Originally Posted by dertsap  sounds as though the blocks on your gantry is toed in , which will cause binding and the rails not be able to align properly |
Yes, I would have thought the rails being out of a perfect parallel line would be the cause, but the question is how do people reliably align the rails, if they are not accurately located.
Imagining replacing the rail on a machine in service and spending most of the day tweaking the rails to run free, in a very hit and miss method. You just want to unbolt the old rail, bolt on the new rail, everything to work and go to the next job.