Hi Wire - Here's the quick calc on 12mm screws. Seems they will do the job. I based the whip and buckling on 39" long is that about right? Cheers Peter
summary 10mm lead screw 12mm dia (so 16mm will work as well)
using a 3Nm motor at stall produces 173kgf
1000mm buckles at 400kgf so motor can't buckle screw
1000mm whips or whirls at 1867rpm and motor can't go that fast cheers Peter (check the math I could be wrong)
the ultimate speed of the motor comes down to the quality of the drive. If the N23 can get to 1000rpm and the lead is 10mm then it moves at 10m/min which is quite fast for a small machine. You have a large spindle on it and it will move the machine around (inertial forces) so you'll have to tie it down... even my small machines with 500W spindles have to be toned down as they can slide around on my plastic bench when I crank up the accel figures.
I would change the bearing beam and make it as low as possible and make the walls a little higher to compensate. Will be stiffer and will contain dust better.