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Eraser

Under-Extruded Layer - Z-axis dropping too far?

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Hi Everyone,

For a long time, I have experienced a singular layer showing under-extrusion in a print. See image below of the same part printed twice, the error occurring both times:

5a33218a3f5a4_File2016-09-20105447PM.thumb.jpeg.28fbddd22cd356c48b242f2c7117f32b.jpeg

5a33218a7d5df_File2016-09-20105401PM.thumb.jpeg.b091a14c54ac4d5e10dbc034566951b1.jpeg

Some troubleshooting information:

 

  • The "layer error" occurs only sometimes (~50% chance) when printing the same part (from the same gcode file) multiple times. It has been getting more frequent lately (used to be ~10% chance). It is not an error in the model.
  • The "layer error" occurs at around (but not exactly) the same Z-position (+- 1mm) between different prints.
  • The "layer error" has different severities - sometimes it presents as a minor flaw, and sometimes it is quite serious (as in the photos above). Sometimes it causes the part to easily split along the underextruded layer

 

I don't have a camera setup on my printer so I can't say for sure what is the cause. However, my best guess is that the bed is dropping too far when transitioning between layers. This suggests maybe there is some problem with the Z-screw or something related to the Z-screw. I have made sure there is plenty of the green lubrication gel on the screw.

Does anyone recognize this problem? Any advice for correcting this problem?

Thanks,

~Nick.

5a33218a3f5a4_File2016-09-20105447PM.thumb.jpeg.28fbddd22cd356c48b242f2c7117f32b.jpeg

5a33218a7d5df_File2016-09-20105401PM.thumb.jpeg.b091a14c54ac4d5e10dbc034566951b1.jpeg

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Looks like yet another case of sticking in the vertical linear bearings / rods.

Short suggestions to remedy: add some weight (500-1000g) on the platform between the rods, wipe off any lubricant off the Z-rods (they should be dry and non slippery), remove the linear bearings and exercise them (a lot) by running them back and forth along the rods until the motion feels smooth and rolling (no skidding).

If you find a good source of components, you could also try to replace the linear bearings with higher quality parts (Misumi parts have been recommended by some).

See here (for example, but there are other similar threads):

https://ultimaker.com/en/community/11123-z-axis-layer-error

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Hi Nick,

same problem here - opened a new topic, saw your topic to late :-(

The bad layers in my parts are around 10-11mm in Z-heigt. Seems nearly the same height in your parts.

I'm in doubt that this is a z-axis lubricating problem or something similar. Different machines with the same problem occuring... This failure seems more like a systematic failure.

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Thanks guys.  @Balden - damn that was a long thread to read through.  I am going to try your suggestions and see if there is improvement.

 

Eraser,

See my recent post here:

Z Layer Error / Skipping Layers - Sharing Experience

In my case the problem was the vertical linear bearings (both of them). The Misumi replacements solved the problem completely.  Some have said the bronze lead screw nut can be the culprit, but even if it has some play, it shouldn't matter if the linear bearings are good.

If your linear bearings are crunchy and the realignment process doesn't fix it, I recommend going with the Misumis.

Edited by Guest
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Yeah they seem to have some issue (anyway is always a good idea to test this by printing long small towers at the same time on different spots of the bed to remove the chance that is other issue).

Misumi has exceptional high quality earings. On one of my printers the problem was the nut I got one from a Pololu distributor and it fixed it.

The main issue is that with a bad nut the bearings must be perfect. With a perfect nut the problem isn't noticeable IMO.

Not so long ago saw a mod that could remove both problems and allow to print with a bad bearing or nut or both also increasing the stability of the system, also is quite cheap.

https://www.thingiverse.com/thing:1674387

The backlash nut I only found it on china sites like robotdigg or aliexpress. The good thing is that is a fairly easy mod, and doesn't involve taking out the long bearings.

