Options for installing linear guide rails ?

[Jonathan]

What do most home builders do, do they not use steel ?

Most attach one rail with a couple of bolts (this is the reference) then assemble the gantry without the ballscrews/drive attached. Push the gantry back and fourth by hand several times so that the second rail is automatically pulled parallel to the first. Put one bolt in the second rail... move gantry back and fourth, add another bolt ... repeat. Then just check there's no tight spots at the end. This is of course not the best way to do it, but it works.
For Y and Z you can do the same. Alternatively as the rails are closer you can attach a DTI to one rail, as Mark suggests, and indicate off the reference surface (there's sometimes an arrow to show which - check out the Hiwin linear rail guide). Look at how much it varies and tap it into place.
Before mounting the first rail you could just put one bolt in, then attach DTI to one bearing block and indicate off the frame to get it parallel to the frame.

Edit: This may interest you if you've not already found it:
http://www.cnczone.com/forums/mechan...nterlines.html