Second CNC table build, here we go again....

[D.C.]

how to weld the closer plates in - only way i can think is to heavily bevel the plates then insert and fill bevel with weld tying into the tube sides?

If you are trying to weld thin steel and thick steel together you need to run the weld pool down the thick piece of metal and then quickly move across to the thin metal, back to the thick metal, down a bit across just enough, back again to the thick piece, down again etc etc

If you try running a bead equally down the joint, the thin metal will blow out or the thick metal will not get proper penetration.

A CNC machine is never going to generate enough force to cause a properly formed weld to fail, it will probably never get above 1% of the failure point. But if you decide to do nice 'stack of pennies' welds that would make an instructor happy you will be introducing a lot of heat into your frame that will cause stress and distortion over time.

Clamp it, tac it, clean it, back weld an inch at a time and drink lots tea so the heat doesn't build up.

This is an awesome video for demonstrating welding heat distortion, well worth 10 minutes.