What sort of machine do you have? There may be a kit out there for it already. Also, is this a manual machine, or a CNC machine. The reason I ask the second part is that there are some drawbacks to ballscrews in manual machines. The main problem is called "self feeding" which basically involves the cutter pulling the work along in an un-controlled manner. This can happen because the the friction on ballscrews is really low (the main reason they are used), low enough that the force of cutting when climb milling is enough to overcome it.
I guess I should also ask why you want to replace your stock screws with ballscrews. You say you want higher tolerances, but ballscrews won't necessarily get you that. The main reason to go with ballscrews on a manual machine is to eliminate backlash so that you can climb mill. To eliminate backlash, you not only need a ballscrew, you also need pre-loaded nuts. Even after eliminating backlash, the ballscrew will not necessarily be more accurate than your stock screws unless you get some higher end ground screws (or your stock screws are extremely worn). A better (and likely cheaper) solution to the tolerance issue is a DRO, which will likely be accurate to within a couple tenths, regardless of what shape your screws are in. You can get a DRO for under $500. If you go for ballscrews on X and Y, and get the low end screws with DIY pre-loaded nuts, and do all the machining yourself, you're going to come in around the same price (and it might not get you what you want). I'm assuming here that you have a bridgeport clone, smaller machines are going to cost less of course.
So, think about why you want ballscrews. Depending on the answer, ballscrews may solve your problem. Alternatively, a DRO may be more satisfying.
