I know this question has been asked before, but no other thread that I looked at answered my question.
Recently i started getting gaps in my top and bottom layers, I can't say exactly what I changed when this started happening. I thought it was under-extrusion, even though the walls were fine, so I re calibrated my extruder to ever so slightly over-extrude, and no effect. I decided to count the lines on the bottom of the cubes.
I calculated that: It's 20x20, so the hypotenuse is ~28.29 (the top/bottom pattern is diagonal)
The cube (yellow) cube has 2 perimeters, so 28.29-2*2*0.4=26.68
26.68/0.4, because the lines should be 0.4mm amounts to almost 67 lines on the top and bottom.
I took a strong magnifying glass, and tried to count the lines with precision tweezers. I counted 32, so there are probably 33, which would be exactly half of what the should be.
The blue cube is sliced in cura 2.5 beta, and the yellow in 2.5.
The setting are the same in both slicers.
Both cubes are in pet g.