Edited by Guest

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The main issue is that with a bad nut the bearings must be perfect. With a perfect nut the problem isn't noticeable IMO.

 

Funny, there are definitely several ways to look at the same problem, kind of "chicken and egg" situation...

I personally would say that a "perfect" nut does not exist (some amount of play is essential for the screw to run through the nut). So what you want is to lower the play in the nut to well below the layer resolution of the printer (for Ultimakers with 40 or 60 microns layers, I guess this translates to nut/screw tolerance below 10 microns or so... Clearly, the original Ultimaker parts do not reach this level of tolerance. I have no idea what the standard industrial tolerances are for such parts: maybe by sourcing screw AND nut from one same quality manufacturer, they will be adjusted to tighter tolerances...?

So in my view, we should accept the fact that there is some play in the nut, and this should not be an issue as long as the weight of the platform brings "natural preloading" by pushing down on the nut and leaning its bottom-facing threads against the top-facing screw threads. When the regular weight of the platform is not sufficient for this, the problem can be greatly mitigated by adding weight (and possibly cleaning dirt in the screw threads), which in my opinion is the simplest of all suggested fixes and unbeatable efficiency / simplicity ratio.

Unless I misunderstand something, the suggested "anti-backlash-nut" fix is also based on preloading the nut threads against the screw threads, which will work even for axes without the weight-load, but in our case we do have some weight and it seems to be a more natural way to obtain a similar effect. Moreover, I personally don't feel comfortable with the (lack of) stiffness and geometrical alignment of the plastic adapter, especially in case the bearings begin to severely jam (wouldn't the whole thing slightly twist?) which in my experience happens about once in 20 or 30 prints. I prefer the nut mounted directly against the platform (metal against metal, very stiff).

Still in my view, the part of the mechanism that is really not sound (and not working as designed) is the "sticking / jamming" of the linear bearings (especially common 10-15 mm after direction reversal of the Z-movement at the beginning of prints). Bearings are supposed to roll, not jam... Many people noticed this and it is clearly due to the low quality of the standard Ultimaker bearings. Sure, a very tight screw/nut (less than 10 microns) will mitigate this by "tearing off" the jammed bearings without noticeable shift in the Z-layers, but to me this is not the proper (intended) functioning of the mechanism.

In my case (UM2+), additional weight (1 kg) together with a lot (really a lot) of exercising of the linear bearings completely solved the problem (I haven't had any visible "bed drop layer" in months and tens of prints at various Z resolutions, including 3 challenging tests at 60 microns). Still, I'd be willing to replace my original bearings with Misumis if I get an opportunity to buy them, but unfortunately Misumi itself only sells to companies, so if any of you knows a reseller who accepts dealing with individuals (and ships to France), I would be very thankful for the hint.

Edited by Guest
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Hi @balden thanks for the commentary. I very nearly ordered the Misumi bearings yesterday. At least the US Misumi site allows individuals to register and order - there is a company field to fill in but you can just write whatever you want there. It's still shipping to the address you provide. The problem for me is that I live in Canada and after exchange rate, taxes, shipping, brokerage fees (they use UPS to ship) it is going to cost me $100 for the two bearings. So today I am going to try cleaning and exercising the bearings really good. If that fails, I'll just have to eat the cost.

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Unfortunately, from France I have not been able to order from Misumi without a company ID. I haven't tried to order from Misumi US but as you say the shipping costs are likely to be a rip off (assuming they ever accept to ship outside North America). Anyway, I'll remember to check that possibility: after all I already uselessly invested in a couple of costly Igus polymer bearings which did not fit well and only made the situation worse ;-).

About exercising the bearings: in my case, I made several runs over several days (probably cumulated about the equivalent of one continuous hour exercise for each bearing). I also made several (2-3) cycles of spraying WD40 into the ball races, then exercising with WD-40 inside, then applying degreasing agent (regular bike degreaser fluid), then rinsing with hot water, drying with hair dryer, exercising again the dried bearing, and repeat...

