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[edited] oops! nevermind!

Here is a possible source of dark spots in a white print (this is a feeder on an S3 or S5 but I guess any printer can have similar filament dust after hundreds of hours of printing. Although usually I think it is filament that sticks to the nozzle, caramelizes, and then drips into the print.

Maybe. Probably. I'm not sure. With PLA, even the cooling that is built into the UM2 is barely enough. When using a large nozzle like the 0.8mm nozzle then extra cooling could help a lot. Maybe add a window fan or desk fan blowing into the printer. Yes - that's probably a dirty Z screw. I would clean the z screw. Just the top 2 inches to see if it helps. You can do a quick clean with a paper towel and a screwdriver but make sure you clean all 3 threads. It is a triple helix screw. 3 helixes (dna is double helix). So make sure you get all 3 grooves. Or you can do a thorough proper cleaning by removing the stepper motor from the bottom and sliding it out of the printer and then putting it all on newspaper and cleaning with a tooth brush and WD-40. If you do a complete clean (only if) then add one pea sized drop of grease. One pea sized drop is enough for the entire screw. Underextrusion. Could also be dirty screw. Maybe. Many things cause underextrusion (such as speed changes, dirty Z axis, and 30 other possibilities). Keep in mind you are using a huge nozzle - 0.8mm is a big nozzle. The quality won't be as good as with 0.4mm nozzle. And large layer height (0.4mm). The quality is better with 0.4mm nozzle and 0.2 layer height. But it takes 4X longer to print. And the quality is even better with 0.25mm nozzle and 0.1mm layer height. But that takes another 4X longer to print.

If the switch moves right core up and down just fine then run the lift switch calibration procedure in the menu system probably under "maintenance".

I think you have to click on the object and get it so it has a blue outline before the scaling option appears. Maybe? Not certain.

Strange. 1) What is that huge black rectangle with the curved edges? I'm thinking that is part of your STL files? That might have something to do with it? 2) Please get to this state as you show and then do menu "file" "save project as" and post that file so I can see what you are talking about. Also what version of cura are you using so I can test with the same version. 3) Did you check the 3 comments listed in your post? A) try selecting everything (ctrl+A maybe) and scaling everything down by 3X to see if it can then slice. It might be the black area extends beyond the edge or the printer. B) Make sure an extruder is enalbed C) Is it possible you modified these 3 objects to be modifier meshes?

By the way, 90% of the time when you are losing steps it is because the set screw on one of the pulleys isn't quite tight enough. You might want to use a sharpie and mark the shaft and the pulley for the stepper pulley and the pulley it connects to (the two pulleys on the short belt). Usually it's the hardest pulley to get to - the one on the stepper motor. Then when the problem happens again you can check to see if the sharpie marks slipped or are still aligned okay. But from your description I kind of ruled this issue out. But I could easily be wrong and this might just be an issue where you need to tighten the hell out of that set screw. You can push the head around which rotates the stepper and you can position that screw for easy axis without needing to take those little side covers off that hide the steppers (or you can remove the 2 or so screws that hold those covers in place).

I usually call this "missed steps" but it can be a pulley that slips on the shaft, or a stepper that is skipping steps. Is it a gradual shift as you move up the print? Or are the large sudden shifts? When you say "switch off and back on" please elaborate - is it that the other axes keep moving or does the entire printer stop and reboot? Or does everything stop and it just sits there for a little while? If you have one axis that stops it's almost never the stepper (despite your past experience). Steppers are extremely simple and will probably last for decades. 1) It could be that there is a lot of friction. When the printer is idle push the print head around and see if the Y axis has significantly more friction. If it does then you need to fix that. 2) It could be the stepper driver. These drivers can partly fail where they just don't have all that much power. But more likely it's another issue: they have heat sensors inside and if they get too hot they shutoff. Usually just for a second or less because they cool again in milliseconds (I guess the sensor is right there on the semiconductor substrate). Is the ambient air temperature higher than normal (UM headquarters is cold - they like it really cold there)? Do you have a towel or something under your printer blocking airflow? You could experiment by removing the larger bottom cover - it's only 2 to 4 screws (I forget but I've removed many many times) and then tilt the printer up with a book or something and use a small box fan or other type of fan to blow under there and it if heat is the issue it will preform so much better. If you have a really old UM2 then it may have a spare stepper driver on for a second extruder (originally they hoped to do dual extrusion on UM2). It's possible to update the firmware to use the spare driver and plug the Y axis into that channel. Building the firmware is not a trivial process but I can point you to step by step instructions if you want to try that route. Most UM2s sold in the last 3 years don't have the extra stepper. It's easy to detect as it is quite a large chip and situated across from each of the corresponding stepper cable connectors.

