Steel Framed Router

[Jonathan]

Critical speeds with BK/BF bearings (approximate):

16mm dia, 10mm pitch, 1600mm long - 800rpm = 8m/min
16mm dia, 10mm pitch, 1500mm long - 910rpm = 9.1m/min
20mm dia, 5mm pitch, 1600mm long - 1000rpm = 5m/min
20mm dia, 5mm pitch, 1500mm long - 1150rpm = 5.75m/min
25mm dia, 10mm pitch, 1600mm long - 1220rpm = 12.2m/min
25mm dia, 10mm pitch, 1500mm long - 1400rpm = 14m/min

So given that in my opinion 20mm diameter 5mm pitch screws are the worst option regardless of the gearing since the critical speed is quite low. Also the moment of inertia of a 20mm screw is 2.44 times greater (since it's proportional to the radius to 4th power) than the 16mm screw. That means the motor has to impart that much more kinetic energy into the screw to accelerate it to a given feedrate. The pitch of the 20mm screw is half that of the 16mm screw it clearly has to spin twice as fast. As the motor torque is proportional to the angular velocity of the screw and the inertia you loose big time since the torque required is now a factor of (10/5)*2.44=4.88 greater between the two options!! This doesn't take into account cutting forces, which would make the figure look not quite as bad ... but still it's a massive difference.

Clearly I can use a similar argument as the above to say why the 25mm screws are too big for those motors...unless you rotate the ballnut. Rotating the ballnut is a win-win situation since screw whipping is no longer an issue and the moment of inertia of my rotating ballnut assembly on the RM2510 screw is about a third of the inertia of my 2m long screw. With that I get 15m/min (well over the critical speed) confortably and I'm limited by the 3nm motors, not the screw. For your size machine I reckon 16mm diameter with rotating ballnuts might be a good option...

If you can find 20mm diameter, 10mm pitch screws then that's potentially good option (though still the inertia is greater as above), but I bet it's going to cost you more than a rotating ballnut setup on the 16mm screw.

This does of course all depend on how fast you want to go. If you're not trying to get the land speed record then the RM1610 screws would be fine...