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Printing through USB seems to have lots of trouble for lots of users of many operating systems. Because of this most people recommend printing only using the UC. But one person did mention that when they bought a powered USB hub all their USB printing stalls stopped happening. I have no idea why. Maybe it was just a bad USB cable? Or better grounding of the USB cable?

Yes what version. There are issues with the latest Cura with: 1) virus scanners - they try to scan the executable part of cura called "steamEngine.exe". You might want to add it to your virus scanner exception list. Or just turn the stupid thing off, uninstall it, and burn the disks. 2) SD card auto scanning. The new Cura searches for SD cards automatically. Some people have dozens of USB devices or other issues that makes this code run very slow. You can disable this feature in Cura.

If you print it sideways you must make the minimum thickness (in the lithopane software - not in cura) be .8mm. If you make it thinner than .8mm then Cura will mess up. If you print the part flat you can make the minimum thickness one layer (for example .1mm).

Here's something I printed myself laid down flat: http://www.thingiverse.com/make:40709

Yes. Do that. Very good idea. Then when the problem moves, describe this to UM and have them ship you a new stepper motor. Oh. And check the wiring to the stepper very carefully. Sometimes one of the 4 wires is loose or frayed or badly connected on one or the other ends.

Wait - check the filament diameter. Maybe I'm wrong. Is it 1.75 or is it 3mm? It should probably be around 2.9mm but there are places out there that sell 1.75mm diameter filament. But I don't think it will work very well in a UM unless you make some modifications.

Okay. Lol. Okay. Well the main problem is the filament diameter parameter in Cura. You have it set to 1.73mm yet I assume it is closer to 3mm. This means it is trying to put out (3/1.73)^2 or 3X more filament than normal. So it is just flowing out all over the place. You should be able to print a lot faster 50mm or 75mm should be fine. And fill density of 20% should be plenty - don't need 55%. Retraction should be 4.5, not 7. travel speed can be 150mm no problem (not 30) Bottom layer at 10mm is a good setting. Cool should be set more to 7 seconds (not 20!) and get rid of minimum speed. That minimum speed (in expert/cool setting) will hurt you some day possibly. But basically the only parameter that was really messing you up was that filament diameter.

Did the fan shroud melt a bit? Mine did and I repaired with kapton tape.

The STL format is defined nicely here: http://en.wikipedia.org/wiki/STL_%28file_format%29 It shows examples. Note that there are binary and text versions. Most readers can handle either format. I'm 90% sure Cura can handle either.

That's a good theory and makes sense. Easily checked in gcode view. You might be able to fix by making the minimum thickness of your lithograph .8mm thick. Less than .8mm and Cura will have trouble. Or print it flat side down. Or it could be underextrusion. Maybe tighten the extruder spring a few turns or slow down the printing speed to 50mm/sec to see if that makes a difference.

Great research. You are such a scientist. In a good way of course. I wish you tried clamping the UM stiffly to something immovable. I'm curious if that is worse than the "middle of your desk". I can see how putting the UM on for example a slippery surface could actually help more than hurt. Another thought I've mentioned in these forums before is it would be interesting to put a weight on the long belts at the point opposite the clamp. The two weights on the X axis should add up to the weight of the print head and same with the Y axis. Then moving the head would not vibrate the UM itself nor the table it sits on, nor the platform. On the down side it would make for twice the work for the steppers. Another thought I had while reading posts above was that oiling the rods might hurt matters. A little friction can help dampen oscillations.

still tiny :???: Sometimes technology tries to help too much. You can click on "gallery" on the top left corner of this web page, then upload your picture there and then when you are making a post click on "my media" to insert it.

download meshlab. It's free. Just loading into meshlab and resaving the STL can fix most problems. How many faces do you have? Meshlab tells you at the bottom of the main window. If it's more than say 1 million or even 100K it's worthwhile to reduce this with meshlab. Google something like "meshlab reduce number of faces" or "triangles" or something similar as the feature is well hidden.

Oh. You printed upright. Now I see it. I understand now - it's during the infill stage - causing strings from island to island. The older Cura might fix that as I mentioned in previous post. Another possible solution is to try "skin" infill only. Set the skin setting to something huge like 20mm. Not sure what will happen - look at the gcode view carefully before you decide to print.

Retracting 9mm is too much as you will get air in the nozzle and then you will get other problems later. But this isn't your current problem. In the lithograph picture, the edges look like that because the tool you used (and I used) has a bug/feature where it leaves you with very rough edges. When it prints those edges the UM makes a horrible sound as it weaves in and out of all those bumps. I don't see the stringing issue you mentioned in the pictures but if you say you see no retraction then I believe you. The older cura (13.04) had a checkbox: "retract on jumps only" which defaulted checked but which I liked to uncheck for certain prints. If you uncheck this then cura retracts on all moves. You could try that. There was a major change in Cura I think version 13.05 - completely rewritten slicing algorithm.

You also might have to extend the two slots that hold the end switch down a little further. You could use a dremmel, or a small file, or a saw or a router bit in a drill.

I assume we are talking about vertical cylindrical holes and not horizontal. I thought the same thing until I looked at the gcode. It's fine. Cura compensates by the radius of your nozzle size so as long as you tell the truth, Cura does the correct thing. So for example if you ask for a 10mm diameter hole and your nozzle is .4mm Cura adds .2mm all around the outside path so that the head moves in a 10.4mm radius circle. So why is the hole too small? One factor is the number of lines segments in your circle. If you have the typical 10 segment decagon, each segment is cutting into the interior of the circle a tiny bit. That tiny bit is significant when tollerances are around .1mm for fit versus not fit. That hurts somewhat. PLA shrinks when it cools. This also causes holes to be smaller. The "pulling" effect (like pulling on a string) tends to stretch the PLA slightly towards the center of the hole as it pulls the PLA around the rim of the circle. Molten PLA isn't like water - it is a bit sticky and pulls against itself.

