I've compared this idea with
https://www.youtube.com/watch?v=ewU8zWnuE3s
(I think Piotr's machine seems rigid enough to perform the job I had in mind)

The bigest revelation I got was that torison in the granite base should not be an issue in this build. In the moving gantry configuration, whole length of the gantry is subject to torsional forces since gantry is anchored to the base at it's ends with blocks on rails. In traveling column configuration, torsional forces act on shorter length - depending on how the bed (which stock is mounted to) is mounted to the base structure. So instead of building a ractangular closed section, ribbed construction would work better. Distance between ribs should correspond to X axis block span.
Deflection due to the weight of the base and the column can be somewhat controled by the distance between the mount points of the whole structure (eg. lengthwise 1/4 free-supported, 1/2 supported-supported, 1/4 supported-free, actually 310mm+780mm+310mm seems like the most balanced ratio)

In Dan Gelbart's lathe video
https://www.youtube.com/watch?v=sFrVdoOhu1Q
he connected two granite beams using precision spacers he made from steel tube. Beams are pressed against these spacers with bolts.

In his other video about large structures
https://www.youtube.com/watch?v=EeEhS3zmnDg
he uses tube and epoxy to connect two plates together.
I'd like to go this route using steel tubes as ribs, but I'm affraid that thermal expansion of steel would crack the granite.
This page
https://www.engineeringtoolbox.com/t...ge-d_1612.html
shows radial thermal expansion of 4"(~100mm) diameter thin walled stainless steel tube for 90degF (50degC) change to be 0.003" (0.0762mm). How would I check if granite would crack under this stress?