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Screwcutting on the Bantam

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william austin27/05/2021 19:53:07
7 forum posts

Hi Everyone,

I need to make some adaptors UNS 3/8" 27 TPI. This is not a standard pitch on offer according to the machine placard. I have a fair set of 13 change-wheels (21,35,36,39,42,49,57,60,66,69,100,120,127) and the original Bantam manual. Unfortunately, the manual is very short on explanation in the worked examples and it doesn't cover the possibilty of compounding (which is used in some placard solutions) of the idler stage or how to choose the gearbox settings.

Can anyone steer me to a methodology?

Tony Pratt 127/05/2021 20:49:37
1637 forum posts
8 photos

What idiot ‘designer’ would use 27 TPI? 🤬


Neil A27/05/2021 21:05:26
100 forum posts

I'm afraid 5/8 x 27 TPI UNS is the standard American microphone mounting thread. Everyone else uses 3/8 x 16 TPI Whitworth.

What idiot indeed!

Sorry I can't help with your change wheel setup.


DC31k27/05/2021 21:05:33
547 forum posts
1 photos
Posted by Tony Pratt 1 on 27/05/2021 20:49:37:

What idiot ‘designer’ would use 27 TPI?

There is a large country to the west of this little island, called the United States of America. The 27tpi thread is quite popular there on pipework.

Neil A27/05/2021 21:20:55
100 forum posts

Just had a thought, although I can't help very much on a Bantam lathe as I don't know how its change wheels are arranged, You might get some ideas if you look at the "Little Machine Shop.Com" web site.

They have a calculator for change wheel setups, albeit for mini lathes, under their learning centre, "how to articles".

This might give you a starting point for your setup. No guarantees.


DC31k27/05/2021 21:23:10
547 forum posts
1 photos

There is a thread here:

where, in the fourth post, a link is given to a very comprehensive data pack for the Bantam.

I am using page 16 (6 of 15 in the pdf) from that as a reference.

It gives a formula for threads not available from the gearbox: DRIVER/DRIVEN = (7 * Z )/(3 x Y * TPI)

Y is 1, 2 or 4. Z is 12 through 7.

If we pick Y = 1, Z = 9 and TPI = 27, the formula reduces to DRIVER/DRIVEN = 7/9.

There are only 18 possibilities with the formula so you can work through all of them to see if a better one emerges but I think the 7/9 above is the simplest.

Now the only gear you have with 9 in it is 36 (9 x 4), so the driver would need to be 7 x 4 = 28, which you do not have. 21/27 gives the correct ratio, but no 27t gear. 35/45 is correct, but no 45t. 42/54 is correct but no 54t. 49/63 is correct, but no 63t.

Any one of the missing gears above would solve the problem, so it gives you a few options if buying or making an additional one is a possibility.

Chris Gunn27/05/2021 21:49:25
386 forum posts
26 photos

Tony, using off standard sizes of fasteners and so on, is not so daft after all. It makes your customers come back to you for spares. A company I used to work for used 11/64 pins, 1 1/16" pitch chain and chain wheels, I could go on, but anything prone to wear or breakage was non standard. For one high wearing part they bought special 10.8mm silver steel, impossible for the customer to buy this in small quantities to make their own parts.

Chris Gunn

DC31k28/05/2021 09:07:34
547 forum posts
1 photos

If you want to go the compound route, you can model the 7/9 as 1/3 * 7/3.

For the 7/3, you use the 49/21 that you have. This then gives the following possibilities using one gear that you already have, the other gear (the numerator of the fraction) being a make-or-buy: 40/120, 20/60, 23/69, 22/66, 19/57.

The big caveat with the compounding is whether the gears will fit in the space available.

RDG, G&M and Bede have Bantam gears.

This would solve your problem:

william austin28/05/2021 09:33:01
7 forum posts

A big thankyou to everyone for the amazingly quick input. I am trying to make an adaptor for a new Propane torch nozzle. The old one is an Australian product that I think is a copy of an Anerican one. That is 3/8" x 27, the modern variety is 7/16" x 27. I agree that it is probably a case of manufacturer entrapment.

