Ballnuts

[coredump]

Yes my deflection example is wrong when the total force applied to the nut is below preload threshold, my mistake.
But having preload rated against cutting force (ie force resulting in the cutter itself) is not enough! don't forget that you have also a motion!
There is force needed to achieve proper acceleration, and this add to the cutting forces.
Once you are above cutting forces, you need to have highest rigidity possible: shortest and strongest spring possible.
Having a very very high preload itself is not always the way to go: higher wear, need more driving power due to loss.

[Jonathan]

Yes of course you need to include all the forces, cutting and F=ma. Taking my machine as an example, mass of gantry is about 40kg, a is 1m/s^2 at the moment (it will do 3m/s^2 but that doesn't help) ... so that means you have up to 40N extra from the acceleration.