Nit-pick print quality question - just curious

This is a question more from curiosity than anything else.

I just printed a case for a Sainsmart oscilloscope board. The case came out perfectly, but I noticed that the sides have ripples in them (not this just print, but this one does show it more than others).

At first I thought it might be some artifact from the extruder hot-end. But looking more closely, all of the ripples are lined up from layer to layer. The side shown in the picture was printed with the head moving left to right - layers are horizontal:

https://dl.dropboxusercontent.com/u/...56/Ripple1.JPG

https://dl.dropboxusercontent.com/u/...56/Ripple2.JPG

Now I'm wondering if these ripples are actually there because the stepper motion isn't actually continuous, but a lot of micro-steps. That would explain why they're lined up. Maybe what I'm seeing as a smooth motion from left to right is actually step-microseconds pause-step-microseconds pause etc.

I'm not trying to change anything, just trying to learn about the process and curious.

Is there any play in one of your axis? Maybe something is loose somewhere, so when the axis moves it's not moving 100% accurately

Everything is tight and moves smoothly. It also does this on sides (Y-Axis too). Everything is printing cleanly and accurately. This has to be an artifact of something, just trying to figure out what.

That maybe resonance on the axes, if you are using belts think of them like the strings in a guitar, they'll tend to oscillate at a particular frequency. You could try to print a test piece and change the printing speed, or clip a small weight on the belt at some times and look if the pattern changes. If it doesn't then it's some other issue.

Measure the repetition interval. For a more accurate measurement, measure across say 10 intervals and divide the result by 10. Are you using belts or using threaded rods for the XY axis drive? Is there a correlation between the repetition interval and the belt tooth spacing or threaded rod pitch?

The repetition is 14/cm (calculated as 0.71mm/ridge) My X and Y axises are both belt driven.

If I put my finger on the print head as it moves across the X axis, I can feel it "step" across, even when visually it appears to be moving smoothly and at a decent speed.

This leads me to believe that it is the stepper motion. For example, in order to move 50 mm, maybe it is moving in units of 0.71mm - perhaps Marlin is telling the stepper to move 0.71mm over and over again to achieve the 50mm.

Note that I have my stepper drivers (X in particular) adjusted to that they aren't supplying more current than they need, but yet enough so that there are no missed steps (print accuracy and repeatability are both good). Rods are greased so movement across them isn't impeded.

The more I think about it, the more I think I'm seeing the stepper microsteps. If so, what can I do to smooth things out? Or is this just the nature of the beast?

Thanks.

Quote:

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Originally Posted by iras

...The more I think about it, the more I think I'm seeing the stepper microsteps.

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Unlikely. Assuming you have a 20 tooth pulley on the X and Y motors, the Marlin DEFAULT_AXIS_STEPS_PER_UNIT setting for X and Y should be 80 steps per mm. This means each microstep would give you 1/80 or 0.0125mm linear movement per microstep. Your 0.71mm pattern interval is in the neighborhood of 57 microsteps, again, depending on what you have for a belt pulley on the X and Y motors, and DEFAULT_AXIS_STEPS_PER_UNIT values in Marlin.

Anyone can jump in with different opinions, but I'm not aware of anything in Marlin that would cause it to pause at an interval like this.

BTW - if the issue is appearing on both X and Y, there could be an issue on the extruder that is affecting the ability of the extrusion to flow smoothly, and this could lead to a pattern like what you have. Perhaps trying another layer thickness or something else that would cause the extrusion volume per distance to change would confirm or rule that out. EDIT: Although having an extruder issue that causes the pattern to be consistent in the z-axis would be unusual.

... it might be interesting to know what happens if you change the print speed.... Is the "ripple" frequency invariant wrt tool speed?

Is it auto bed leveling? The ridge might be where it is moving in all 3axis at the same time making it go just a fraction on a mm out of line where the bed is high or low. Had a friend with an i3 see the same thing

Get it figured out?

Quote:

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Originally Posted by iras

The repetition is 14/cm (calculated as 0.71mm/ridge) My X and Y axises are both belt driven.
This leads me to believe that it is the stepper motion. For example, in order to move 50 mm, maybe it is moving in units of 0.71mm - perhaps Marlin is telling the stepper to move 0.71mm over and over again to achieve the 50mm.

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This might not be drive belt related. It might just be the extruder motor doing steps and you are seeing it the print. You could over extrude by a small amount and see what happens. I would expect the frequency of the ripples to increase, but for them to be harder to see because they will 'smear' together if this is the case.

I thought I could just live with this, figuring that it was just what 3D printers do. But the more other printers I see, the more I know that not to be true. And it should be fixable.

Here is more information on the mechanics and geometry of my printer:

X and Y axises are configured identical. Motors are 200 steps/Rev. Driver configured for 16 microsteps/motor step. (3200 microsteps/rev). 18 tooth gear with a 2mm belt pitch. So for Marlin, I have both X and Y set to (200 * 16) / (18 * 2) -> 88.8888

I measured the repeat of the pattern more accurately. It is 83 ridges over 60 mm, so .723 mm/ridge.

I idea that it might be extruder related didn't really occur to me. But now thinking about it, I would have thought that the PLA would be too "wet" at the point of contact to be effected at all by tiny steps of the extruder motor. And with the ridges to precisely lined up from layer to layer, I still keep thinking it has to do with X and Y motion. If it was related to the extruder, then I'd think that there would be no reason why the ridges would line up vertically along straight walls in X and Y.

I don't think it's a resonance problem - although the parameters of the X and Y motion are the same, the mechanics are completely different, and the pattern is the same for both X and Y walls.

Another clue! If the vertical wall is 45 degrees off the X and Y axises, the spacing of the ridges expands accordingly!

Still scratching my head.

I'd like to see the gcode file for a print that does this. If you post it to 3dprintboard, change the filename extension to txt so the system accepts it.

Attached is the G-Code for a small PIR sensor case about 50mm square by about 40mm tall. I've printed a few of these and all of them exhibit the same problem.

The Gcode file is much to big to attach (1.8M), so I split off the beginning (the first few layers have holes and are basically solid, so too much Gcode to send) and the ending very top few layers which are essentially the 4 vertical walls about 50mm long and a couple of mm thick. It might be the second file that is most useful. THanks.

The files are too small for what I wanted to look at, but here's what I was thinking. I've seen cases where a repetitive vertical pattern on the outside is caused by infill. What I wanted to do is upload the gcode file to the online viewer at gcode.ws and scroll up and down through the print layers, observing where the print head should be moving in straight lines and where it is zig zagging around for any infill. Try viewing the full file in gcode.ws yourself. The slider on the right side will move you up and down the print layers. The slider on the bottom can be used to view the nozzle movement for the selected layer. If gcode.ws shows the print head only moving in straight lines for the vertical walls on your print, you at least know the issue is something in your printer, not a slicing artifact. If gcode.ws shows an infill pattern in the walls, gcode.ws might indicate whether the infill pattern is at about the interval you're seeing. If so, try changing the infill percentage and see if the repetition interval on your print changes.

It's been forever since I've worked with slic3r. The settings summary at the end of the 2nd file probably answers the question, but how many perimeters are you printing? You might try printing with another one. Or try printing the external perimeters first. I think you're currently printing them after infill, which could help cause any infill pattern to show through.

Good suggestion ..... but I don't think it's infill related. It prints 3 border along all edges before going inside to do the infill. And given the thickness (thin-ness) of the walls (6 side-by-side extrusions) that there's no infill in that area. I've been watching to see.

It sure seems like it almost has to be the stepping interval, but the math doesn't add up.