Angle between x and y axis should be 90 degree but it is not. How can I fix this ? I used the things to make x and y moving pipes to make them parallel to non-moving pipes, but there is still small angle offset. My fixed pipes are parallel to each other. I think the middle assy makes this small angle offset. I need to make an adjustment to middle assy but how ?
Take the 4 long bolts out and them put them back while it is still on the rails. Tighten them evenly, not one at a time but each one a a few turns at a time.
That usually solves it.
Take a look at my post “The broken circle”. I’ve resolved my issue…
I found why angle between x and y is not 90 degree or x and y are not perpendicular. My prusa i3 was the problem, because x and y were not perpendicular 
I am very surprised, I see it after 2 years of usage. You can print a rectangular then check with a setsquare