Rotating Ball nut

[Jonathan]

But what is much better acceleration? I use 1.5m RM2010 and happily achieve 4000mm/sec^3 at 16.5m/min with just Nema23 4Nm(video posted in my Strike thread) I don't run at that because its harsh as *$&£ but if a standard setup can achieve that I have to wonder why bother increasing it further when its already excessive?

If the inertia of the system is reduced, less torque is required to achieve a certain acceleration. To reduce the inertia you can either make the gantry lighter, or reduce the mass/size of the rotating components. Clearly optimising the rotating components is preferable since reducing the mass of the gantry will be detrimental to the overall rigidity of the machine. Since reducing the inertia of the rotating ballnut assembly to be less than the screw is not difficult, it's logical to do that as even if your motors will achieve the required acceleration you gain a greater factor of safety which is always good when it comes with no added cost. So to answer your question directly, it's to get the inertia low enough, or if it already is, then increase the factor of saftey on the torque requirement to make the system more reliable, or reduce the cost of the electrical components.

The moment of inertia of one of your RM2010 ballscrews is just over half the inertia of my RM2510 spinning nut, so if paulus used that design with the same motors and drivers he can expect to get about half the accelleration you get, assuming the mass of the gantries is similar. Similarly, if he uses the design pictured in the first post, then the acceleration would be less than 1m/s^2, which is a bit too low really. Either way if drumsticksplitter has put the numbers in the motor calculation spreadsheet, then he will have found that the 4Nm motors are ideal so long as the inertia of the rotating nut assembly is minimised.

For this size machine, then since RM2510 ballscrews are being used the only realistic option is to rotate the nut, since the critical speed would otherwise limit the feedrate to around 5m/min.