No - I totally trust CNC Kitchen. He has fantastic videos and seems extremely honest. You might want to subscribe and watch one of his videos every day. They are just - really informative. A testing site like CNC Kitchen - if they lied/compromised and got caught - it would ruin their channel forever. They have too much to lose. Although I'm pretty sure that the effect on a 0.4mm nozzle would be smaller than on the 0.6 nozzle that was tested in CNC Kitchen. And if you go to a 0.8mm nozzle it's so obvious that the multi channels help that even *I* noticed it.

1) You have retract before outer wall enabled. Turn that off. That's probably the issue. 2) You have outer wall speed at 15, inner wall speed at 15 and infill speed at 20. You should make them all the same. I definitely wouldn't go slower than 20 but 25 should be fine and print significantly faster. Having these different speeds causes problems - when you stay at the same speed for a while, there is soon a balance where the pressure in the nozzle is perfect so that you get a nice extrusion speed to match the head movement speed. If you speed up the XY then it will underextrude until the pressure balances at a new, higher pressure. If you slow down the XY speed then it will over extrude for a few seconds until the pressure balances at the new, lower pressure. So every speed change causes some over or under extrusion. This could be the problem also but probably not - my money is on #1. See screenshot below showing all your speeds. I only care about printing speeds - mainly inner wall, outer wall, infill, top/bottom. 3) You messed with maximum travel resolution and maximum deviation. I've never touched those things. I'd put them back to default settings just in case this is causing a problem. At least one time for a test. Then you can put it back to how they are now. I'd change all 3 things. It's okay to change many variables at once until you get it working then undo the changes one at a time to figure out which variable it was. By the way #2 above was specifically called out - saying a value of 0.05 is definitely a problem that can cause ugliness and it should be closer to 0.4 (default for UM2 is 0.5 but I guess default for his printer was .05 which was a problem). Here's his post:

You need to watch this video which proves that the multiple channels is MUCH faster and lower resistance than one big channel even for 1.75mm filament. Bontech worked closely with 3dsolex when developing the CHT. Faster even than the volcano nozzle. I have it queued up at one of the comparison graphs.

Maybe you should just post your project file so I can see all these things myself. In Cura do "file" menu then "save project as" and post the resulting file here. It will give us your STL file and it's position and any thing you did to it like scaling. It will also include your printer settings and your profile and your settings overrides. >jerk and accel I don't like accel/jerk reduction (if you disable it chooses max possible values which I prefer). Those are meant to stop ringing but cause other problems like the bumps you show. You have to decide if you care more about precision or looks. And you have to decide if ringing is worse or if those bumps are worse. Rarely I have issues with ringing - like if I have lettering on the side of something that needs to be legible. In those cases I might enable accel/jerk. These should be off on a UM2. By default. Because UM2 doesn't have much of a ringing issue in the first place - it's more for the heavy print heads of the UM3/S3/S5 printers. >The small hole appeared matched the location where the next start point of new layer. Excellent. You narrowed down the problem. I'm not sure what to do but if you post the project file the first thing I will do is make sure "coasting" is disabled.

Basically, burnt, yes. Caramelized might be a more accurate term. Oxidized. It's probably building up on the *outside* of the nozzle and dripping down into the print. It's worse with some nozzles and materials than others. Try cleaning the nozzle as it is doing the initial purge just before starting the bottom layer. It's also possible it is caramelizing inside the nozzle but this is rare with PLA (quite common with PVA). I'm not sure how to fix this. 3dsolex has some "ICE" coated nozzles that is supposed to prevent this. I'm not sure if that works or not. I don't have this problem much so I don't pay much attention.

Last photo I reply to fist. You circled a layer in black that sticks out. It doesn't go all the way around benchy. If it did I would say you need to clean your z screw. Some of the fainter horizontal lines are probably caused by a dirty z screw but most of them don't go all the way around. The issue with dirty Z screw is that the bed doesn't always move down by exactly 0.4mm. Sometimes 0.3mm or 0.5mm. Because of dirt on the z screw. If it move > 0.4mm then you have an underextruded layer and it sticks inwards a bit. If it moves < 0.4mm then you are extruding too much for that layer and some squeezes out and you have a layer that sticks out. However most of those lines don't go all the way around so it's something else. Not sure what.

