Oddity milling aluminum

I milled my first aluminum today, and while it went very well, there is one oddity that I"m trying to figure out.

Attached is a picture of my first test piece, and there’s some horizontal striations that always occur at the same Y-Axis locations across all 3 squares.

Any ideas as to what causes this? Maybe at those points there’s more friction or something on the rails?

Could be rough spots on your rails.

Do the belts for that axis seems any tighter or looser than the other axis?

Could also be chatter, looks like a single pass cut (the toughest), if you get into wider cuts when it is only cutter with half the bit that will probably go away. Like it you cut a solid square the first cut is tough all the rest are must smoother, faster.

All 4 belts seem to be pretty similar in tension.

That was 5 or 6 passes at 0.3mm DOC at 5mm/s.

I’m not overly concerned about it… my current aluminum milling needs are all through holes/paths, so I won’t even notice it on the finished product. But, if I can fix it, all the better.

It probably is rough spots on the rails… I might take some high grit sandpaper over them and see if it makes any difference.