I prepared the blank ready to thread. It took me six different variations of gear sets and swapping about to find one that would physically fit on the 7x12 to turn a 6tpi thread. I almost gave up but the last try worked.
Well there is no way this little lathe is going to do this under power. Sadly not with a hand crank for the headstock either. I tried turning the chuck by hand an the change gears are under so much strain all they want to do is jump out of mesh. I have to physically hold them in place with the leaver and it takes some effort. I quickly came to the conclusion that this was going to bust something if I didn't give up quickly. So I've left it at that.
Feel rather defeated now ....

I sorta worked out that for one inch the headstock needs to turn 6 times but the leadscrew would have to turn 17 times at 1.5mm pitch. That's some gearing up going on there at 1:2.83
But the other way around if I cranked the leadscrew by hand as Andy suggested I would be turning it 17 times per inch to the headstocks 6 turns. That's a nice thought, but will it make it easier on the gear train?
Another alternative is to try the Union lathe. I've not done anything with it as yet and it's currently in bits. I'm waiting to finish building the inside of my garage before setting it up.
The thread chart only goes down to 8tpi but I do have the gears for 6tpi.
The leadscrew is 8tpi, so the gear ratio is quite small 1:1.33
Would that make life easier for the gear train which is a whole lot beefier than the 7x12 to start with.
I'm assuming a lot here, I've never had the Union lathe running, let alone cut a thread with it.
Maybe I should just admit defeat