While exercising, I deliberately applied slight torque on the bearing (wrt to the rod) in order to increase friction between the balls and rods, between the balls and races and between the balls themselves. You'll notice that the jamming almost always happens when reversing travel direction (hence bed drops essentially happening at the bottom of the prints and much less frequent afterwards when the first jamming has been overcome and the bearings are rolling again) so I concentrated the exercising around the points of distinct jamming, sliding back and forth over a few millimeters: the intent was to apply many friction cycles to progressively wear off any roughness preventing smooth operation inside the ball races.

At first you'll be desperate because it takes very long for the feeling to become noticeably better. But after a while the function will start to improve, to the point where there is almost no jamming any more (or very slight and easily overcome by the platform weight).

I read somewhere on the forum (or maybe another forum) that such low-cost bearing are sometimes shipped with a layer of some sort of anti-corrosion agent that makes them slightly over-tolerance and prone to such jamming. It definitely feels like this exercising helps wearing off some friction-causing layer somewhere: I don't know if this is truly a protection layer or if those low cost bearings just need to be "run in" at the beginning...?

In any case, I think that Ultimaker should be aware of this problem now, and check the operation of their bearings more cautiously (or use higher quality bearings): my UM2 (with "2+" extrusion upgrade) is 2 and 1/2 years old now, so in my case it may be a "youth problem" on the early models. I have had this problem on most prints from the very beginning until last June. I have no idea if this has been robustly fixed on the more recent UM2 models.

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Hi @balden,

My experience is essentially identical to yours:

 

  • 14 month old UM2 with UM2+ upgrade installed 3 months ago
  • Bearings jam when reversing slowly. Can be overcome with force
  • Jam seems to be avoided by torquing the bearings, or running them horizontally

 

For the community's benefit, I have uploaded a video showing the jamming that occurs about 10mm after a slow reversal (which is where the layer skipping is happening) -

password is "ultimaker"

 

I WD40'd them. I don't have degreaser, but I'll go to the store and get some now. Its going to be a long day...

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Thanks for the video Eraser. I feel like watching myself and my bearings mid-June. Be patient and you'll get rid of this jamming.

It is worst when the rod is vertical because there is no load on the ball bearings so the balls start to skid easily when the least resistance is encountered along the ball races. At first I was able to jam the bushing even when the rod was horizontal, and then progressively the sole weight of the bushing would become sufficient to avoid the jamming in the horizontal and then increasingly oblique rod slopes (the bushing was able to roll back and forth freely and reverse direction without jamming just by rocking the rod up and down), and eventually up to the vertical.

Also, don't forget to add weight on the platform (under the hood between the rods). In my case I sawed an iron bar in small chunks then piled them up and taped them on both sides of the screw. It's very simple and can be easily added almost at any time since it requires no (or very basic) disassembly.

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So, I did the following:

 

  • Sprayed the bearings and rods with WD40
  • Exercised the bearings on the rods (up/down about 10mm) for 10mins each
  • Sprayed the bearings and rods with degreaser
  • Let degreaser work for 5 minutes
  • Rinsed the bearings and rods with hot water
  • Dried the bearings and rods with hair dryer
  • Exercised the bearings on the rods (up/down about 10mm) for 10 mins each

 

Then I repeated all of those steps again 2 more times.

Unfortunately, my bearings are still jamming on reverse. I think there is some improvement, but I'm tired after doing this several hours, so I'm going to break down and order the Misumi bearings.

Cheers

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Unfortunately, my bearings are still jamming on reverse.  I think there is some improvement, but I'm tired after doing this several hours, so I'm going to break down and order the Misumi bearings.  

 

Eraser,

Sorry to hear that Misumi charges so much for shipping to Canada, but even at $100 (is that CAD or USD?) I think you will be happy because the problem should just go away.