I can follow the comment from DC31k and I guess 7/3 is a constant either from the gear head or inherent in the gearbox.

I have 100/120 and 127 that could be compounded in the idler gear position to extend the combinations but I don't understand how to include that other than an iterative process. I have two options for a new gear, 1/2" Aluminium plate (which I have) to make a complete one or cast some HDPE (alias melted milk bottles) onto a smaller gear. For both I would need a gear cutter and for the former, to cut the 8 tooth spline.

Can anyone suggest the tooth form for the gear? I could do the spline on the lathe by indexing the chuck.

william austin28/05/2021 09:36:47
7 forum posts

Sorry about that, while I was composing the 09:33 post, I didn't spot DC31k's 09:07 pop up. My apologies

Brian Wood28/05/2021 09:45:07
2435 forum posts
37 photos

Hello William,

I think you may find the tooth form will be DP and perhaps 14.5 degree pressure angle. I am going by the vintage here but I may be wrong on the pressure angle, it could be 20 degrees making for stronger gear teeth by virtue of the slightly broader tooth base.

Owners will be able to confirm my thoughts


Emgee28/05/2021 10:56:22
2146 forum posts
265 photos


Bantam toothform is 16DP and I cut some for my machine with 14.5 deg PA and they mesh and work OK.


william austin28/05/2021 14:17:14
7 forum posts

I think I will try to go down the path of a new changewheel.

I only have two DP16 14.5 PA cutters. A No 1 135 to rack and No 7 14-16. I know neither is perfect.

From the point of view with the workholding, a 54 or 63 tooth gear would do best for me. The No1 cutter looks much more suited to the 54/63 than the No 7 but that is a guestimate. The wheel would mesh with 100/120/127 depending on space.

Since the gear will be in Aluminium, it will hopefully bed in fairly quickly. I just really need to avoid it jamming but with enough mesh enable three adaptors in brass.

Does anyone have any thoughts?

John P28/05/2021 15:25:55
309 forum posts
208 photos

If your lathe has a gear chart listing for module gears .3 mod has a
linear pitch of .0371" . 1 inch divided by 27 = .03703".


william austin28/05/2021 15:45:14
7 forum posts

Hi John,

It is an Imperial lathe but does have "dp" setting list and at one end of the spectrum are 40, 44 and 48 and the other 7,8 and 9. I have no idea what they mean


william austin28/05/2021 15:47:46
7 forum posts

Looking at the placard picture, my guess is for worms

JasonB28/05/2021 15:49:02
21284 forum posts
2416 photos
1 articles

Those would all be tpi. - threads per inch.

Andrew Johnston28/05/2021 16:21:09
6215 forum posts
676 photos
Posted by william austin on 28/05/2021 15:45:14:

...does have "dp" setting list and at one end of the spectrum are 40, 44 and 48 and the other 7,8 and 9....

If the numbers are under the heading 'dp' then they refer to the DP of the gear for which a worm thread will be cut. Given that the DP of the gear is integer the tpi of the worm will be irrational.


Pete Rimmer28/05/2021 17:11:01
1042 forum posts
58 photos

Where abouts ar you William? My lathe does 27tpi on the gearbox you're welcome to use it if you can get to North Kent.

Otherwise I could hob a change gear for you.

Incidentally if you have a chart for DP threads see if theres a way you can add a multiplier to achieve 85DP that's the exact pitch for 27TPI


william austin28/05/2021 18:31:14
7 forum posts

Hi Pete,

Many thanks for your kind offer. I am near Witney, to the west of Oxford, so a bit far just now.

I think that I may just cut the teeth on a disc to see how well it meshes using the cutters that I have. If it works, then progress to the spline. After all, the gear is not going to do much work, it just needs to stay in mesh.

I think that the spline may be more difficult to fashion.

If my crude gear attempt fails, then I may take you up on your offer and send you a disc or two.

As far as chasing the 85 DP target goes, I think that I am in the original boat by lacking the multiples of 9


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