There does seem to be a bug. I tried a 3mm tall cylinder and at .001 layers it worked out to 3000 layers and was fine. When I tried just 10% more (.0009mm layers) it just did 3 slices. But 3000 layers is ridiculously high. How thin is this rocket? If it takes a minute per layer it could take you a few days to print this. Or a few weeks. I suggest you do .2mm layers. This will keep you under 1000 layers. It's kind of silly to print with less than .2mm layers as you only improve the resolution in one axis. The other axes are restricted by the nozzle size (.4mm). Of course now lots of people will tell me how much better the quality is with .1 or .06 or whatever layers. It may be true that it's harder to see the layers but you won't improve the resolution in the plane of a layer much beyond .4mm resolution. Until you have printed many dozens of prints and know all the tricks - it's crazy to print something that takes more than 3 hours as there is a high probability something will go wrong.

There is no slice button anymore. It just does it. It's so fast it probably finished before you noticed. To save gcode, right click on the second button in top left corner. To see gcode (slice) view click top right button.

First of all, why post .2 megapixel pictures. Can't you upload higher res images? It's hard to see in these. I looked at them for a good long time the first time and don't see anything new this second time. Well except that the antennas on your UM robot are printed much too hot - they need more cooling - usually you can increase the "minimum layer time" in the "cool" section to at least 7 seconds to fix this. And make sure the fan is on. You get melty, droopy layers when you don't let a layer cool enough before the next layer goes down. .06 thick layers is nice and thin and if you are going to print at a leisurely 70mm/sec (I would try 200) then you might as well lower the temperature to around 190C. That might help those antenna on the robot and other droopiness. Anyway - to your issue... I guess I've never paid attention to the color of the plastic. I guess it hasn't been an issue for me. I will pay more attention in the future. It seems hard to believe this is related to the fan as while you are printing, the fan blows more on the top of the print than any particular side. Sure 10 layers later, the fan blows briefly on the "fan side" but not the other 3 sides. But it's hard to believe the color is changing many minutes after the layers have been laid down. It really looks more like an issue with infill. When the head comes over to do infill it touches the outer edge and reheats it. Of course this should happen on all edges, not just the left side. Maybe the fan is blowing on the nozzle? (it shouldn't be) That might cause one side of the nozzle to be cooler? Arrg - this seems unlikely to me. I mean across the 2mm area of the tip of the nozzle it should be all about the same temp no matter how hard the fan is blowing on the nozzle. Anyway, hopefully someone else will look at these (tiny) pictures and recognize this issue. I certainly don't.

Everything I wrote about the belts causing oscillations applies if it's the bed instead that is oscillating except it's much easier to fix the bed mechanically to make it stop oscillating than the belts and print head.

Or clamp the frame of the UM to a 400 pounds weight.

f=ma. force = mass X acceleration. The mass is the weight of the print head. So there is a direct correlation between the amount of force on the belts and the acceleration. Twice as much acceleration means twice as much force on the rubber belts which means more stretching (potentially twice as much stretching). When the acceleration stops suddenly at the corner it's like being in a car stopping at a stop sign. You really notice it when the acceleration stops suddenly at that moment when the car goes from .00001 miles per hour to 0 miles per hour because at that moment the acceleration goes through a very fast change. Same thing when the print head goes around a corner. This very high change in acceleration is called "jerk" by physicists but Marlin's meaning of the word "jerk" is different. Anyway, at this moment of high true jerk, (the corner) the acceleration on one of the axes goes from stretching the belt to zero force. Like pulling back on a guitar string (the force) and letting go (no force). It vibrates, or rings at it's resonant frequency. You can lower the frequency by adding mass or loosening the belts. You can increase the frequency by removing mass or tightening the belts. But note that adding or removing mass also increases or decreases the force on the belts so it's usually better to remove mass! There are ways to mechanically dampen the harmonic oscillation - all involve increasing friction. For example if you glued a small box half full of sand (of the same weight as the print head) to the print head that would absorb a large part of the oscillating energy and it would probably only give you a single ripple (or not even that). You could design belts with material that don't store energy very well. For example belts that don't stretch. Think of a ball designed to not bounce very well. Like a wood ball. Or a tennis ball half filled with sand. If one type of rubber ball bounces to 80% of it's original height and another only bounces to 20% of it's original height - the second rubber would be the kind to use to reduce ringing. Another way would be in software: decrease the acceleration gradually instead of suddenly at the corner. Aka "true jerk". It would be nice if Marlin worried about true jerk (optionally) for people who don't like the ringing and are willing to print a little slower. People tried this in firmware in the past and the problem was the print head pretty much came to a complete stop at every vertex. Fine for a square but for a "circle" with 100 segments this is crazy slow. But maybe cutting the acceleration in half is good enough for most people. Right now Marlin defines "jerk" (roughly) as the maximum change in speed at a vertex. This lets it go around a circle fast as there is very little change in speed at the vertexes of a circle. Maybe Marlin needs also a similar setting for the maximum acceleration occurring just as you approach or leave a vertex (and max true jerk to achieve that acceleration).

Nice. Do you have a larger version of the final photo (the one that shows difference due to acceleration)?

Well okay, but you could instead just lower the velocity and acceleration, right? And if the problem is not related to velocity (likely true) but is only related to acceleration than it makes more sense to reduce acceleration than to rotate the part. But I see your point.