I have finished building my CoreXY with 370mm cube size.
For control MKS board with smoothieware is used.
There is an issue with one corner as per image. Any ideas? This is with acceleration se to 3000. Only at 1000 it is looking good.
I don't know what kind of filament you used, or how big the print is, but my guess is that you sliced with the seam "aligned" and the seam is at that corner. That is where each layer starts and stops, so the extruder spends a little more time there than the other corners, which tends to heat up and soften previous layers of plastic. If the print is small, those previous layers are still pretty warm. Try printing two of those boxes at once and see if it improves.
Since that's the start/stop of each layer, tweaking the retraction settings may be useful. But I'd check the time/temperature issue above, first.
Thanks on your interest.
It was printed in PLA at my usual 200degrees and you are right seam is in that corner.
I am printing very tall rocket from thingiverse now Nd once completed I'll try again with the cube but this time from Cura3.0 not from Repetier Cura with random Z seam.