I've built this machine 100 different times in my head!!!

[Jonathan]

Cutting aluminium will be fine regardless of which screw you use since it's only possible to use around 0.6-1.2m/min. With regards to resolution 5mm is better, but it's not a big difference and you're unlikely to notice it. If the critical speed is 500rpm, then yes that means you can get 5m/min, but not 7m/min since that would be exceeding the critical speed. The critical speed formula should be treated as a guidline, since it doesn't take into account the support from the ballnut and relies on a constant you select for the end-fixity, which is a bit of a guess. However I still wouldn't be happy with operating much above what the formula states, even if it looks OK. Have you seen irving's motor calculation spreadsheet on this forum - it may help clarify things.

[JAZZCNC]

Has Jon says the critical speed is a guide and the reality is that you'll possible double that and still get away with it.?. . My 1500mm x 20mm screws do.!

Regards me using 1:2 mostly well that's not exactly correct. Since the last time I changed back to 1:1 it's been left that way and when I do change back it's going to be 1:1.5 because I don't the speed. I don't really need the torque or the resolution either if I'm being honest but that little overhead helps when I do need higher feeds.