This is part of my Baby-Huey large format 3D printer project.
A fixing system similar to a rawlplug for plugging into the end of a square tube. It is a filleted square plug with a tapered hexagonal hole to hold a nut. The design file is parametric so nutsize, tubesize and taper can all be changed with a simple parameter edit.
It is my first freeCAD project so maybe not the most elegant design but it is a start.
This isn't going to be super strong but should work OK for something like a 3D printer frame.
expansion-plug-a.stl is sized for a 25mm square tube with 3mm wall thickness. The nut is 13.5mm face to face and the taper is 5 degrees.
I intend to resize for metric but didn't have any suitable nut and bolts on hand.
I think ABS would work best but my test print was in PLA and it seemed to work perfectly. I had to give it a little rub with a file to get it to go in. I The round hole was a but tight too and I drill it out.
The expansion seemed to work but I haven't used it for anything serious.