Measure the heel and toe distances across your machine, make sure they are equal. If not adjust the end clamps.
1.) What distance exactly should I measure “From heel to toe”? The distance between the rails and the table on the complete length and check if they are equal?
2.) And what kind of end clamps is the tutorial talking about? The plastic clamps that are holding the 2 shorter rails together?
I believe it refers to the distance across X between the front of your YZs and the rear to ensure they are parallel. If they are \ / or / \ when looking from above you might need to adjust the end clamps that connect the YZ to the X rail
Just measured mine and they are off by about 3/16" But only the fronts clamp to the tubes. So how do you change the heel/toe angles to bring them parallel?