Rotating Ball nut

[Jonathan]

I have a bit of a problem doing that because my ballscrews are under the machine

Move them - you don't really gain anything by keeping them there and you have a lot to gain by adding supports to the frame.

, I did have a 4nm nema 23 but I thought it would be way underpowered for the 2510 ballscrew...

If spinning the screw, then 4Nm is definitely insufficient.

I first liked it because its very compact and maximises the travel of the ballscrew, which is important if I want to be able to to cut 8' x 4'

Your screws are 2800mm, 8' is 2438mm, so surely there's plenty to spare? My design occupies 173mm of the ballscrew.

I have 2 x 12Nm nema 34's for the X (1 for each ballscrew), do you think I would get away with the rotating ballnut I found with my motors?. I know with them being bigger motors that the moment of inertia is greater than say a 4Nm, but I plan to drive them with 68V 8A drives.

If you stick with those motors then the mains voltage drivers would be much better - 68V would be the bare minimum. Try putting the numbers in irving's spreadsheet to see which is best. You'll need to enter the inertia of that assembly, which will be about the same as a 3000mm long RM2510 ballscrew, so put that in and you'll get reasonable results. The inertia of my design for RM2510 is equivalent to 1250mm of RM2510 ballscrew, hence much better acceleration is obtained than if the screw was rotated. Thinking about it, you could still use one smaller bearing with the design you found, if the bearing on the locknut end is moved off the end of the nut. Not sure how much difference it would make without drawing it.

Also it depends on what you're cutting - how fast do you really need it to go? If cutting wood most of the time, which seems likely given the dimensions, then it's important to have decent federates. The same is not true if cutting aluminium.