Leadscrew is... ...Pitch 3.0mm Dial Indicator gear 24 teeth.

OK. Thanks.

For the 1.5mm pitch you describe in your initial post, you do not need the thread dial indicator at all. You could remove it from the machine altogether and still cut that thread properly.

If the leadscrew pitch is 3mm, every leadscrew pitch contains exactly two 1.5mm pitches. That means wherever you engage the half nuts, the threads will line up.

You can do the same exercise with other metric pitches. For instance, the 3mm leadscrew pitch contains exactly three 1mm pitches. So for 1mm pitch threads, the indicator is not needed.

If you go through every reasonable metric pitch, you will find roughly 50% of them do not need the indicator at all.

The following pitches do not need the indicator:

0.2, 0.25, 0.3, 0.5, 0.6, 0.75, 1, 1.5, 3

For other pitches you do need an indicator.

Let us pick 1.25mm pitch (M8). How many 1.25mm pitches fit into a multiple of 3mm pitches? The lowest integer that will divide 3 and 1.25 is 15. Thus five 3mm pitches contain twelve 1.25mm pitches.

Hence, for this pitch, you would need a thread dial indicator that has marks every 15mm of travel. Since each tooth on the gear attached to the dial counts/measures 3mm, you need marks that occur once every five teeth of the gear. You also need a gear that has a tooth count divisible by five (so the marks do not change every revolution). Your gear has 24 teeth, so it is not suitable for 1.25mm pitch. If you want to cut that pitch, you have to use the full reverse method.

So what pitches can you use your current TDI for? 0.4mm, 0.8mm, 2mm, 4mm and 6mm. The synchronisation distance (q.v. Cleeve) for these threads is 6mm, 12mm, 6mm, 12mm and 6mm respectively, so your dial will work if its marks occur once every four teeth of the gear.

Finally, let us look at the gear requirements for pitches not yet mentioned. 0.35mm needs a gear with teeth that are a multiple of 7; 0.45mm multiple of 9; 0.7mm multiple of 7; 0.9mm multiple of 9; 1.25mm multiple of 5; 1.75mm multiple of 7; 2.5mm multiple of 5; 3.5mm multiple of 7; 4.5mm multiple of 9; 5mm multiple of 5; 5.5mm multiple of 11.