Curious delta problem: slanted prints

I've got a home built delta printer, loosely based on Rostock, controlled by a RAMPS 1.4 board running Marlin firmware. It works fine, and has done for quite a while. It prints straight and level. The calibration is correct. Until, that is, I print something larger than about 175mm in the x or y direction. As soon as I do this, the whole print takes on a slant, as if a constant x increment were being added at each layer. The slant is uniform across the whole print. It's like printing in italics. If the print dimension is slightly smaller, say 150mm, there is no slant at all. There are some pictures on my blog (http://robinsonia.com/wp/?p=335) if you want to see. I have tried test prints sliced with both Cura and Slic3r; both behave the same. The on/off nature of the problem makes me think it is not a hardware problem - I would expect distortion to appear gradually if this were the case. My suspicion is that there is an issue in the Marlin firmware, and that somehow it is losing track of the motor steps, or perhaps there is a cumulative rounding error which only becomes important above a certain size. I've googled, of course, and no one else seems to have encountered this issue. I've recalibrated, rechecked all my settings and made sure the the Marlin firmware is up to date. Smaller items print fine. I'm basically stumped, and I really want to print some bigger items.

Do any of you experts have any thoughts on what might be the cause of the problem?

Very curious! Here's a test suggestion: after making a print which goes slanted, but before homing the heads (remove any final G28 from the gcode and replace with a move out of the way of the print) if you try a move to the centre of the bed (G0X0Y0Z1) does the head go to the centre or is it offset like the print?

Cheers,
Robin.

Good suggestion. I'll try it tonight.

I would think it's one of your steppers skipping encoder steps. Check your belt/tendon tension, leveling, and hotend temperature, in that order. I've seen something similar on my Rostock MAX, though my situation may be different from yours. I have prints where the initial 5-7 layers have a gradual shift towards a tower. It's usually towards a tower with a belt that's over-tightened. I've also seen cases where the nozzle catches on a previous layer due to inadequate leveling or an excessively high hotend temperature, and it drags the nozzle just enough to cause an encoder skip.

Do you see the same error if you slow down the movement/print speed, increase the layer height, or start the print with a sparse raft?

Thanks for your suggestions. I did wonder about skipping steps. What puzzles me most is that the error is so consistent. I would have expected skipping steps to be a bit more random. When the problem manifests, each layer seems to be offset by the same amount. I have indeed checked the leveling. I've checked the tension to ensure there is no slippage twixt pulley and fishing line, though I had not considered it might be too high. The carriages move very freely (everything uses ball bearings), so I don't think that the the torque required is high enough to cause the steppers to missing steps. The motors don't even get hot, which implies to me that the torque is not excessive (the extruder drive does, because that's working hard). The hotend temperature is between 210 and 220C. I'm printing PLA. If it were dragging or catching, I'd expect the same effect on an "identical, but slightly smaller in the x direction" model. In fact, the issue goes away completely if I reduce the x size from 200 to 175mm.

Speed and layer height seem to have no effect on the slant. I have not tried printing with a raft - what is your line of thought there?

Can you try printing the part on a different part of the build plate? You shouldn't have this issue with fishing line, but with belts, there may be a portion of the belt that's degrading or has debris.

With a raft, which is generally printed relatively sparse, I think you mitigate the problems of printing the first layer too close to the bed. If your bed's not level sufficiently or the height's not quite right, your first layer will be squished, and you may see some excess material extending into the next couple of layers. I would think that this could cause the hotend nozzle to get caught and potentially lead to skips. At least, that was my reasoning when I had this error. I was used to seeing some early layer shifts whenever I printed at 0.1mm layer height, but they would go away if I used a 0.2mm raft.

The fact that your problem only shows up for bigger prints would indicate to me that it's either an issue with your pulley/fishing line or your bed. Even though your motors don't get hot, your stepper drivers may be getting too hot. I've read that as a potential cause on some other forums. Of course, totally spit-balling here. Hopefully someone more experienced on this forum can chime in soon. Good luck!

I had a similar issue with my printer. In my case the problem arose when printing higher than a few centimeters. It seems like Marlin FW has some problems with round/sqrt to much After switching from GT2-20 timing pulley to GT2-16 timing pulley those problem vanished.
I always have an older Repetier FW ready in case such problems occur to verify if the problem due to FW issues.

zsquared,

I tried your suggestion this morning - without homing the head after the print, I moved it to a reference position (x40, y0, z10 so that it did not collide with the print) where I had previously placed a marker for comparison. The head was indeed about 3mm offset in x. After homing the head, its position was once again accurate. You can see from the photo below the degree of slant I'm getting. It looks like the same offset is being added each layer. Given that not all the layers are the same (the first 10 or so have much more material in them), I would expect the error to vary if it were mechanical, such as slipping wires or skipping steps. I can't help thinking it's the firmware.

