Pure guesswork, but would expect the Leadscrew on a lathe of that size to be 3mm pitch, so for 5 tpi it would need to be rotating faster than the spindle by a ratio of 1:0.59.

With 3 tpi to be the coarsest thread that can be cut, this is about 8.4mm pitch, needing the Leadscrew to rotate about a third faster than the Spindle. (1:0.3543307 ratio).

Do you have a 127T or even a 63T changegear? That should allow you set up a train, (and the Norton box) to cut Imperial threads, of various tpi.

As a starting point, 5 tpi (0.2" pitch) at 0.59:1 would seem to need the 127T gear driving a 75T, ( or a 63T driving a 37T) AS AN APPROXIMATION in an otherwise 1:1 train. Greater precision can be obtained by moving away, slightly from the supposed 1:1 ratio.

But then I could be wrong! Anyone else got any suggestions?

Howard

As you say 'the max tpi is 3' it implies you have parts and information for cutting imperial threads so is your question about the possible strain on the parts in cutting a coarse thread? Tables and gearboxes are sometimes more for completeness of the list rather than implication that it is a good thing to do – rather like car speedos going up to 150mph.
If that is your concern then one way round it is to push or pull the saddle manually while using the gear train to impede its movement to the correct pitch rather than providing the force needed to pull it along.

I never fixed it, I just thought I had found a solution, but it only lets mods rotate images. You may or may not be able to see the rotation I've done, you can probably see the random spaces.

Neil

I selected the text behind the picture and pasted it into notepad to view it.

Technology, eh!

Edited By Muzzer on 02/06/2017 14:32:38

For the sake of clarity, the bit that got overwritten in this post as adjusted above was that the table of threads came from the handbook for my MK 2 Bantam, so it shows how to set up the change wheels for 5TPI as requested.

There does seem to be a little bit of confusion going round – if the lathe can cut everything down to 3.5 TPI as shown in the manufacturer's table, then all those finer threads (of which 5TPI is one) are available. Cutting coarser threads – numbers below 3.5 TPI – might be difficult and certainly needs extra care or a different approach. But a Mk 2 Bantam is a pretty robust beast, and I wouldn't expect to have too much difficulty doing 5TPI on it. Mind you it's a lot of thread to whittle out, it's going to take a bit of patience (and a lot of concentration) to make a reasonable job of it.

I'm sure I'm not the only one who'd like to see a piccy of the finished result.

Looking forward to seeing it. Can we have some " in progress" pictures too please?

Good luck Simon