Hello @EdChamberlain ,
Sounds good. My concept is pretty simple and uses 3 pairs of steppers, each pair is in lock-step. By mounting the steppers in gimbals, allowing them to move freely about a fixed point, the rods stay in parallel and act the same as in a standard, linear delta -- only varying the length of the rods instead of the height of each carriage. This also limits the number of degrees of freedom to 3 (x, y, z) as the effector stays horizontal.
The Stewart platform concept would work as well and would have about the same parts count. Its kinematics are already part of LinuxCNC and are relatively well known -- just not part of the available kinematics for our 3D printers. A 6DOF Stewart platform as a 3D printer has been done at least experimentally. https://www.youtube.com/watch?v=qGyiXFGvkqE#t=23
Here's the same concept as a 6 DOF milling machine. https://www.youtube.com/watch?v=G_UmhUjZhNo
My interest is in simplifying the linear delta by getting rid of carriages and belts and by stiffening the overall machine by using an octahedral frame.
Here's some more info on hexapods.
Enjoy!
Todd.