Hi Bill,
Thankyou for your very interesting post ! As you say a sneaky idea, certainly a new slant, to me anyway, on making a worm wheel. It gives me more confidence that I can make it work.
Actually the picture raises a question, do I need to mill a groove in the rim of the disc ? Or will the tap simply cut deeper as I advance the cross slide on each pass. I had calculated that I would need to go approximately 3 mm deep to get a 40% of the tap diameter to get the maximum thread coverage.