Homebrew PCBs

Hi Guys

I am going to start building a small CNC milling machine (X = 7", Y = 
7", Z = 6") for machining PCB's and I am wondering what I should be 
using for the Feed Screws?  I am not sure if a small machine like this 
warrants installing Ball Screws?  What about using a good stainless 
threaded rod and a split nut to adjust the backlash?  I have a Sherline 
Mill with Stepper mounted directly onto the screws and I find the travel 
speeds to be very slow.  Sherline use 18 threads per inch on their Feed 
Screws.
What are you guys using for your feed screws?  When using directly 
mounted Stepper Motors, what would be a good thread pitch for reasonable 
travel speeds?
Thanks for the help.

Bill