Hi John,
I was thinking that it would amount to an effective 5 tpi, just using a standard thread gauge, it came up with 10 tpi, then there's the splines etc on tother end.
So I would really need to do it 5tpi, then get a 5/8" x 5 tpi LH acme tap, far easier than grinding the correct form, then screwcutting in the lathe, for me anyway..
If I have to end up screwcutting, I've come across a suggestion, that might make things a little simpler regarding the wear on the screw, recut the existing screw to match the worn section.
What I'm saying is, grind a tip to the worn area form shape, then cut the unworn section, which I hope would then end up all the same along the screw.
Doing this would I hope, effectively strengthen the phosbronze nut, due to the wider thread section required in the nut, to compensate for the widening of the gaps in the thread form of the screw..
Any suggestions or pointers to flaws in my thinking would be appreciated.