Whether to use bronze or Delrin is less of an issue for me than to use 1/2-13 all-thread rod or ACME rod for the drive screws.
All-thread rods typically have very rough threads that will excessively wear anything softer than the all-thread rod itself. If you use all-thread rods, run them through a 1/2-13 die to clean up the burrs and then clean up the tips of the threads with fine sandpaper.
Low cost 1/2-10 ACME rods from use-Enco.com are much better and are not so much more expensive than all-thread rods. They will run a little faster for the same rpm.
Diameter of the rods depend on the screw length, rpm, mounting assembly design, and how straight the rods are.
CarveOne
CarveOne
http://www.carveonecncwoodcraft.com