If you really need to hold 1.5 micron (plus or minus approximately 0.0000590551") accuracy over 2 meters, I doubt that any ebay/aliexpress ball screws are going to work for you. In fact, that's something that even the most expensive ball screws will have trouble achieving. I don't want to dampen your enthusiasm for building a machine, but starting with more realistic tolerances will make it a lot easier to accomplish without costing you a large fortune. Here's a link to a ball screw manufacturer's resources page: https://dynatect.com/resources/ball-...ign-resources/ Their top grade precision-ground screws (DIN/JIS C1 grade) are specified as accurate to 5 microns over 300 mm. You might call them and ask what it would cost for a screw that would meet your specifications - if it's even possible. Since even a small variation in temperature will throw measurements off by more than that, the ball screws would require internal cooling, and the whole assembly would need to be operated in a climate-controlled environment. Are you sure you really need that much accuracy? Most people here are ecstatic if their machines can hold +/- .0005" overall; if you want more than that you have to be willing to pay for it.