sizes don't need to be screw cut unless some one just wants to do it for fun. I wouldn't bother. Save it for bigger diameters.

John

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JasonB04/03/2016 18:48:08
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I'd hazard a guess that its for th piston rod on the Stuart beam which you do want a true as possible and being stainless a bit harder to cut with a die and not have it slip in the chuck putting marks all down your piston rod. Though skimming the piston to finished dia once its on teh rod is the best bet to get things concentric

As suggested screw cut it 80% at 0.8mm pitch and finish with a die if you are not happy that your die will do it straight off

John where are you buying M4.5 taps and dies, I've never seen them

Ajohnw04/03/2016 18:52:10
3631 forum posts
160 photos

Just as an example Jason

**LINK**

It's part of the extended series. Not sure when the size steps exceed 0.5mm.

LOL Forgot to mention I haven't bought any of these as I have a decent range of BA. However I did know that this size is about. 

John

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Edited By Ajohnw on 04/03/2016 18:53:25

phil burley04/03/2016 19:19:24
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thanks everyone, its in stainless steel and my die is well worn , I wasn't allowing for a bit of an error , just comparing the exact pitches that I could produce , please excuse my beginners ignorance .Yes it is part of a ST beam engine

regards Phil

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