Hansi, once I squared up the frame as well as I could with a tape measure, and before bolting it down, I ran the Z axis back and forth at a slow speed from end to end several times, watching for any movement in the rails. Then I ran the Z almost all the way to the back, turned off power, and manually turned the Y ball screws until each side was up against the end blocks. Then I powered up and ran the Z to the front to be sure they touched the front end blocks. Then I repeated the slow run back and forth looking for any movement in the frame. Then I screwed the support blocks down, being careful not to move them while running the screws in.
I have, like you, wondered if there was enough error from start/stops, rounding from direction changes, etc. to cause any significant , or even noticeable, change in the alignment of the axes. I repeat the back and forth test every once in a while, and have not found any errors. I make qute a few aluminum parts with bearing pockets, alignment holes, etc. and all seems well. True the parts are relatively small so small errors might not be showing up, but I mic the parts and have been pleased with the results.
On an sort-of-unrelated topic, while testing the LongMill’s ability to do aluminum, I was reminded that you can’t always believe the specs from the manufacturer of the end mill you’re using. I use the Sienci 1-flute 1/8" mills for aluminum and they work great. Because of their shape, it is very hard to accurately measure the diameter of the cutting surface. I tried it at 0.125" and the diameter of the holes came out 0.012" too small, so I dropped the mill diameter to 0.113". Then they were 0.008" too big. I finally settled on 0.118 for the end mill and it’s right on. The reason I bring this up is that now that the cutter D is firm, I am getting very repeatable results, which leads me to believe the LongMill is behaving correctly. I’m almost done with the assembly of a drag knife, all the parts of which were made on the LongMill from 3/8" aluminum plate except the shaft that I turned on the lathe. I’ll get assembled pics up in a while.
Good talking to you. Stay safe.