Hey Bill,
how do you measure deflection under load? What I mean is ideally the overall deflection at the end of the bit in relation to the workpiece position, which would mean 1. bit deflection, 2. spindle shaft deflection (presumably to neglect), 3. Z slider deflection (presumably of big importance), 4. the Z assembly deflection (due to X carriage linear bearings play), and 5. the X gantry deflection (due to Y carriages linear bearings play) and 6. Y machine’s feet deflection on the table (assuming that the workpiece is attached at an absolute and undeflectable position to the machine bed, otherwise this would be point 7).
Or, which would be easier, at least the overall deflection of the spindle case in relation to the workpiece position (remove the two first points in the above list then).
I am not yet decided on how to attempt to measure this under realistic work conditions. At the moment, I think of measuring at least a small but relevant part of it, i.e. the deflection of Z slider and Z assembly on X carriage to the X gantry (or Y carriages), by attaching a laser to the spindle pointing upwards and an image sensor running on a separate axis parallel to X axis that catches the laser beam and would record the deflection of the spindle.
Another point: Did you take into account bit deflection. Bits get dull and the more they get dull, the more resistance the material offers them, and the more they can bend. In addition, the hotter they become, which contributes to the bending.
Do you use the shortest bit possible?
Did you click on the links mentioned above regarding Z slider?
What about ball screw backlash? You did not mention it.
PS: By the way, did you read the thread Hole Diameter is inconsistent in my Wasteboard?