Okey so I have done some metal work.
I made one for each side. And they should hold the wheels axis perfectly parallels to each other. At least that is the theory.
I have done some 40x40cm drawings again and this time the pen tip stops exactly where it started!
But it still doesn’t draw 90 degrees angles. The square is not square. It is still a parallelogram. It has gotten a bit better tho. But not enough.
So my guess: adjust physical end stops until square is square.
Even tho my metal brackets put them in parallel. The bearing on the Z axis doesn’t touch the pipes anymore. @vicious1 should I be worried about this? The machine runs true now, but as you see on this image, one of the bearings doesn’t touch the pipe anymore.