This is the upper-arm casing for Sophie's double-jointed arm. It is intended to fit over an internal 'bone' of 20x20mm aluminium extrusion. I designed it with internal flanges to achieve this.

(However, due to what I think is a meshing problem with the CAD model in Fusion 360, one of the internal flanges always fails to print - it only comes out as support structure. Not that this is a major issue, since the other three flanges are enough to hold the casing onto the aluminum extrusion, plus it leaves more space inside the arm if I ever want to run electrical wiring through).

I tend to print with raft enabled so that the bottom of the arm casing doesn't warp.

The length of the aluminium upper arm 'bone' is 178mm.

The shoulder joint is available to print here: https://www.thingiverse.com/thing:4295678

The elbow joint is available to print here: https://www.thingiverse.com/thing:4296037

I hold the elbow joint on by drilling a hole through the end of Elbow DJoint part 'Charlie', then inserting a screw through this so that it bites into the aluminium extrusion. I then cut a small notch into the upper arm casing, just above the back of the elbow joint. The upper arm casing rests upon this screw and won't fall off.