Rotating Ballnut - design ideas

[Neale]

2. "Rotational Mass" refers in this case simply to the mass which requires acceleration and deceleration to control a rotating screw assembly.

It's not obvious until you do the arithmetic but the inertia of a rotating screw like this can be high - you can easily get to the point that the inertia of accelerating the ballscrew exceeds the inertia of the gantry. As the length of screw goes up, the critical speed - the speed at which whip in the screw becomes excessive - goes down. So you need a bigger diameter screw, which increases critical speed again. But the rotational inertia of the screw goes up with something like the 4th power of diameter. In other words, you can increase the screw diameter to increase critical speed and hence gantry speed, but the increase in inertia will have a massive effect on acceleration and acceleration in turn has a big effect on cutting performance if you are doing anything other than straight cuts. This is what is behind Nick's comment - whip, critical speed, machine speed and acceleration, screw diameter - these are all linked and the art of the engineer is to find an acceptable compromise between them. Or use a different solution, which is why bigger machines use rotating ballnuts or rack and pinion.