3 very different machines you list there, with different approaches required to CNC each. The small travels of the X2 and the fact that going would be slow in steel and Ti would limit your options based on what you want to make with it. If you have space for either of the other 2, I would be leaning towards them.
As for CNC, the smaller X2 with no vertical leadscrew is arguably the hardest one to convert, although kits are available. And it is bar far the most converted machine so there is a lot of information. To answer your questions in my opinion:
1. There are certainly a lot of X2 users running stock ACME screws, mainly as it is the cheapest machine and the trickiest to squeeze the common 16mm ballscrews into. It can yield very good results, if you bear in mind you may have to adjust out the backlash occaisionally as the nuts wear. Probably less people using stock screws on the bigger machines. The price of ballscrews is a smaller fraction of the machine price in their case, and the performance to be gained potentially is even larger, so it makes sense.
2. You can get excellent accuracy with the stock screws and DROs, but that doesn't help when it is running under CNC. Mach can not take information from linear scales for positioning. Then you have to use software compensation, which assumes equal backlash throughout the travel range and can do undesirable things when you want to do 3D contouring work and are using CV mode in Mach. Plus of course the table is not rigidly constrained on the axis if there is backlash, even with compensation turned on, so this will compromise surface finish.
3. I guess point 2 answers this, personally I haven't had too much luck with backlash comp and things got much better with it turned off and with double nuts on the ballscrews to remove it mechanically at source.  |