I'd use thicker panels for the outside walls of the torsion boxes. Mainly for screw holding capability.
I'm hoping your axis arrangement is the same as the one I try to maintain that is X,Y & Z from bottom to top.I would attach a 3/4 600mmx195mm on the front to bolt the profile rail for the Y axis.
For the uprights or supports for the Y axis gantry beam. You might get additional stiffness with a boxed design or go to 3 layers of plywood. I have to ask though why a second machine? Do you have shortcomings on the original you are trying to correct? If so highlighting those issues may make it easier for people to focus on design issues for those problems.Here's the unfinished CAD of my design.
For the sides I was thinking of laminating two 3/4 birch ply together that would give me 35mm sides.
Seriously all machines are flawed, nothing is perfect machine tool wiseI'm planning to use the Makita 1.25HP small router I already have on my current CNC
1 - Is this a valid design or is it flawed?
Given that I see some of your linear bearings appear to be mounted such that they aren't supported well. The X axis being pretty obvious.
The gantry box section looks to be a bit thin too. Frankly I'd look into steel for the gantry beam. By the time you build up a strong wood gantry beam you will be close to the expense of a square steel tube.
It depends upon what you are trying to correct. Physically it is easy to do a three panel laminated ion and not worry about it.2 - Will 35mm sides be strong enough?
Well this is always a challenge. You could integrate the uprights into the gantry beam design but this isn't efficient when it comes to material usage. Another option would be to do an upright that is T like in cross section and use the upper T as a flange to connect to the beam. The reality is there is likely to be a million ways to do this.3 - What would be the best way to attach the sides to the gantry beam?
I don't see a huge problem with your design beyond what is outlined above. Frankly I'm not a fan of wood machines to machine metal as I generally expect to use some sort of lube / coolant. However if you are machining dry with little issues right now don't let me stop you.I'm mostly cutting wood and dome 1/8 alu sheets with my current CNC and want to do the same with this one.
Thanks
In any event you need to outline why you want to build a second machine and what are the problems on the first you want addressed. At first glance I'm concerned about rail placement and gantry size.