High speed spindles on conventional mills?

[routercnc]

In my view a high speed router with high feed rates is good for cutting wood, and if sturdy enough will cut aluminium at a lower feedrate. Steel is not generally suitable although small amounts with smaller cutters (say 4mm) is often possible.

A low speed, high rigidity mill is great for cutting steel, and will also cut aluminium. But the main spindle speed is quite low so the feedrates end up being low for aluminium. If you want to cut aluminium faster on a mill you can add a high speed spindle. But then you can be limited by travel, especially in Y, so the larger format router can be a better option for aluminium.

Perhaps simplifying it: You can 'cut' wood, aluminum and steel on both a router with high speed spindle, and a milling machine with low speed spindle. But the mill will end up being slow for the softer materials, and the router will end up struggling with the harder materials due to lack of rigidity and lack of low speed torque.