I plan on converting my Grizzly Mini soon. I want to avoid having to modify the castings, ie, drilling holes for whatever, or threading for too many fasteners. So, I want to use the rack and pinion already existing on the mill and couple directly to the stepper motor. What are some of the pitfalls to this kind of movement?

There's just way too much backlash in the rack and pinion for it to work. Plus, I doubt it is an accurate rack either.
I put a ballscrew down the side of the column and it wasn't much work. Not like if you put it through the head casting. On mine, I drilled and tapped 2 holes in the top of the cast iron column (with a cordless drill) and mounted the ballnut off a bracket which connects to where the gas strut mounts to the head, avoiding drilling more castings. Not that it is hard, drilling with a cordless power drill and hand tapping the threads is actually very easy and worth it.

I'm currently looking at ballscrew retrofit kits for my mini mill... check out kdntool.com and cncfusion.com. If you want to stick with the acme screws there are alot more options.

It just seems that enough people have done the design work for you on the X@ that unless you *want* to design your own system, you should at least buy a set of plans instead of rolling your own.

-Jeff

OK so, skratch that idea. I guess it a threaded rod then. Thanks