Consistent layer offsetting in Y-axis on some model features

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Consistent layer offsetting in Y-axis on some model features

Postby Jivix » 2018-Jul-Tue-19-Jul

Had to make a post about this since I can't find any issues similar to this not just on the forums, but the internet at large.

I've noticed this issue to a lesser degree on many prints over the past few months, basically the layers are not consistently aligned along the y-axis on TWO different Printrbots, one a Simple Metal and one a Simple Maker, both using the Rev D printrboard. Previously, I had chalked up this issue to the y-axis slop on the Printrbot and that there's nothing I can do about it, but this particular print has made me change my mind about that because it only occurred on 2 of the four legs on a printed table.

Things I have tried so far:
-Increasing/decreasing Y-axis belt tension.
-Manually applying pressure to the y-axis to remove any slop.
-Changing slic3r settings, including layer/perimeter settings, support material, and print speeds.
-Using different brand of filament.
-Connecting directly to printer or using Repetier-server
-Tried different High resolution and low resolution models alike, also .stl and .amf
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Consistent layer offsetting in Y-axis on some model features

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Re: Consistent layer offsetting in Y-axis on some model feat

Postby RetireeJay » 2018-Jul-Wed-16-Jul

So do I understand this correctly? When Y is at the middle, the pillars print nicely, but when Y is significantly toward the front or back, they look bad? If that's the case, I'd be looking at the mechanical support of the table on the Y axis. On Printrbots, the bed is cantilevered when it's sigificantly toward the front or back, but balanced nicely when it's in the middle. If the bed is literally developing a slope toward the front or a slope toward the back when it's away from center, that might explain at least part of your observation.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
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Re: Consistent layer offsetting in Y-axis on some model feat

Postby Jivix » 2018-Jul-Thu-12-Jul

It's the opposite. The orientation of the model on the 3rd image is the print orientation. The legs in the middle of the print bed (in relation to the Y-axis) are the ones with the issue.

It happens to a similar degree both on large prints and small prints, indicating this is not a mechanical issue because it would scale with the y-dimension of the model and/or print speed, which it has not been shown to do. This happens even with extremely low print speed, the printer is deliberately offsetting the layer by up to 1mm.

Also I have leveled the bed in relation to the hotend to within a 0.2mm tolerance across the board. This print bed has no sag in the middle, unfortunately that probably isn't the issue.

This issue has been a real head scratcher for me, at the moment I'm suspicious it's a Gcode generation error on Slic3r's part. Will try printing with Cura and see if the problem goes away.
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Last edited by Jivix on 2018-Jul-Thu-13-Jul, edited 1 time in total.
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Re: Consistent layer offsetting in Y-axis on some model feat

Postby RetireeJay » 2018-Jul-Thu-13-Jul

Circular objects are a little difficult to analyze in G-code. But if you designed the same thing with rectangular legs, then the G-code becomes a lot simpler, and you can fairly easily look for repetition of the same X and Y coordinates on layer after layer.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
User avatar
RetireeJay
My next printer is...
 
Posts: 4935
Joined: 2013-Jan-Wed-13-Jan
Location: Greenville, SC
Reputation: 495

Re: Consistent layer offsetting in Y-axis on some model feat

Postby Jivix » 2018-Jul-Thu-14-Jul

It looks like Cura is producing the exact same poor results. This leads me to believe there is a problem with the .stl file being used. I made a similar model and exported it as a .stl file and will try printing that next. If it still produces errors, I'll change the legs to squares and see if I can reproduce the same issues.

Update: It's definitely something wrong with either the .stl file or the way the geometry is being interpreted by the slicing program. The .stl was generated in OpenScad by a friend. If I change the scale of the model I can change the travel behavior. What could be causing this. Rounding error?

Oh, and the same travel errors are also there if infill is turned off.
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Re: Consistent layer offsetting in Y-axis on some model feat

Postby Jivix » 2018-Jul-Thu-15-Jul

Well, my version of the model printed without issue. However, when we generated a model with square legs we ended up with the same issue as with the rounded legs. I'd really like to get to the bottom of this, as it crops up here and there with Solidworks-generated models as well, despite there being no visible issues with the .stl or .amf file in Meshmixer or in Slic3r before slicing.

Models with extremely high resolution seem to be less likely to produce these issues than low resolution. Doesn't Slic3r and Cura try to guesstimate curved/circular surfaces from triangular geometry during the slicing operation, could that be the source of our problems?
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Re: Consistent layer offsetting in Y-axis on some model feat

Postby RetireeJay » 2018-Jul-Thu-20-Jul

It's just hard for me to believe that the slicer does this. Can you post your STL and your G-code for me to take a look? (Use "upload attachment" just like for a picture, but don't "insert inline"). If the site gives you grief about the file types, you could try zipping the files.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
User avatar
RetireeJay
My next printer is...
 
Posts: 4935
Joined: 2013-Jan-Wed-13-Jan
Location: Greenville, SC
Reputation: 495

Re: Consistent layer offsetting in Y-axis on some model feat

Postby Jivix » 2018-Jul-Tue-13-Jul

Update on the issue. My buddy and I are currently suspicious that there is a partial slip on the Y-axis drive pulley under certain conditions, and the STL files which have uneven travel moves layer-to-layer are simply revealing the issue that is present. In other words, high acceleration Y-axis jerks might be making the toothed gear slip 0.25-0.5mm, but it's only a problem when printing one layer in one direction and one in the other. This could explain why only the middle legs are exhibiting the offset issue, because the legs on the furthest extent of the Y-axis are traveled to in the same way regardless of slicing.

However I also have been doing maintenance on my print server so I have not had time to test out this theory. I will let you know as soon as I can.
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