Spent the week printing giant frame halves for my desktop CNC build

I built somthing similar last year, a gantry router with printed corner blocks bolted onto 40x80 extrusion, and the print orientation made a bigger difference for me than material choice. My first set of blocks were printed flat on the bed and they flexed along the layer lines in a way the simulation never showed. Reprinting them on thier side with more perimeters instead of infill stiffened the whole frame up noticeably. These days I run four to six perimeters on structural parts and barely bother with infill at all.

On the PLA versus PETG question, I ended up going PLA for the frame pieces after reading that PETG creeps under constant load, which a machine frame has plenty of. My shop can hit the mid 30s in summer though, so if yours gets warmer than that you might want to think it through. I've had no issues in two years of hobby use. One thing I'd suggest before committing to the rails is bolting both arches together loosely, checking diagonal measurements corner to corner, then tightening in a crisscross pattern. I skipped that on my first build and spent a weekend chasing a twist that turned out to be in the frame, not the rails.

I went a similar route with a printed frame for a small laser cutter, big hollow sections bolted over 2020 extrusion, and the thing that bit me later was the bolted joints loosening up over a few months. Plastic under the bolt heads slowly squashes down, especally PLA in a warm garage, and what felt tight in January had visible play by summer. I ended up redoing all the joints with big fender washers on both sides and a habit of snugging everything back down after the first month or two. That fixed it and it has held since. The other thing I noticed with big flat printed parts is that they can come off the bed slightly crowned and you don't see it untill you bolt them down and measure across. Mine looked flat until I laid a straightedge diagonally and found a hump near the middle. Shimming against the extrusion sorted it, but I'd check that before committing to rail mounting holes.

Two days per half on an old bedslinger is dedication. I ended up splitting my parts smaller so a failed print only cost me an evening, though I get the appeal of fewer seams to worry about.

Lurking here a while and this is making me want to finally start my own build. My printer is only 220mm on the bed so a kilo per half is way out of reach for me, I'd have to split things into maybe six chunks and I worry the seams would be the weak point rather than the material. Curious how you plan to mount the rails, are you tapping directly into the printed plastic or running bolts all the way through to the extrusion? I tried tapping into a printed bracket once for a camera slider and the threads stripped out the second I snugged them down.

The bit about rechecking diagonals before tightening everything down matches what happened to me. I built a printed frame for a small mill conversion and skipped it, then the whole thing leaned hard enough that my cuts drifted. When I finally put a dial indicator on it the runout traced back to a twisted base, not the spindle. Now I check both diagonals with a tape measure every time I swap anything heavy on the frame. Also came out cheaper than I expected once I stopped chasing the problem in the wrong place.