I don't see any problem using a nylon rack. I wouldn't worry about accuracy either. There's no cutting force anyway. Just use something with a larger tooth so you don't risk shearing off the teeth or grinding them up when the machine does a hard acceleration/deceleration in corners and whatnot. Maybe the money you can save by not using metal can use re-directed to using a helical gear rack instead of a spur. That way you can have smoother motion with larger teeth.
As far as I can tell, the most abuse that the racks in MY cnc plasma takes is from the horrible, evil, gritty dust that plasmas are so good at making. It's about two years old, and the racks look the same as when they were new.
I will do a little preaching now - hopefully you already know this, BUT////
When you're building this thing, remember, (unlike PracticalCNC) that the little quarter inch shaft (hopefully larger!) that the spur gear is mounted to IS IS IS IS the part of the machine that has all the force of moving the gantry applied to it. Keep your bearings close to the gear, and if you can, support it from both sides. Just my two cents. Shield the hell out of your wires, especially if you're using servos, and do everything you can to keep the plasma dust out of delicate areas, like your bearings and encoders. Spending the extra few bucks on sealed components is well worth it. And again, same rant as every other guy in here gives, USE GOOD LIMIT SWITCHES! They'll protect you and your investment. At least use a micro switch with a roller on it, like from All Electronics.  |