anti backlash for cnc: ball screws, bearing blocks, and
There is at least one highly regarded low end CNC mill being sold today that uses ABEC3 bearings to mount the ballscrews. OTOH, if you want the best, ABEC 7 is the way to go. Note how small the actual differences are between these two grades: 0.00008″ runout vs 0.00003″ runout. Half a ten thousandth!