As a mechanical engineer, I would have used an anti-backlash nut in the design rather than relying on the weight of the platform to pre-load the bronze nut, but I realize that Ultimaker is most likely under tighter cost restraints than I would be (I work on medical equipment).

Also, as neotko alluded, if the nut is perfect, then it should just force its way past the bad spots in the bearings without jumping. To me, though, the anti-backlash nut is just a band-aid to deal with junky linear bearings.

The Thingiverse anti-backlash nut is interesting, however, and I may give it a try just for fun.

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I received my Misumi LHFSW12 bearings. The first thing I noticed is that the bearings are covered in oil. This concerned me, because I understand that lubrication can cause the bushing to slide instead of roll along the rail causing the very problem I am facing.

So I did a little googling and found a Misumi spec sheet on the bushing. https://us.misumi-ec.com/pdf/fa/2012/p1_0265.pdf. This document says that the oil is Anti-Rust oil, and recommends degreasing the oil, applying Lithium soap based grease to the ball rows immediately, and then every 6 months. I am obviously not going to take apart the machine every 6 months to grease the bearings, but ignoring that, did other people apply grease the bearings before installing? Am I misreading something?

Thanks guys!

~Nick.

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I have some on three machines, never did a thing but just installing them.

Btw don't skip the bed readjust one screw at a time on each bearing, up/down 5-8 times. Boring, but is a must do. Seen a video of a guy assembling a umo+ and he did just raise the bed once and was adjusting the tight of the bearings without going from one side to the other, and ofc his bed dis move badly... Patience, and one at a time. I still remember how much backpain I had after installing this bearings on two machines on the same day...

Edited by Guest

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@neotko,  

Did you just install the bearings still covered with the Anti-Rust oil or did you use degreaser to remove that first?

Can you give me a step-by-step on this "bed readjust one screw at a time" process you are referring to (or point me to a webpage or forum post)?  I was not aware of this at all.  I want to do it right.

Thanks so much.

~Nick.

Edited by Guest

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I just wiped the oil off the outside of the new bearings and installed them. There wasn't a lot of oil on mine to point that I even thought about degreasing them.

For aligning the new Z-bearings, I have seen a video of someone doing it properly (in Dutch with English subtitles as I recall) but I can't find it right now. The basic procedure is to leave all 8 bearing screws loose and move the bed all the way up and down. Tighten one of the bearing screws and then move the bed all the way up and down again. Now tighten one of the screws on the other bearing and move the bed up and down again. Repeat this process for the remaining 6 screws alternating between bearings as you go.

Honestly, I think once the first two screws are tightened, alignment doesn't change, but moving the bed up and down a bunch of times will help you ensure that the bearings glide smoothly.

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I didn't remove anything on the bearing... But I used the C-(cost effective version) check specs, maybe they don't come with that.

Indeed is what @rowiac says I think it does matter, I did the adjustment quite a few times while changing nut, cleaning motor, untighted the shafts holders, a lot of stuff. And I did fell different when doing it right than just skipping a bit.

 

Um2 uses a 'guides' to hold the bed in position (google ultimaker 2 assembly manual) they also explain that part. For a manual with photos look for "ultimaker original bed upgrade manual".

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@rowiac and @neotko,

Thanks so much guys. With the video and the assembly manual, I think I have everything I need. I'm going to give it a go tonight.

Re: bearings. Misumi doesn't sell the value bearings in North America so I have the regular ones. They come package in a plastic bag inside a box, and there is actually a small pool of oil in the bag, so it's a lot of oil. I'm going to try following the Misumi instructions - degrease the oil and then apply lithium grease.

Cheers!

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Eraser,

In my experience, grease is for slow moving bearings, whereas oil is for high speed operation.  The printer platform is very slow moving so you should be good with the grease. You just don't want the grease to be so thick and sticky that the platform will not drop of its own weight to take up the backlash in the nut. Let us know how it works out.

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