Heat buildup in the spindle lengthens it. The longer the distance between bearings, the greater the accumulative effect of heat, which then results in a change in preload.
I wouldn't use a plain bushing in a modern machine. Rather, a cylindrical roller bearing used at the outboard end can provide good centering without requiring any lengthwise preload consideration.
ABEC1....do you have to look hard for those?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)