New member here. Currently building the popular Kossel Delta 3D printer. One cost savings in the new versions was to eliminate the linear slides and use the 1515 or 2020 Al framework with roller wheels. The MPCNC ingenious feature (IMHO) is its use of cheap electrical conduit (aka EMT) as structure and slide surface available in most big box hardware stores. Using common roller skate bearings and (mostly) 3D printed parts are the other features that have me interested in building one (and signing up for this forum).
You will be hard pressed to come up with a better/cheaper structure as metal EMT. The only thing I can come up with in this regard is the possibility of using PVC tubing. Granting it is flexible in smaller diameters, but bigger sizes are very rigid, but more costly. One of the advantages of PVC if course is the availability of off-the-shelve connectors. I wonder if 3/4″ PVC could be filled in such a way to make it rigid enough for CNC printing. One inside another? Speaking of off-the-shelve connectors, who says you can’t use PVC connectors with EMT? (Assuming you can bolt/screw fasten).
It might have been suggested, but I’d love a Delta printer made from EMT which would push that printer’s price down another roughly $50 worth of 1515 or 2020 Al structure. Use the EMT for the vertical supports only because the main three corners would end up pretty big if the side rails were EMT and not something like Al bar stock.
My only other closing thought would be MPCNC with bigger servos for bigger routers. Granted a small router can cut almost everything a bigger one can, just at a slower rate. And with CNC, you can let the machine be patient while it cuts rather than impatient humans (like me).