To tension,

I'm curious how you have screwed into the end of the sttel box section on your gantry?
I had 6mm plate profiled out and tapped with 4 x M10 holes. These were then trued up to the ends of the box section and for the moment tacked into place. They will be fully welded once I know this layout works...

To Paulus, That indeed is very interesting and also worrying for my 12Nm motors. I was looking at the 150v drives to start with, but I got a guy in china to spec the drives and power supplies to my motors, like he did on a plasma table I recently built and he recommended a 68v 400w system. He also sold the 150v drives btw... However, I do think myself these would work best and would save having to buy power supplies for every drive.

This whole thing with acceleration and critical speed is going way over my head... I was hoping to achieve a 2:1 ish reduction to my 2510 screws to buy a bit of resolution and torque. I've sourced taperlock pulleys, because I was sick to death of damn grub screws coming loose on my last build. Therefore the smallest pulley available is a 34 tooth, which with a 72 tooth pulley gives me 2.12:1 reduction. The problem being that the 72 tooth pulley is 114mm, and going by Jonathon's recommendation to keep the moment of inertia low with small diameter components, I'm faced with another problem...

I've seen this:

Click image for larger version.  Name: techno lc ballscrew.jpg  Views: 5702  Size: 25.0 KB  ID: 8356

Its from a Techno LC router (google search), which is a similar kind of design I was looking at. I know you can't see any detail of the ballnut and bearings, but there is a quite a big ass pulley... This particular machine is servo driven, I wonder if that's why this design works for them?