Quote Originally Posted by Jonathan View Post
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
Ok. I am happy with that for getting the rails parallel to one another, but what about the alignment of the first rail. For example, how do most people get the rail level across it's full length making sure there are no high or low spots and ensuring the rail is linear rather than like a piece of spaghetti ?