I am working on my Z-axis, I am using IKO linear rail and bearings. My Z-axis is made out of 3/4" x 6" aluminum. The rails are mounted on 3/4" x 1 1/2" bar that is bolted to the 3/4" x 6" plate. I have my rails parallel (within .001). My problem is when I start to tighten down the plate (made out of 1/2" x 4" aluminum) that ties the two trucks together, the trucks start to bind when rolled down the rail. You can tighten a bolt here or there and eliminate some of the binding, but not all of it. I noticed that on the underside of one truck someone (previous owner) etched .010. Does this mean that they had to shim that side to allow for smooth movement? I am about 99% sure that I have my trucks parallel with each other, I used a square to set them up. I didn't think I would need to shim, but at this moment I am out of other ideas. Any suggestions?