Unresolved Intermittent Z Ribbing?

The Z artifacts on the round test.stl print looks to be the classic Z Ribbing caused by periodic variation in the Z height of each layer. The pitch of the ribbing also looks like it matches the pitch of the Z lead screw. Most of this Z variation on the stock Robo is due to the helical coupler geometry.

There is a package of fixes to reduce the Z height variation on the Robo.

http://forums.robo3dprinter.com/ind...facts-known-issues-and-fixes.2576/#post-21295

The Z Resonance Test

TBH Make magazine calling this model a "resonance" test does not make a lot of sense to me. IMO the test should be called an "alignment and Z stability/ height variation" test as it will

• expose any X/Y misalignment/variation (as a loss of layer registration or model not square) and
• Z Ribbing (caused by vertical Z height variation) or
  
• Z Wobble (Caused by horizontal variation of the Z Axis)

Your "resonance" test print will have the same periodic pattern of Z height variation as is very obvious on the round test.stl print. The exact same coupler geometry mechanics will still be causing the Z height variation.

I suspect the reason the Z Ribbing is not as clearly seen on the resonance test print is that the walls are very thin and there is very little infill. For example, the test model slices in Cura to a single outside and inside perimeter separated by some infill.

So any variation in Z height will mean individual perimeter layers randomly squeeze outside or inside in very tiny amounts (in other words there will be some random variation in Z layer registration). As this print has little infill and no thick perimeter, the infill/perimeter is not being squeezed in the same way it is being squeezed on the round test.stl model when the Z height varies.

I would suggest that if you look carefully at the resonance test layers under high magnification you will see a pattern of Z height variation matching the periodic pattern on the round test.stl model. Maybe holding a strong light behind the model would show the pattern up as well.

Alternatively you could try printing a solid vertical square bar. ie one with a thick perimeter and infill. It's a good bet that the Z Ribbing will show up just as clearly as it is on the round test print.

Bottom line is if you do the package of fixes for the Z Ribbing, the print quality on both test prints should improve.