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Is this backlash or something else?


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Posted · Is this backlash or something else?

Okkk, here we go:


I have had an issue cropping up where my UM has had trouble printing certain layers with the lines touching, especially printing solid layers over infill. I suspect this might have something to do with backlash, as I recently slightly loosened one of my short belts to stop a loud snapping sound that was happening as the belt kept riding up on the edge of the stepper motor junction. Here is what some of my prints are looking like:




I plan to attempt to re-tighten both of my short belts, and see what happens, as I don't get much of a note when plucking either of them. I also tested out plucking the long belts and they seem very loose to me (at least compared to the pitch from plucking in the belt-tensioning video I've seen around these forums).

Now, my questions:

1. Do you think what you see in those pictures is likely caused backlash? The first layer always goes down fine, and it is only higher layers (and tops) which seem to show what gaps between printed diagonal lines, which to me also looks like it could be underextrusion.

2. Is there a belt tensioning device or technique people would particularly recommend? The belt tensioning video seems to indicate I should only adjust the tension of the long belts by spreading the nuts on the pulleys. Obviously, I think the best way to tension the short belts is to firmly push the motor down and then tighten the bolts holding it there.

3. I've seen people mention Steps-per-E when others have had similar-looking issues. I confess I do not understand that setting in Cura, or if I should be doing anything with those settings. Any input in this area would be great.

Thanks again in advance.


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    Posted · Is this backlash or something else?

    It looks like you're definitely under-extruding. The question is whether this is due to settings like steps-per-e or print speed, that affect the amount of plastic being squeezed out, or whether it's simply caused by backlash putting your infill lines in the wrong place.

    You say that the first layer looks good - but that might be due to it being extra-squashed, if your bed height is slightly off, so that the first layer spreads out a bit to fill in the gaps. How does the second layer look? Perhaps you could try printing a largish circle, and show us what the first and second layers look like? A good clue for backlash is that the infill diagonal lines don't meet up with the perimeter lines.

    You mentioned a twisting problem with you short belts... that might be due to the belts catching on the frame, and or the pulleys not being in line properly. I'd definitely check that out, and try tightening the belts a bit, and see how that affects print quality.

    For long belt tensioners, you can start with something as simple as:


    For the short belts, I find this (or derivatives such as the one with the spring) to be good (although the push down and tighten approach is a good start):


    Steps-per-e tells the printer how far to advance the filament in order to feed 1mm of filament into the printer. You tell the slicer the diameter of your filament, so it knows the cross-sectional area, and hence the volume of plastic for every 1mm that gets fed in.

    The gcode that Cura generates tells the printer how far to move the head, and how many mm of plastic to feed in for that move (in order to get the correct volume of plastic fed in, and hence squirted out of the nozzle). The printer executes that by turning the motor the right number of steps, based on the steps-per-e setting. If your steps-per-e is wrong, then the printer may feed more or less plastic than it should into the Bowden, and so force more or less plastic out of the nozzle. You can fine tune the setting, but usually other things are more important. For a standard Ultimaker, the value is normally around 830 (which is set as a default in the firmware). If you specify a non-zero value in Cura's preferences, then it will add gcode to tell the printer to use that value instead.

    Another thing that can affect extrusion amount is the speed and temperature that you are printing at. What settings are you using, and what layer height?

    Finally, what setting are you using for filament diameter. Is it right for the filament you are using?


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    Posted · Is this backlash or something else?

    This print looks pretty good. Those gaps could be caused by not having the "shell" thickness an even multiple of 0.4mm. Or it could be backlash. Or something else.

    But it *does* look like play (aka backlash). Having two diagonal lines touching in infill followed by a gap is also another symptom of play and I saw that in your other photo.


    adjust the tension of the long belts by spreading the nuts


    Yes. That was enough for me. Takes 5 minutes what are you waiting for? That's all I had to do. I think I only tightened 2 of my 4 long belts. You should definitely hear at least a low pitch when plucking the long belts and when the head is pushed to a corner. You'll have to loosen one of the pulleys so the tension is spread evenly on the upper half of the belt and the lower half of the belt.


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    Posted · Is this backlash or something else?

    In response to illuminarti's questions, here are my Cura settings, currently:

    Settings 1

    Settings 2


    As you can see, I'm not printing very fast (~50 mm/s), and am using a layer height of 0.2mm, with the shell thickness a multiple of my 0.4mm nozzle. My filament is 3 mm filament from Ultimachine, and it is generally around 2.8mm in thickness, give or take 0.1mm, I have the Cura setting set to the default 2.85 diameter setting.


    I first tightened the short belt by pushing firmly down on the stepper motor, and retightening the screws. Immediately the clicking noise that I first detailed here: http://umforum.ultimaker.com/index.php?/topic/2560-can-anyone-explaindiagnose-this-loud-crackingclicking-sound/ , returned. It is fairly clear to me that the short belt is riding up on the frame of the UM, and snapping back down, but I don't understand how to prevent that. It just started one day after I had been printing for months, and even after I re-centered the belt on the rod, it drifted back over almost immediately and began clicking once again. Strange, but not the main problem here.


    I printed a large circle. Here are the pictures of the first three layers:


    First Layer

    Second Layer

    Third Layer


    I then had it print some infill, and here are the first three layers printed on top of the infill:


    First layer over infill

    Second Layer over Infill

    Third layer over infill

    You can see that the layers on top of the infill are much more sparse than the first three seem to be, although you can see some clear gaps (after the first layer) between lines in the first group of layers nonetheless. Additionally, the infill diagonal lines do seem to meet up with the perimeter lines. The top of the circle does seem quite sparse, however.

