Steel/Aluminium design/build, sorta smallish, cuts ali hopefully

[JAZZCNC]

I make my living with welding and have big strong (200ton tested) jigs I could use and know how to hold weld distortion to minimum :)

Well your sorted then and you'll understand why I said what I did thinking your DIY welder with minimal resources.

But still I like your idea for the bed more. The massive amount of steel I designed there was in a hope that I could maybe use 15mm ali bed and stop excessive movements with the frame underneath.

Yep it would have been but personally I'd only buy enough to cover the cutting area has Ali plates expensive and even with ali plate you'll want the edges supported. My steel Bed frame is larger than yours with less supports than you first drew and with 16mm ali bed and I don't have any probs with movement. Unless your throwing engine blocks on it then there's very little forces acting in a downward direction while cutting and you really want to limit plunge cutting to a minimum for spindle and machines sake has it knocks shit out of them.!!

So with the Y you basically suggest a structure shaped like a PFC?

Yep just like a PFC ( Parallel Flange channel for those wondering.!!)