P1010192 by squishyheadboy, on Flickr

RostockMini4tw,

Thanks for the info. It's interesting that your problem went away when you switched to a smaller pulley - that should mean more steps per mm movement, which in turn means bigger numbers of steps overall. My immediate thought is that overflow or rounding errors ought to be more likely. I have tried to use Repetier firmware, but it has a separate problem for me. My drive pulleys are not exactly equal diameters (I have measured and calibrated them all), so I use a slightly different steps/mm setting for each. Marlin is happy with this. Repetier allows me to set the values, but then blithely ignores them and just sets all three to the same number when it actually comes to do its calculations. I haven't had the stamina to go through the whole of the Repetier source to try and fix this. It took me long enough (and several increasingly frustrating calibration cycles before looking at the source code) to realise what was happening.

I think you are right that Marlin is the cause of the problem, though. I guess I'll have to dive in and see if I can spot where it's happening. Ugh. Debugging embedded code is horrible.

Since we are spitballing the issue, I would venture to guess there is some binding somewhere. Once the steps are skipped it is skipped for good. See if you can spot any binding in your movement. In my printer there is some very slight binding when the carriages are nearly at the home position. In my case I will try some sand paper or increasing tension on the rods to cure it. Also keep in mind print speed, the slower the printer moves the more torque it has, the faster it moves the less torque it has. Can you try to print at a slower speed to confirm the notion. If at slower speed there is no shift or the shift is lower, you have a binding issue.

I am using 20tooth pulleys on mine, I do not see an issue with shifting layers. Although the head of my printer is very heavy in comparison to most. I have mine sandwitched in aluminum and am using aluminum plates to hold the j-head. I suspect around 50% increase in weight.

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I think it has to be the firmware. Skipped steps won't give you a linear distortion like this on a delta printer.

Edited 1 time(s). Last edit at 07/27/2014 02:07PM by RFZ.

I just took a look at the pic. Your not cooling the hot end at all it looks like. I don't either. However I can say that from the pic it looks like the hot end is slanted. I bet your getting distortion on the hot end carriage. I had problems with distortion on my printer as well. Also it was unable to hold a z - axis height because the hot end was moving with heat. I cured it by using aluminum sheet. Dismount the carriage and inspect it for warping. To me the plastic used looks like PLA which distorts after about 55-60c.

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Quote
squishyheadboy
RostockMini4tw,

Thanks for the info. It's interesting that your problem went away when you switched to a smaller pulley - that should mean more steps per mm movement, which in turn means bigger numbers of steps overall. My immediate thought is that overflow or rounding errors ought to be more likely. I have tried to use Repetier firmware, but it has a separate problem for me..

It was more like GT2-36, which I had earlier, means 44.4444 steps/mm and GT2-16 means 100 steps/mm which should result in less rounding errors see also [forums.reprap.org]

Did you tried the options in Configuration_adv.h like comment out "#define SLOWDOWN"
Do you have any kind of bed-leveling enabled? If so try to disable. I had terrible results with Repetier firmware with this advance bed-level thing enabled.

Quote
jaguarking11
I just took a look at the pic. Your not cooling the hot end at all it looks like. I don't either. However I can say that from the pic it looks like the hot end is slanted. I bet your getting distortion on the hot end carriage. I had problems with distortion on my printer as well. Also it was unable to hold a z - axis height because the hot end was moving with heat. I cured it by using aluminum sheet. Dismount the carriage and inspect it for warping. To me the plastic used looks like PLA which distorts after about 55-60c.

Jaguarking1, thanks for your input. The hot end is being cooled, it's just that the fan is out of shot in the picture. You are right that the tool head PLA, My earlier designs did have issues with overheating of the hotend holder, but this one doesn't have that problem. It's perhaps not clear from the image, but there is quite a bit of separation between the hot part of the hotend and the mountings. The hot end does look slanted in the picture, but that's just an image artifact, down to perspective from which I took the picture. I'm pretty sure the print slanting problem is not caused by tool head distortion, because if it were then it would happen to all prints, not just those over a certain size.

Here's an update. I have tried to eliminate as many potential causes of error as I can. I've written a python script to generate clean, simple, compact G codes, so there is no possibility of the error being introduced by a slicer. I'm moving the head without extruding or heating, so there is no possibility of heat distortion or the print head dragging on previously-deposited layers. I have set up a calibration block, and at the start of the each 'print' it just touches the print head. At the end of each print, the head is moved to the same position. If the print is 180mm or less in the x direction, all is well. If it's 200mm in the x direction, there is an increasing error. There's a short video of the problem on YouTube at http://youtu.be/NDURTVJJcOs. The only difference between the failing print and the correct print is the size in x. It's not the absolute location of the print head (I have printed smaller movements at both extremes of x with no problem), but the length of travel. I am now fairly sure it's the firmware having an issue with counting too many steps. Which is a pain, because I can't easily fix it.

I'd be willing to try your Gcode to see if my printer does the same thing. I'm running a Marlin FW that supports FSRs, and have the RAMPs board too. Would be at least another data point if nothing else...

Dave

I'm very late on this topic but what was the solution?
I have exact the same problem and I'm desperate
Hope you can help me

The solution which finally worked for me was to change the firmware on the printer. I had been using Marlin, but now I've changed to Repetier host firmware. It took a while to get the calibration settings right, but now the prints are not slanted, however big they are. I am convinced that the problem is a bug in the Marlin firmware.

What version of Marlin is that?

[deleted]

Edited 1 time(s). Last edit at 11/06/2016 04:45PM by Matt Gilbert.