Why not simply shim the material being sanded, similar to what you would do with a thickness planer, so the top is level, I can't see how it works otherwise with only X and y axis movement.
This is a good question. I'm thinking of building something like this to save time when finish sanding large flat panels. My routine is twelve passes at 100, 150, and 220 grit using just the weight of the sander and the suction from my vacuum to get to a ready-to-finish surface. For big projects, this is a huge time sink. Imagine if I could just set it up and push go, coming back every few minutes only to change the grit or the workpiece, for a fraction of the cost of an expensive floor model belt sander, which seems to be the only commercial alternative.Yeah I'm missing the point. Why go to all the trouble to build a CNC machine just to handle an orbital sander? Seems like an enormous waste of time to me. A big part of finish sanding is "feel", and I doubt you're going to be able to duplicate that in a mashed-together rack that holds a hand-held electric ROS.
I don't mean to be negative, just missing something I guess?