Thanks Johnathan

My current power supply is 40V so it looks like upgrading those should help out alot with regards to the steppers being able to cope.

I suppose I should of been a bit more specific with my pretty fast comment :) my current machine rapids at 10000mm/min and I am pretty happy with that so if i go for 20mm dia at 10mm pitch on the X and 16mm dia also at 10mm pitch on the X and Z I should be able to achive those speeds.

At first I will be direct driving the screws as it will be alot easier to mount the steppers. Once the machine is built I will be able to make brackets and motor mounts for a timing belt/pullys with the machine.

I did consider 2 screws on the X and I do agree with you, the only reason I went with the single was so I didnt have to buy another driver, stepper and ballscrew, the cost is mounting up as it is, if there is any way to get arround using 2 screws would be a big help. I have seen some pretty good ideas on ways to help prevent racking which I may try out, one that sticks in my mind is a system using pullys and cable similar to those used on sliding patio doors I will see if I can find the link. Failing that I guess I will have to let some more moths out my wallet and buy the extras.