QUOTE: once I work out what size gear and how many teeth it might be able to made on a 3D printer. Any thoughts on this idea?
Sounds as though it would be well worth a try for gears for occasional use, since very little power is needed to drive the indicator. FWIW I just had a look at the indicator on my imperial lathe, which has an 8TPI leadscrew, which is close to 3 mm pitch, and the gear is just a straight toothed pinion with teeth 10 mm wide. The teeth appear to be cut a bit deeper than the common spur gear form used for the lathe gears. The whole gear is very roughly finished, so obviously no great precision is required. I can't see on my thread indicator without pulling it apart if it's spindle is tilted to the helix angle of the leadscrew. I suspect it is though, to give a bit more tooth contact with the leadscrew, which I think might be a good idea with a plastic gear.
It will be interesting to see how you go with this - don't forget to keep us informed!
Frank.