At first I thought the screw couplers were loose but that wasn't it. The machine is a mini-mill with a g540. Reliable up until today. The code being run was CAM generated. Large files with many small moves. The commanded movements were mm to 4 decimal places, well beyond the capabilities or resolution of this machine. My normal programs are dimensioned in inches to 3 decimal places.
So the question:
Could the precision used in the gcode be the cause of the loss of position ? As an accumulative effect of very small moves and a very large file? One axis was out close to 3/16".