Metric Bantam lathe and 19tpi threads

Has anyone any settings for making a 19tpi thread on a Bantam metric lathe? I know there is a formula in the manual which is a bit obscure (to put it mildly) and my maturing brain won't cope with it too well. I'm sure someone else must have wanted this for a BSP thread sometime before I did.

Francis

Mine's an imperial Bantam, or at least the main leadscrew is, so I've only done it the other way round.

First of all, though - have you got the 127t and 100t gears? And do you have most of the other gears mentioned in the manual?

No point getting too clever if you haven't. I believe you can do it with another combination but the manual assumes you have those. If so, I'll see if I can figure it out for you with my manual.

Murray

Yes I have the 127 and 100 gears and most of the other gears mentioned on the table to cut standard (i.e. not 19tpi) imperial threads. Thank you for having a look. I found the formula a bit odd as it has 2 other unknowns apart from the one you actually want. I'm sure that I could have at one time worked it out from first principles but the grey matter isn't quite so flexible now.....I'm told it's a common failing.

Hi there,

Try Lever A = position T (value1)

Lever B = position 5 (value 12)

The formula in the handbook now becomes 224/19x12x1 = 18.6667/19

Multiply top and bottom by 3 and you get 56/57. You were always going to have to have 57 in there somewhere!

so the train becomes Driver / Driven = 56/120 x 127/57.

I've taken this from the handbook for my Mk 2 Bantam, I assume the Mk 1 has the same gearbox ratios.

Try it carefully on a scrap piece before committing to the real thing!

Muzzer - could you please check my arithmetic!!!

Rgds to all

Simon

Edited By Simon Williams 3 on 30/04/2016 11:26:32

Not sure which gears you have, but if you have a 3mm pitch leadscrew, to cut 19tpi you can use a 20T on the lathe spindle and a 45T on the leadscrew with whatever sized idlers fit in between. Will have an error of a couple of thou per inch but close enough for pipe threading work.

Hi Simon - yes, looks good to me, although the lever numbering is different. I have a Bantam 1600 (MkI) which may differ from a MkII. The gears you suggest give the required pitch pretty much spot on.

The formula's the same and you need to get the product of the 2 levers to be 12 but mine are identified like the illustrations ie levers Y and Z (1 and 2 in the pic):

Murray

Edited By Muzzer on 30/04/2016 14:40:17

Thanks for these thoughts. Simon's calculation works, but I haven't got 56T or 57Tt gears, but it provoked a rush of blood and I now have a glimmer of light in realisation that the tpi required has a relationship with the driven gear, which led me to gearbox 5 (=8 in the calculation) and lever in positn B (=2) which gives me gears 28/38. This is now one gear down as I have a 28T in stock. I assume that the gearboxes etc on the Mk1 and Mk2s are the same but the terminology has changed a bit.

Hopper I understand now how you achieved your calculation because I have just found the 'nthredp' program which is what I suspect you used (or something similar). I haven't got a 20T gear but now I'm 'cooking on gas' as regards the gears I have got, so thank you for the inspiration (the leadscrew is 6mm but this is easily dealt with now).

Thanks to both of you for your time and efforts.

Does anyone know precisely what the gearbox ratios are so I can make allowances for this in the calculations? I'm a bit loathe to take the gearbox apart to count gears!

ps thanks Murray, I've just seen your post which went in as I was writing mine

Edited By Frankiethepill on 30/04/2016 14:48:34

On a related but parallel matter, I had a sort through the change wheels I've squirrelled together, and realised I have some of 16DP which match the originals supplied with my Mk 2 Bantam, but I've also bought some with 14DP without realising they were different, sold to me as suitable for "The Bantam".

Both have the same splined centre. Could it be that the Mk1 had 14DP wheels?

I've got a hope that I can mix and match them, provided the same DP's mesh. With the centres being the same I can in theory put one of each DP on the idler spindle (say), and use the corresponding meshing gear on the g'box and on the mandrel. Bit of a faddle, but needs must. Always supposing they are the relevant no of teeth of course. I've yet to try this experiment!

Any thoughts? Simon

Pity your machine has a metric leadscrew and presumably a metric-specific set of ratios. Mine is imperial and I've made a spreadsheet to work out every possible pitch and feed for every gear and gearbox setting. You're welcome to a copy but I can't answer your question about whether the gear ratios are the same and you'd have to change all the numbers about to change from imperial to metric.

Somewhat envious that you have a metric leadscrew. I think and work metric where possible. Bizarrely, my top slide and cross slides are metric - I believe this isn't unusual but for me, screwcutting is a little painful when a metric thread is required.

Dunno what DP the MkI uses but you could mix and match them as long as mating gears are same DP surely.

Murray

Actually, I have the parts list showing the gears used in both English and Continental (metric) boxes. I'll see if I can figure them out. Have you got the full set of manuals and parts lists? And which version do you have - MkI or MKII?

Edited By Muzzer on 30/04/2016 19:07:39

Simon

My Bantam 1600 Mk 1 gears are 16DP, same apron as in Muzzer's pictures.

Emgee

Edited By Emgee on 30/04/2016 19:54:57

Edited By Emgee on 30/04/2016 19:55:23

Mmm. Interesting little snippet there, thank you Emgee. I wonder what I've got! Are these gears common with a Student I wonder?

I've been sniffing about in the parts manual which shows the guts of both the metric and imperial ("English" gearboxes.

