I'm trying to battle the z-ribbing that I've pretty much always had on my

I’m trying to battle the z-ribbing that I’ve pretty much always had on my Eustathios, V2 and I’ve just had a thought that I would love someone here to explain to me.

Z-ribbing is where your layer height is incompatible with your z-axis full steps, this results in a small cumulative error that occurs as your z axis steps through your layer heights. This error creates thicker and thinner layers in a regular pattern as you progress through Z. This is supposedly easily solved by picking a layer height that matches the full steps of your z-axis.

On the Eustathios with 16:32 gear ratios and 2mm pitch leadscrew this results in layer heights of 0.1920 or 0.2080. I’ve tried these, and 0.2 to no avail, and its definitely ribbing and not wobble since its not shifting the layer but making the layers fatter or thinner as we go through Z. Its also not filament diameter issue since that would change depending on the length of filament extruded in the layer and no matter what size object I print the ribbing stays the same frequency.

So, this got me to thinking. The idea that you should always be on a full step for every layer is sound, the motors are much more accurate on full steps and will hold positions reliably there. But we zero at the top of our axis on Eustathios. This means that when we get to Z=0 we’ve moved the motors to get there and there’s no guarantee we’re actually at a full step when we zero out at all. So could we be introducing this issue in a way that would mean we’re constantly on a microstep and hence not having fully accurate steps per layer no matter what we do?

If this is the case how can we correct it?

You can work without micro-stepping, a bit radical but … you asked for it.

However if the problem is due to micro-stepping and not stopping at full steps, should not change the frequency when you change the layer height?

Sadly I’m on a smoothieboard so I can’t, at least not without soldering on the board and I’ve yet to find any instructions about how to accomplish it anyway.

When printer homes gets in between steps, it can’t hold it ther so the error will be only in first layer. On the next z change it will skip to the next step up or down, depending of the direction so error will not be accumulated. At least this is how I understand this issue. Also as I know z-ribbing is an issue caused by the bend z shaft or leadcrew that creates misalignment in horizontal layers creating waves on the side of the part. Correct me somebody if I am wrong

Thats definitely possible, I’m not sure if the stepper will fall down to the full step or not while engaged though. Could potentially fix this by homing to zero, then disengaging the motors for a while to let it settle to a full step, then re-engaging them. That could be something worth trying.

What you’re describing in terms of z-shaft or leadscrew bending causing misalignment in layers is what I’ve heard referred to as Z-banding, and you can determine this by seeing the layers shift from side to side. What I’m referring to here though is Z-ribbing which you can see on all sides of the part as layers get thicker and thinner.

@Ben_Delarre Smoothieboard also has some headers for each steppers signals, so you might want to try with an external driver to see if that effect goes away without micro-stepping on your motor (I am not convinced though).

One issue I’ve always been meaning to address with Eustathios is that the distance X between the two z axis rods and lead-screws is fixed. So if it is not perfectly aligned and it will put a side load on the ball screw. Ideally those would be allowed to be aligned maybe making one of the extrusions longer so I can mount to the underside.

@Eclsnowman Latest Prusa i3 parts have changed the hex hole for the Z nuts for a channel that holds the nut but allows side play https://github.com/prusa3d/Orignal-Prusa-i3/blob/master/Printed-Parts/stl/x-end-idler.stl

Now that I think about it, what @Eclsnowman mentions would cause a repeatable pattern with no scale change for different layer heights, right?

Hmm, side load on the leadscrew? I’m not sure I understand enough about the acme profile to make a judgement on that. Can anyone confirm that side loading an acme leadscrew will result in a repeating pattern of variation in layer height?

I’m more referring to the stack tolerance of everything bolting together setting up perfect alignment of the rods which is unlikely. Mine is pretty good but depending upon the thickness of the printed parts and how everything bolts together and how accurately the frame extrusions were cut could potentially put the lead screw in a side-loaded condition. So then the lead screw nut will not rest fully down on the threads but will be riding on the side of the threads which might oscillate back and forth as the lead screws rotate.

Hmm. That would explain why the banding is at the leadscrew pitch as well as why it doesn’t change with layer height.

What would be the change in the design to fix this? Separating the leadscrew nut mount from the z rod bearing mount would make it easy to align the top and bottom of the rod and leadscrew along the z axis, but across x there would still be some potential for misalignment.

Full-step-multiple increments between layers are what you care about, not necessarily only doing layers on full steps. The microstep angle error is very consistent – if microstep X is off by Y degrees, then all microsteps X+n*[full_step] will also be off by Y degrees. Angle error doesn’t cause print flaws if it’s always the same angle error for every layer.

That makes sense @Ryan_Carlyle thanks for the explanation. I guess then its most likely the issue Eric has laid out.

Yeah, I don’t think this is microstep angles. See @Whosa_whatsis post here: https://www.evernote.com/shard/s211/sh/701c36c4-ddd5-4669-a482-953d8924c71d/1ef992988295487c

Yeah that post was what started me thinking about this.

I think this might actually qualify as a new entry, its not Z wobble, but its inconsistent Z movement based on the geometry of the screw and the fact that the nut is riding predominantly on one side of the screw.

@Eclsnowman if I widened the Z nut mount holes to allow some horizontal adjustment along X for the nut position on the mount would that alleviate this?

Make the holes a bit more slot like, loosen off the nut mount screws, let it fall back into its natural position and then tighten them up?

I’ll try and model up some ideas over the next couple of days and throw them out for the group to look at

Well for my specific issue it may have just been my idiocy when setting the machine up originally.

When I first set it up I was having lots of problems with z binding on full length travel. So I shimmed the left side z-rod mount and it seemed to fix it. BUT, I just removed the shims, can still get full length travel without binding, and it appears to have fixed the z banding!

Best test cube print ever.

This doesn’t look like what I was referring to when I wrote about Z ribbing. When that occurs, you’ll have several layers that look pretty much the same, then one that is bulging or inset, as if it was over- or under-extruded. It happens due to rounding errors in the number of steps per layer, or because. Depending on the rigidity of your mechanisms, this may take a layer or two to get back to “normal”, but it shouldn’t look this sinusoidal. I suppose if you had really poor microstep positioning accuracy AND your layer height was not an integral number of microsteps (particularly if it was off by one microstep, like 17 or 31 microsteps, or something like that) you might get something like this, but I don’t think microstep position error is quite that sinusoidal either (@Ryan_Carlyle would know that).

This looks like a form of classic Z wobble, but it could be one of those situations where instead of wobbling in circles around the Z axis or side-to-side (in the X/Y plane), it actually wobbles vertically. This is more rare, but I’ve seen it happen. If, for instance, you have a threaded rod with the end cut-off crooked and then shoved into a flexible coupler so that it’s directly up against the motor shaft AND the rod is at a bit of an angle going to the nut, that uneven end will press up against the motor shaft and stretch the coupler more in one position and less 180 degrees from that position. This would result in a sinusoidal patterns of over- and under-compressed layers with a pitch matching the lead of the screw (which is the same as the pitch for single-start screws, but different for multi-start screws like most of the acme/trapezoidal ones people use). There are also some devices that are designed to isolate Z wobble in the X/Y direction that end up translating it into a (usually smaller) wobble in the Z direction.