Discussion over on my gear cutting thread on Western Thunder has had input from several members who are experienced in cutting their own gears and part of the discussion was around worm and wheel gear sets. 

We discussed that it is possible to use a length of say M6 x 1.00 threaded stud to make the worm and use an M6 x 1.00 tap in the lathe chuck to cut a wheel with at least one member having done this in the past. having seen this done in a video sometime ago it was of interest. Then I used a thread gauge to check a Roxey Mouldings Worm and Wheel gear set which came with one of their fold up gearboxes. The metric 1.25 thread gauge fits perfectly but the diameter of the worm is 6.3mm so I think that it’s actually more likely to be 1/4″ x 20TPI rather than M6 x 1.25

We also discussed that using a BA profile of 47.5 degrees would be better than using the standard 60 degree of a metric or US imperial thread. Having checked my BA thread chart I was a little surprised to find that 0BA is 6mm and has a 1.0mm thread pitch. The key difference between Oba and M6 is the angle of the teeth being 47.5 as opposed to 60 degrees. I have lots of 6mm bar recovered from scrap printers and toner cartridges so some trials are in order.