Good evening Rich,
You are absolutely right ! I'm well aware of the fact that using a CNC-like rig to perform measurements is more than a weird approach !
My background is "measurement", rather than "machining". I have some feeling for the need to add , as a minimum, all the error contributions in quadrature to have a coarse idea of the total error budget. The "old" system can reproduce its position within a volume of 50 x 50 x 50 micrometers ^3. This number is validated with an optical laser tracker survey instrument. The absolute position (in the 2 m x 3 m x 0.8 m volume) is in the 80-100 microns range, owing to the fact that the laser tracker error contribution is around 25 microns. The whole rig must move. We should blow dust off, and try to up-grade what can be. New encoders, perhaps new servos using a safer "inhibit" functions, and get rid of a "silly" controls SW. We have, in theory, internal resources to re-write all the SW, but since CNC controls technology is around and provides some sort of "standard" to move 4-5 axis around in space, why not use it. This is an attempt to think out of the box, not sure we will implement all these concepts, but at least we should envisage all the possibilities.
Regards, Philippe