I disagree with the consensus that this is definitely ringing. I think it could be because of the discreet extruder steps. When the extruder makes a step there will be a slight change in extrusion speed/thickness.

Most of the time these small bumps don’t line up and you’ll see, at most, diagonal lines on the walls. Every once in a while, they align vertically because of the coincidence of the geometry and extruder step frequency.

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Graphite.

That’s awesome. I can’t wait to see the results. That’s quite a collection of card readers, also. It will be interesting to see if they all survive, too.