Anyone got a clue as to why I would suddenly develop Z ribbing that

So I got the M6 screws in and same problem, exact same periodicity so it is definitely not mechanical. I was watching the temps as @Mike_Downey suggested and it seems like the shrunken layer begins when the temp drops, then as it heats up it overshoots by 10 degrees and gets the fatter layers. Then as it drops it is undershooting by 10 degrees and ends up with an over cooled layer which shrinks in more. Now that I think about it I just replaced the thermister because my old one was reading 30 degrees higher than actual which meant I was printing ABS at 200 to 210. I may have messed with the temp residency times which could be causing the big swings. Gonna change the settings back to stock and see if it makes a difference.

These are the thermisters that I bought, they are accurate within about 3 degrees. http://www.amazon.com/gp/product/B00GD471PO/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1.

a bad thermistor can create a shift in temperature, but they shouldn’t cause the oscillations you are seeing.

I would start by retuning your PID settings. Its not hard and only takes a few minutes.

http://reprap.org/wiki/PID_Tuning

Bad thermal coupling of the thermistor to the heater can cause temperature swings. Correct PID tuning should be able to get rid of that, but autotune won’t work if the problem is bad enough.

I have never used the PID before. I ran that and updated the fw which got rid of the temp swings but the bands looked the same. So as @Whosa_whatsis said about the bed expanding and contracting I set the bed to a temp it couldn’t reach meaning it would stay on and not cycle. That almost made the problem go away but its still there just much smoother. Gonna run another test where I shut the bed off completely after the first layer (don’t have PLA) later tonight.

Try changing the Z axis steps from"2560" to “2560.0”, otherwise Marlin handles that axis as integer instead of float.

Tried changing the Z axis steps to 3200.0, and sliced it in Cura. No difference. Printed with the bed turned off the entire print and again, super smooth but still with the pattern. Its so smooth though that I can now see exactly where it stops and starts and the pattern repeats every 3mm exactly.

Oh and the last print with the 3mm bands was done at .2mm layer height.

I have PID enabled for the bed and hotend now and that has really smoothed out the prints along with a thicker glass plate to dampen any flexing. I just printed the Y-corners for the i3 I’m building and the pattern is still there but spread out way more. Rather than a periodicity of 2 or 3mm, its more like 6 or 8mm repeated. Next I shimmed up the hobbed bolt bearings because they had just a tad bit of play and I’m printing out the x carriage right now. The problem is still there but I’m starting to think it’s extruder related.

I’m using 0023