Hi Pio- Not sure if your following the Milli thread. I pursued the exo-portal design and got the weight down to 730kg with all axes 100N/um. It could come down more with some more time. The portal design has merit and on very large machines (and I have seen it on small routers) they use a W axis which is a rising gantry design. This means the Z axis height is always small so its very stiff. Here's a large mill with that feature. There are a couple of lifting gantry machines in the forum. I've run out of ideas now so good luck with the project. Peter

https://youtu.be/f8nTNY0FO6o