It may be obvious to a seasoned threading gearbox designer but certainly I was surprised to see that there are some interesting similarities and differences between them:

• The internal ratios are the same. You can deduce this from the operation manual but when you check the numbers of teeth on each gear you can confirm it quickly enough.
• The initial pair of gears is 21:49 (a ratio of 3:7) which explains why the external default driver gears are 35 (driving) and 30 (driven).
• The imperial gearbox is driven "top down" ie the top "intershaft" is driven by the input gears, then the initial 1:2:4 mesh / dogtooth ratio is selected, resulting in the middle "driving shaft" being at one of those 3 ratios. Then there are 3 pairs of sliding mesh gears that drive the bottom shaft, giving 6 ratios containing the ratios 7, 8, 9, 10, 11 & 12. Finally, the feed and lead screws are driven at 1.5 times the bottom shaft via some idlers.
• But bizarrely, the metric box is driven in the opposite direction. The bottom shaft is driven by the input gears, using the same 21:49t reduction but with the 49t gear on the bottom shaft instead of the top. All the gear ratios are the same but of course the final drive is now coupled directly from the top shaft to the leadscrew by means of an additional splined (but static) dog. There is an orphan 24t gear on the end of the feed screw that is simply left in as a spacer.

By way of a sanity check, with a 4tpi leadscrew, the manual says you need to select "A1" ratios to achieve a 4tpi thread. In this setup, the 3-way ratio (1:2:4) is set at the "highest" position (actually 2:1) and the 6-way ratio is 24:36. The external gear train is 35t (driving) and 30t (driven). This gives an overall ratio from spindle to leadscrew of 35/30 (external) x 21/49 (input) x 32/16 ("A" x 24/36 ("1" x 36/24 (fixed final ratio). Ideally this would reduce to 1.00, which it does. Phew.

Makes sense now. And if you want to do any unnecessarily complex gear ratio calculations, the internal ratio info is all there in the parts list, apart from the 21t input gear which is fairly simple to deduce. I can show the construction and the various gear ratios etc on a cutaway view if anyone's interested.

Murray

Crikey Muzzer, that's quite an analysis there! I think I'm going to laminate it and put in the back of my Machinery Handbook. Thank you for that work!

Rgds Simon

Have to say I'd been meaning to get to the bottom of the gearbox ratios at some point, so this was just a trigger. Previously (as mentioned above), I'd made a spreadsheet to work out all possible threads and feeds available with the subset of gears I possess and I'd based that on the handbook formulae. Every time I saw the exploded parts view I wondered how the ratios achieved the range but the illustration was so exploded it was hard to figure out how they went together. In those days everything would have been designed with pencils, drawing boards and slide rules.

Somebody on this forum or the homeworkshop forum copied me a Clausing Colchester (USA) document showing additional views that are not given in the UK manuals. Sorry I forgot who was kind enough to give me them but they are pretty helpful in understanding how the exploded parts fit together. Surprised to find the metric version works sort of backwards but that must be something to do with the different progression of pitches, fractional vs metric. It can't be that difficult to work out...

Murray

Edited By Muzzer on 02/05/2016 11:36:37

Incidentally, I've just noticed what Hopper suggests but this assumes the gearbox is easily set up to give a direct (1:1) ratio. This isn't the case unfortunately, as there is a 21:49 reduction on the gearbox input. If you want a direct ratio you have to drive it with a 35:30 external ratio which screws up (sorry) the calculation.

If you have a 6mm pitch leadscrew and set the gearbox to cut a 6mm thread, isn't that a 1:1 ratio?

Edited By Hopper on 04/05/2016 11:06:55

That's a bit too hitech for me. I referred rather to Martin Cleeves' wonderful little book "Screwcutting in the lathe" (Workshop Practice Series, Tee Publishing, 7 quid or so). I've been screwcutting on lathes both metric and imperial at home and work for donkeys' years and still find something useful every time I look at that book. If you really want to get your head around obscure aspects of screwcutting, such as cutting metric on imperial and vice versa, I don't know of a better starting (and finishing) point.

Evenin' all

Just had a very similar problem on my own Bantam-cutting a 26tpi Cycle thread. Worked it all out with the aid(?)of the formula in the book. According to my sums (Quiet at the back, there!) you need a 28 t driver and a 57t driven (leadscrew) gear compounded with 120/127t idlers as 28/120 x 57/127. The gearbox levers are pos B and pos 1. These gears are standard Bantam for the English and non-gearbox models. The 57t is non-standard for the metric box unfortunately. Incidentally, for the 26tpi the settings are 28t and 39t, compounded as above, levers at C1. Couldn't get hold of a 39t so made my own. Unfortunately some of these gears are a bit scarce, but occasionally turn up. There's a 39t on fleabay at the moment. There was a query somewhere else about tooth sizes. They are 16DP on the 800/1600-otherwise known as the Hawk/Eagle/Condor. Hope this helps.

Sandy

Thanks for all the comments, and also to Muzzer for his exposé on the gearbox contents- fascinating and curious. I have been playing with figures and also the 'boxfthreadp.exe' program which is an oddly written thing but once you understand its Idiosyncrasies it works quite well. I have calculated the gearbox ratios for the Bantam to put into the program and when you enter the list of gears you have it seems to work well. One odd thing is that it can give a set of results which may look odd, but if you reduce the % error you specify, even down to quite low tolerances, you get a completely new set of results which may be more useful to you. I have tried cutting a few threads, including the desired 19 and 26 tpi BSPs (why do the Brits seem to make seemingly simple things so quirky?) and it seems to work.

I can supply the gearbox ratios as a text file the program can use if anyone wants it- send me a PM.

Francis

Hi Frankiethepill

I have just bought an old Bantam 600 with metric tread cutting i like your littel boxfthreadp.exe' program

but dont know how to fill it out do you have an example of a compleeted so I know what to do

Yours in sport Anthony