To prove I'm right, when it is drawing those circles push the bed up (or the nozzle down) just slightly - the thickness of a sheet of paper is enough so it shouldn't take much force at all. And you should find those circles stick properly.

those curls are "bed adhesion" issues. The typical fixes are: 1) Clean the bed from oils 2) heat the bed 3) brim (unrelated in this case) 4) leveling/squish 5) glues/treatments like magigoo 6) special surfaces like PEI The most important of all of the ones above is #4. In my opinion you need a little more squish. I don't know your particular printer and how you level but you should level a little closer. Or there is a hack in Cura where you can increase the flow for the first layer. If you put 0.3mm thick of filament into a gap of 0.3mm (where the nozzle is 0.3mm above the bed) versus if you have a gap of 0.15mm then you get something like 10X more stick. It's really hard to believe until you've experimented with it. Cura also has options to make the line width farther apart but that doesn't help with squish at all. I think that defaults to 120% or something. It's not going to help with squish. Only initial layer "flow", of cura features will help you here. But really you should fix this on the printer - have it level a little lower/closer.

Do you know how to read schematics? The schematics for the UM2 are here: https://github.com/Ultimaker/Ultimaker2/blob/master/1091_Main_board_v2.1.1_(x1)/Main Board V2.1.1.pdf The 2 cooling fans are 12V fans with 24V across them but the fans are in series so it usually works where each fan gets half the voltage. But some brands of 12V fan will not work that way - they do not like being put in series. 1) Are the stepper motors also broken? If the relay, K1 is broken (it broke on my UM2) then everything will look fine except 3 things won't work (no 24v reaching them): steppers, heaters, side fans. Everything else will seem okay (ability to read temperatures and use the menu system and so on). Hopefully this is the case as this is super easy to fix. 2) You can turn on the side fans manually with the menu system (if I remember right) on the printer (if not you can certainly do it with the "tinkerGnome" version of the UM2 firmware.

3dsolex sells a very similar nozzle - a diamond nozzle tip like this. But with the "race" technology (multiple passages for the filament to flow) which speeds up printing a lot.

Well according to Greg, it sounds like it will work great.

I hate to repeat myself but 1) Did you check if the fan was at 100%? 2) Did you go into cura PREVIEW mode and check the layers with the dimples to see if they coincide with where it does the outer layer? If so look at the order of the inner, outer, infill and consider changing that. I think by default it does the outer layer last? Or first? I forget. I would probably try setting all the printing speeds to 35mm/sec (infill, wall, inner, outer). Leave the travel speed high. You lowered it to 130mm/sec which is probably fine but as a general rule: lower travel speed means lower quality print lower printing speed means higher quality print UM2 should be fine at 250mm/sec travel speed. If not you may need to oil the gantry rods.

@GregValiant - you didn't mention if you have ever printed PLA with this all metal hot end from micro-swiss. And was it like 2 hours of PLA prints or > 20 hours of prints?

Be aware that most "all metal" hot ends won't print PLA. Or not well. Some will. Most won't. Molten PLA sticks to all metals in a bad way and if this happens up where the PLA is cooler than 100C, say around 60C, and if temperatures are going well above and below (due to retractions or just changes in print speed) then the filament can stick to the side of the upper portion of the hot end and just get stuck there. PLA is pretty much the only material with this issue so if you are printing nGen, CPE, PETG, ABS, PP, or Nylon then it should be fine. Hot ends with a very short heat break don't have this problem because although the PLA can stick right at the heat break, the length of the heat break is short enough that the feeder can usually overcome the sticking force.

@kayazuki - pictures please. It's unclear if you want to remove extra lines only on the top layer maybe? Or if you have hollow cylinders with air inside. And are you printing a single cylinder at a time? Really a picture would be very helpful. If you are printing hollow cylinders with an open top and you don't want internal stringing there are 3 things to try. The first is "combing". Make sure that is enabled. If the cylinder is very large it may be disabling combing for some layers but in layer view if it is doing "combing" there should be zero internal travel moves inside your cylinder. If there are some let us know (another photo would help) and there are a few settings that override combing that we can look at.

It looks like some of your parts are suffering from "not enough fan". What material are you printing?

I'm not sure but it could be where that outer wall line starts and ends it's loop around benchy. If that is the case sometimes you can fix those by just slowing down or by playing with the order that the walls are printed or increasing fan speed (although it should already be at 100%). You could verify by going back into cura and looking at the layers near those holes and seeing if that's true. This is typical quality for a 0.8/0.4 combination (not as good as say 0.4mm nozzle with 0.2 layer height). What are your printing speeds for this print (default, outer wall, inner wall, infill)?