Yeah that was where I was going wrong :). The 4Nm motors from cnc4you will give me cutting speeds of around 3000 mm/min and rapids around 10000 mm/min (which is as high as I can go due to the critical speed of the ballscrew), on a 48v supply with a 3x margin. 68V is way above the ideal voltage for these motors (3.0mH inductance) so I think I'll stick with 48V rather than turn my motors into workshop heaters :)

I'll probably get the CWD556 DSP drivers from there too.