Shoulder Double Joint

A double-revolute joint which serves as a shoulder joint.
Consists of three parts; Alpha, Beta and Charlie.

The 'Alpha' part is currently designed to slot over a piece of wood 29mm wide and 17mm high. It has a socket depth of approx. 15mm.

The 'Charlie' part is currently designed to slot over 20x20 aluminium extrusion.

Of course, if you have CAD software like Fusion 360, you can modify the .STL files of both these parts so that they will fit whatever you like.

The 3DP nut and bolt in this joint have been adapted from

https://www.thingiverse.com/thing:193647

by mike_mattala.

However, they are pretty weak. Especially if only printing in PLA.
Don't over-tighten or the top will snap off the bolt.

It's a good idea to use 25-30% infill at 0.15mm resolution for the main parts of the joint, then use finer 0.1mm resolution and 50% or higher infill for the nut and bolt, to give it a well-defined thread and ensure that the bolt is as strong as possible.

For my o