This s what I have been looking at. Some is not relevant to my lathe as he is discussing a 4mm lead screw but all those numbers and then he talks about a 'prime number'. So what is the relevance of the prime numbers to the number of teeth?
I have 3 gears on the stem of my metric thread cutting dial and they are 28, 30 and 32 teeth. Those give you the prime numbers 2,3,5 & 7, which combined with a few different gears will give you all the metric thread pitches you will need. Not quite as straightforward as cutting TPI on an imperial lathe, but 95% of the world's machinists cope. I can take a photo of my thread chasing chart, ie dial teeth v pitch, if that will help.
4mm Leitspindel:
Thread pitch - Gear - Segment on dial (to engage in)
0,45 - 27 - Segment 3
0,5 - 20 - Segment 4
0,6 - 27 - Segment 3
0,75 - 27 - Segment 3
0,9 - 27 - Segment 3
1 - 20 - Segment 4
1,2 - 27 - Segment 3
1,25 - 20 - Segment 4
1,5 - 27 - Segment 3
1,75 - 21 - Segment 3
2 - 20 - Segment 4
2,5 - 20 - Segment 4
3 - 27 - Segment 3
3,5 - 21 - Segment 3
4 - 20 - Segment 4
So that means I would need a 20, 21 and 27 Tooth gear to get all the thread pitches my lathe allows for....
can anyone confirm this??