What is the weak link in machine precision?

I am wanting to retro-fit a knee mill for prototyping aluminum and
some steel tooling dies. Many of the parts are complex surfaces and
require 3 axis simutanious machining. I need to keep the machine
accurcy in the 0.001" to 0.003" range. Quill will be locked and knee
will be moved for z-axis. Extreem speed is not a requirement.

I am assuming that the software will be running closed loop and the
culmitave error will be less than the mechanical error of the machine
elements. Yes / No? Replacing the acme threaded rods with ball
screws (0.0006"/ft) seems to be the first step. Z-axis ways - anyone
experence excessive wear or degration in performance by using the
knee in constant motion? I (again) assume that the ways need to be
chromed to help resist wear, or do they not wear for the first 2000
hrs of use (or some other vage number)?

Anyone with measured performance on their machines?

Thanks.