Cutting an unlisted thread on colchester student

By all means experiment, but I doubt Colchester got it wrong. The good thing about gearboxes is they provide instant access to common ratios,. The bad thing is that they may not provide all the ratios you need. Covering all possibilities would need a complex gearbox and cost is usually an issue. I don't think it's unusual for ratios to be missing.

As a compromise, my lathe has a 3-speed gearbox allowing me to switch quickly between TPI or metric triplets like 0.75, 1.5 and 3.0 pitch. But, if want to cut any other thread, I have open her up and alter the change wheels. Takes ages and I get covered in grease.

It gets worse. In practice my 3-speed gearbox doesn't help much because the next thread I cut almost always needs a different change-wheel combination! Your gearbox is much better than mine, but it still has the same problem - sooner or later to get the wanted ratio out you have to alter the input ratio. On some lathes this isn't possible at all, or it needs change wheels you don't have, or - if you can't find a table - requires wet-towel and paracetamol mathematics. Jason and Brian's suggestions look good, but you would need to source the gears.

How is threading done by changing pulleys? Presumably a toothed belt?

Dave

Just to update this thread with a solution if any else needs to do this

(I still cant get my head round it )

My thread plate

I used Brian Woods suggestion of swapping driver and driven gears

So i used the 35 as the driver and 21 tooth as driven

then with the levers set DA and the gearbox for 20tpi/1.5mm setting and the imperial/metric lever set to imperial

and leaving the half nut engaged i get a lovely 3.5mm thread s far as i can measure with a pitch gauge over 6 threads but it certainly works with commercialy availble nuts

disengage the half nuts and it wont go back on the thread previously cut

Many thanks to all that offered advice

Ian

Hello Ian,

Thank you for bringing us up to date with the outcome of your experience. It is always gratifying to hear that suggestions have worked, as planned, and that you are pleased with the outcome.

Happy threading

Regards Brian