Point taken - I knew I was pushing my luck running it like that, but was eager to get some movement after the machine had been dormant for so long.

I'm running the motors in bipolar parallel and the drivers are PM752's and IIRC the PSU (Zapp PS806-5) produces 68V.

Funnily enough I have ordered another stepper (wanted a spare and could always use it for 4th axis assuming I can get the 3 running first), so I could cut the extension down and run 2 motors - would I need another driver or wire them both in parallel off of the same X axis driver? I also thought running 2 motors was a bit risky if one starts losing steps for some reason?

I could also go the two belt method you suggest - I have a 3rd pulley coming also, which I was going to use for tensioning. The problem I foresee with that is finding the right length belts will likely mean some kind of custom motor mounting plate, which I'm not setup yet to make - if only I had a CNC machine ;)

Good to know that the heat isn't immediately fatal to the drives, thanks for that.

I'll have a think over the weekend and see which option for "sorting" out the drive to the second ballscrew I should go for (read easiest).

8-9 m/min sounds scary enough, let alone 16-17.........