The bottleneck of this project will be the spindle. So a certain realism should be applied when designing the machine.

Why? I agree that you could build a frame any style for a mill. And make it rigid enough.

You have exactly 3 general choices:

-use cheap Chinese water cooled spindle up to 3kw /2 phase/ or even bigger 4-5kw. 18-24k rpm. The problem is that the bearings of the spindle are not made for steel machining though from time to time you could do that. No low revolution power. I would never try to use mine spindle on steel

-Use the BT30 spindle 8000RPM from ali express. Now this spindle screams for column Z. Otherwise how you will mount the motor and the tool change and balance the weight? Not possible. Not so cheap though as only the spindle side will be >2000eur with the motor and changer. Not so fast for High speed machining


-use the design like the german gantry machine example and add bigger than 10kw spindle. That will work best for what you want but the spindle will be very expensive.


So as you see, it is like i said before- you want steel -you make a mill, you want up to aluminum -you make a router. You want both-yes it possible, but prepare some $$$, cause you will need BIG motor to be fast and at the same time to has power on the low rpm.