    In response to gr5, I did not tension the long belts with the screws, but only because I am not 100% familiar with the system, as I did not assemble the machine. It will be, in fact, the only part of the machine I have not had to take apart and reassemble. I was unsure about one comment: what is meant by "loosen one of the pulleys so the tension is spread evenly on the upper half of hte belt and the lower half of the belt." As you know, on each of the sliding blocks, it looks like there are 4 bolt/nut combos, and so I'm guessing I should just loosen each an equal amount until I get the desired pitch from my long belts? I also printed the long belt tensioners, and will put them on if I feel as if I still need more tension out of the belts.


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    Posted · Is this backlash or something else?

    One other quick note: the top of the circle does look quite streaky. It seems as if it would do about 6 or 7 lines touching properly, and then a 5 or so that were much more sparse. Very strange.


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    Posted · Is this backlash or something else?

    To be honest, now I see the evolution, your prints look great :-)

    Printing over the top of infill is always going to be patchy, because the filament doesn't have anything to support it for most of the travel, so the filament bead catches on the lines of infill, but then kinda gets stretched out in between. Actually your first layer over infill is going down really well - the threads don't look to be breaking at all, just getting a bit thin.

    The second and third layers gradually get better, but you're still printing on a rather shaky under layer, so they aren't as good as when you're printing flat on the bed. You just need more layers to get a really solid top layer. Just make the top/bottom thickness be at least 4 or 5 times the layer height, maybe more. Then you'll get solid surfaces. Everything else looks really good about your prints.


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    Posted · Is this backlash or something else?

    Regarding spreading the tension... the long belts basically are divided into three parts:

    1) from where the belt is gripped by the sliding block, leftwards, towards a pulley.

    2) from where the belt is gripped by the sliding block, rightwards, towards a pulley.

    3) the long part between the two pulleys.

    When the pulleys are locked in place, adjusting the tension in one section doesn't affect the other sections as much, because the reduction in effective belt length only happens in that one section, since the belt cannot redistribute itself around the pulleys. So you need to loosen the grub screws in the pulleys, so that they can rotate independently of one another and the shafts in order to allow the tension to equalize all the way round the belt. Then you need to do the screws back up, really tightly, after making sure that the upper and lower parts of the belt are directly one above the other, and that the belt doesn't bend sideways as the head moves close to each pulley.


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    Posted · Is this backlash or something else?

    I agree that your parts look better and better all the time and adding another layer or two will make it look even better. Your backlash is low enough that it's probably not worth tightening any more at this point. Or you might get a tiny bit of improvement.

    You *might* still have some underextrusion. I'd like to see a photo of your feeder on the back of the UM and also could you please measure your spring accurately? illuminatri's is 11mm and mine is 11.5mm (when closed).

    Your bed was a little high on the first layer so you got plenty of extra plastic to fill in any gaps. Then your second layer wasn't as overextruding because there was a little more room, by the time the 3rd layer is started, things are normal/nominal so that when the 3rd layer starts printing over the 2nd layer there is now exactly .2mm gap to print in as it should be. That might explain why the first 2 layers look so good even if you *are* underextruding slightly.

    I could be wrong though - the remaining layers actually look pretty good. If you had more than 10% underextrusion your part would look worse. Still it might help to either increase flow by 5% (or lie about filament diameter by 1%) or to tighten the spring on your feeder.

    Personally if I were to print a simple cylinder like this with no opportunity for stringing I speed up to 100mm/sec, .2mm layers 240C. The 240C would let that plastic flow like honey!

    But when printing "real" things I can't always do that due to stringing when the head hops over "gaps" in a layer so I print cooler and slower.

    YOU WILL HAVE MORE PROBLEMS. There are other things I don't think you've run into so keep posting your issues. For example "my holes are too small", "the part warps and lifts at the corners", "my nozzle is clogged" and much more. Everything has at least 2 solutions so don't panic.


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    Posted · Is this backlash or something else?

    Hah, well that is good to know that they are looking better! I agree, I think leaving a 0.6mm top is probably low for a 0.2mm layer height. I will change that in the future.

    And now I understand what I'll need to do tension-wise to loosen pulleys if I increase tension.

    Regarding possible under-extrusion:

    Feeder spring: I had previously had severe under-extrusion issues (that turned out to be a clogged nozzle), so I tightened the spring back then, however I have a question. Previously I was confused about what precisely to measure: the spring alone, or any other part of the screw/washer. When I checked, the spring alone (feeder to the base of the washer) distance was around 13.5 mm. I tightened it a few turns, and got it to about 12.6 or so, but if felt very tight, and I have always been able to observe consistent indentations in the filament going through the bowden. If you think it could not hurt anything, I will plan on getting it closer to 11.

    Flow: gr5, you indicated that I could increase flow by 5%. By that do you mean adjust steps-per-e, or adjust the flow rate on the ulticontroller? I assume the former, since I thought the ulticontroller flow rate percentage only really increased the speed of the print head and extrusion mechanism proportionally.

    As per usual, thanks for the help.


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    Posted · Is this backlash or something else?

    In the ulticontroller there are 2 different settings you can adjust.

    On the home screen, twiddling the knob changes the speed - that increases both the head speed, while keeping the amount of extrusion constant (it now happens at a faster rate than before, since the head is moving faster, but it's still the same volume of extrusion per line segment as before).

    If you go into the Tune -> Flow menu, you can change the amount of extrusion per line segment compared to what the gcode calls for.

    You can also specify a 'Flow' setting directly in Cura to have it request more (or less) plastic than it otherwise would, when it creates the gcode file.


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