@Keith Davis fascinating concept, the OpenXY using cartesian coordinates.

I wanted to specifically congratulate you on something I've been thinking about for some time, and you basically just spelled it out here with this paragraph:

I plan on using 8mm threaded rod for the Z axis. The 4 pulleys mounted on the rods will be 20 tooth. The pulley on the Z axis motor will be 16 tooth. That gives a 5:4 ratio. An 8mm threaded rod moves 1.25mm per revolution. (1.25mm/5) * 4 = 1mm, so one revolution (200 steps) of the motor = 1mm movement. Hence, each motor step will produce a 0.05mm layer movement, guaranteeing that layer height (in 0.05mm increments) will always move by full steps eliminating the microstepping problem for Z axis. I've experienced one caveat to this concept - the #define DISABLE_Z true in Marlin has to be changed to false to eliminate the jump from energizing the Z axis motor between moves.​

I think this is a underestimated statement by many. I'm still new to 3D printer design, so perhaps this is a common/obvious setup. But I haven't seen it spelled out to date.

And if anyone gets this design build, videos! My only slight concern is amount of movement of the idlers on the X gantry when Y moves forward and back. It seems the same as a CoreXY with stationary idlers though. Just an interesting concept, moving those idlers to the movable X assembly.

 

Love having a fixed plate design. Modified my cartesian printer into a corexy that ive been using for over 3ys now. Wanted freedom of access to the extruder so I designed it with an open face. You have some interesting concepts that I'll consider in my next build (about time for a new one).