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

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

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

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

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

I've been sniffing about in the parts manual which shows the guts of both the metric and imperial ("English&quot 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&quot x 24/36 ("1&quot 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

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

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

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

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

I have a Bantam 600 with metric thread cutting (Condor) gearbox. These are the gear ratios that I have put into the superb boxfthreadp.exe program – they seem to work well for me – tested on 26tpi

I deduced these by counting 10 turns of the input shaft and then measuring the saddle movement on the DRO and working out how many turns that equated to of the screw cutting shaft at 6mm pitch.

Hope this helps others.

I am stunned by that little program, it seems to enable me to calculate the gear sets to plug all the gaps in TPI threads to allow me to cut all BSF, BSW, UNF, UNC, BSP, NPT and BA threads using the standard wheels included with the metric lathe.

Of course only some of the gear combinations will actually fit into the lathe but the program produces a huge number of alternative combinations so it is straightforward to select those gear wheel that will fit and mesh.

"may I ask if you could be kind enough as you have a metric Bantam like me to measure the leadscrew whether it is 4tpi or 4 in 24mm (6mm pitch metric)."

Dear Pence,

I have just measured mine, and yes it is 6mm pitch.

Regards, Mike

Edited By Mike Lunch on 01/07/2019 10:18:25