I built a MPCNC with a long axis with 60" 1/8 wall aluminum tubes. All bolted down, it’s still pretty strong, but does benefit from midspan supports. I came up with this interesting design.
Inside the tube slide these inserts which conform to the inner diameter, and inside of it is inserted a rectangle of sheet steel. The idea is to distribute the force of the threaded rod across the surface of the PLA.
This version does nothing to counteract a lifting force. I intended to make a second version that fits snugly inside the tube with a nut and a similar captive clamp for the table.
It is essentially a snug fit cylinder that is open on one side with a nut sandwiched between two steel plates. When the threaded rod is screwed into the nut, the rod will press on the top plate while the nut presses on the bottom plate - expanding the plug inside the rail.
The base has a nut with two washers to hold it inside the PLA. Height is adjusted up and down by turning the base. This is still printing and I can update with pictures.
The way I did mine was to use nylocks under the tube. I wasn’t worried about lifting forces either. Just snugged the nuts up against the tubes so they wouldn’t sag. Threaded rod still sticks up into the tube.
I don’t know if this is a particularly good solution, but this is what I designed back when I had a Burly. It used four nuts and two washers. The top and bottom nuts were in nut traps and glued with a drop of CA glue. The hole in the side allowed a 1/4" dowel to be affixed to position the inner piece inside the tubing.
we didn’t do our job 