First time build - Steel Frame CNC Router

[JAZZCNC]

I see a few things I'd look at. (I realise now why it looks strange.? You have drawn box section with square edges and normally they are radiused which makes the tubes look larger.1)

Gantry mounting is weak point. No end support will let it resonate with only lower bolts supporting the upper rectangle. You are using only fraction of the plate to sit it on and with only 4 bolts inline. You would be better extending gantry width to cover the whole bearing plate width and have square 4 bolt pattern.

Has I mentioned in reply to your PM I'd look at ways to protect the ballscrews from debris. The screws are a little high and funerable. I'd be looking to add gantry sides which drop down the sides to allow screws to be lower and drop on the inside of the screw so covers can be added without interference from bearing plates. The gantry side also strengthens the gantry.

Regards welding then you have a lot of tubes all intersecting at the same points which means a lot of heat will be generated there. Idealy you should try to offset any tubes so the don't all meet at the same points. This will reduce warping and stress.
You also have tubes you don't need the Centre vertical ones for instance. The diagonal ones are enough and it's less welding, less heat and distortion. (see pic)

Mig welder is far better than Arc when comes to heat so if you have access to one I'd use it.

I'd also extend the Top rail 100mm past the end this will allow spindle to pass end giving more flexabilty to machine.

Regards PMDX then you want 126 BOB not 122 it's far better. If you go with Belts then buy Cslabs IP-M.