Mark 4, finally.

The Mark 3 design is essentially complete, but it lacks rigidity - the long vertical smooth rods flex far too much, which is exacerbated by the relatively short arms.  The closer the arm is to parallel, the more force is exuded onto the tower bars.

There were also issues with the torsional skew of the machine as well as normal XY skew.  Although that was solved to a large extent with the large boards, it made the machine rather cumbersome, by which I mean heavy.

Some of the parts have started to arrive - I'll be posting initial assembly instructions and BOM shortly.  There will be another, more professional set of instructions after I've built the first machine, but I wanted to have something out there for the adventurous.