Hi Psiron,

I've been an Engineer for about 16 years, and into DIY cnc for about 2 years, so you can decide how to take any of my thoughts on this one.

I'm struggling to imagine the Y gantry since alot of it is missing, but it sounds like an open section 'aluminium channel' which is bad for torsion. Closed sections are much stiffer. Any chance of a picture to help explain your idea?

Linear rail for the Z axis will be fine. Tried and trusted solution. Consider mounting this in reverse with the blocks stationary and the rails moving. This allows the lowest block to give maximum support all the time. The conventional way round with the rails fixed means that the lowest block is always further up rail to allow for the plunge. Sorry, this is difficult to describe.

It looks like the only thing stopping the gantry falling off the rails is the belt tension, and it's own weight. Not sure about that one. A high speed router coming off the rails could be nasty. Watch out when plunging! Maybe there is a lower bearing restraint missing from the CAD.

As for the binding, there might be more to that than Russ suggests. In theory there will be a cantilever effect (binding risk), if the product of all the forces leads to a net rotation. During high speed traversing and changes of direction when not cutting, the force applied by the belt is likely to be well below the C of G of the heavy bits on the gantry, therefore there is a net rotation, and a binding risk. This could be overcome with gentle acceleration values.
But during cutting, things are a bit different. The router inertia would have less effect (not moving so fast) and there is now a reaction force at the cutter. This might be quite close in Z to the current belt height. If they are the same height there is no net rotation and no risk of binding. If you move the belt up to the bearings, then you would be some distance from the cutting, and a rotation would occur. I've given this lots of thought as you can tell! This could do with a good picture, but the football has started so I'm off . . .

Agree with Ross about the support on the shafts, they are loaded by the belts and may start to whip.

Also agree that a bit more CAD would help . . .

Thanks
Barry