First Up: The spindle
Since the spindle is the most important part of a Mill, I needed to address it first.
The spindle slop was unacceptable to me and led to a great deal of noise at high speeds. It would also lead to poor finishes if run this way.
The first image shows the spindle as removed from the mill. There was no preload between the angular contact bearings. In fact there was negative preload leading to the play in the spindle shaft. Interestingly, there was a spacer in the spindle body that matched the spacer indicated in the first picture. I assume that the original intent was to separate the lower and upper bearings with the spacer for increased stiffness. After re-assembling the spindle with the bearings separated by the spacers the play was not reduced.
I then started to add shims to the outer spacer to increase the effective preload on the bearings. A value of about 0.002" was found to be the best trade off between stiffness and excessive heat for my spindle.
On bearing grease: The manual for the mill indicates Kluber Isoflex as the grease, but when I added Kluber Isoflex NBU 15 to the bearings (due to some assembly issues) the grease caked up and was not effective. I had to completely degrease the bearings and re-apply the Kluber Isoflex NBU 15. I suspect a grease soap incompatibility between the Kluber product and what was in the bearing.
My next step on the spindle journey is to replace the main angular contact bearings with some super-precision bearings I obtained through e-bay. I have yet to do this as the shimmed re-built spindle has been working well.