Swarfing, I'm this chap. Johan posed the same question on the ME forum, and below is a copy of the rely I posted there.
Andy
COPY:
Though your lathe will cut both metric and imperial threads, it has a metric leadscrew, so a threading dial will only work for metric threads.
I made a crude threading dial for an imperial lathe, shown here:
http://andysmachines.weebly.com/roll-your-own-threading-dial.htmlAs you will see, the leadscrew is used as a worm to drive a gear, which turns the dial. For imperial leadscrews, a single gear is all that is needed , with the same number of teeth as the threads per inch of the leadscrew, or a multiple of that TPI. Thus, for my 12 TPI leadscrew, I made a gear with 24 teeth. If that dial only had one mark, I could engage the half nuts each time that mark came round, because the leadscrew would have turned 24 times, moving my carriage 2” along. If it had two marks, I could make quicker progress by engaging each time either of those marks came round and the carriage had moved 1”, because imperial threads are all expressed as the number of threads in one inch.
Life is more difficult with metric threads. Unlike imperial ones, there is no conveniently small distance into which all metric pitches will fit. If you have a gear with 15 teeth and a single mark on it, the carriage will move 30mm for one full turn of the leadscrew. The 11 standard metric pitches you might want to cut are 0.5, 0.6, 0.7, 0.8, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5 and 3mm. Only 0.5, 0.6, 1.0, 1.25, 1.5, 2.0, 2.5 and 3mm pitches will divide exactly into 30mm. That leaves 0.7, 0.8 and 1.75 mm pitches; those will divide into 28mm, so a gear with 14 teeth will cover them (and would also cover 0.5, 0.8, 1.0 and 2mm pitches).
So, you will need both a 15T and a 14T gear if you want to cover all the 11 metric pitches mentioned earlier. The thread dial indicator could have interchangeable gears, or both gears could be permanently mounted on a single shaft which could be moved along to bring either of them into mesh with the leadscrew. Indeed, as the entire indicator will can be made so it can easily be attached to and removed from the apron (see my web page), it might be easiest to make two separate indicators, one with a 15T gear and the other with a 14T gear. Of course, you might decide that you could easily use dies to cut 0.7 and 0.8mm pitches, and that you wouldn't want to cut 1.75mm (as used on M12 coarse) very often, so your needs could be met with a single 15T gear.
As to the number of marks on the dial, two examples might help:
1. Imagine using a dial with a 15T gear to cut a 3.0mm pitch. The dial goes round once as the carriage moves 30mm, and 3.0 divides into 30. But it also divides into 15, so you could have two marks on the dial, and engage the half-nuts when either came round.
2. If you were cutting a 0.5mm pitch with your 15T dial, 30mm/0.5mm = 60, so you could put 60 marks round the dial and engage the half nuts on any of them.
But too many marks can be very confusing, and I have four around my dial. That means that for the finer threads, I may have to wait longer than necessasy for a mark to come round, but in reality little time is wasted.
Going back to my first remark, the reason you will not be able to use a dial to cut imperial threads is that none of their pitches will divide exactly into a conveniently small number of millimetres.
I hope this helps!
Andy.
