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ceramichammer

Calibrated steps/mm and filament diameter set correctly causes Ultimaker to over-extrude.

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I am using a hobbed bolt similar to this. http://printrbot.com/wp-content/uploads/2012/07/hobbed.jpg

The bolt was changed out from the stock bolt when I was away so I had just assumed someone had performed a calibration. The machine (more or less) functioned correctly for a few weeks at the default 865 steps/E with the non-standard bolt. We began having issues with the filament diameter which sent me on a wild goose chase troubleshooting the hardware. Using Repetier, I calibrated, checked, and rechecked the steps/mm value to be 980 steps per mm. These measurements were taken before the extruder drive so the deformation and elongation of the filament due to the bolt does not matter. If I use the correct parameters for the Steps/mm and filament diameter, the nozzle physically scrapes across the over extruded layers on every move and within a few layers, the bowden tube will pop out or the filament will be sheared off by the hobbed bolt. The bowden tube has a heavy duty clamp and I'm amazed at what force would have to be applied to rip it out of the hotend. Also, the hobbed bolt is a much more appropriate drive and it does not grind or slip. If there is that much backpressure, it just shears off filament. I'm using Cura 13.01 and standard print settings. I've had to back the steps/mm down to 900 as well as bump the filament diameter +0.1mm above nominal and even now, I still hear scraping every once in a while which to me means it is over extruding. These numbers account for almost a 15% difference between settings that yield a decent print and the settings that they should "theoretically" be. It just doesn't make much sense to me. I know that with this drive bolt, it extrudes 1mm for every 980 steps. I also know the diameter of the filament to +-0.01mm. Is this a problem with the slicer? Where else could this error be coming from?

 

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When you calibrate the steps per e in Repetier, does that store the value in the controller's memory? Or do you have to set it every time in your gcode. (And if you are then slicing with Cura, have you also set the correct steps-per-e value in Cura's preferences).

If we assume that the steps-per-e is correct, and the filament diameter is correct, then the only other variable on the input side would be the extrusion multiplier/packing density. That should be at 1, or very close. If it is, then by definition (and your verification that a 100mm extrusion command moves the filament 100mm, and the filament really is the diameter it says), then amount of plastic being fed into the system is 'correct'.

So that leaves the Bowden and the output end of things as the problem...

Even with the 15% over-extrusion you calculate, it surprises me that things are failing quite as catastrophically as you mentioned merely as a result of that. I could see prints maybe being a bit blobby with that much over-extrusion, but I wouldn't expect it would be too hard for the excess plastic to ooze out of the way, unless maybe you're printing something large and solid. What material are you printing, and at what temp?

Incidentally, this doesn't really explain the over extrusion/scraping aspect, but it sounds like you're putting a lot of pressure on the feeder end of things... when you talk about shearing off the filament, do you mean that it's snapping the filament? Filament is typically pretty soft, and i could see it slipping or chewing up the filament.. but shearing it off seems a bit unlikely in the normal course of things. In some ways, I think that slipping/grinding is a preferable failure mode to something that is going to put enough pressure into your feed system to shear the filament or pop a well-secured Bowden tube. This makes me think that the pressure plate on the feeder mechanism must be very tight - in which case it is probably deforming the filament pretty severely - is that enough to cause it to get stuck in the tube, or at the point where it enters the head? When you aren't having catastrophic problems, what does the filament in the tube look like? It should have some teeth marks from your bolt, but still be basically round, and not chewed up, flat-spotted, etc.

The other thing to consider is the physical capability of the head. What speed are you printing at, and what layer height. The standard 0.4mm nozzle is limited to depositing about 10 cubic mm per second... which corresponds to 125mm/s at 0.2mm layers. If you try to go faster than that volume per second, then you will get a pressure build up that can cause clogs, and in turn pop the Bowden. This could happen at lower speeds too, if the nozzle is partially blocked. But none of that wouldn't really explain your 'scraping' scenario.

So, next I'd verify that the bed is properly level, and homed? If the home position is off, so that the head is jammed down into the bed when you start printing, then that's going to cause scraping of course, and make it hard for even the right amount of plastic to escape.

Finally, is there a problem with the x/y or - most likely Z - calibration or mechanism? In particular, is the problem really that you are over-extruding, or could it be that the extrusion is right, and the head isn't moving as much as it should on each layer change, so that it drags through what it just extruded, rather than making full space for the next layer? Have you tried moving the bed up and down a set distance at the move speed you have set in your slicer, to confirm that it moves reliably? While you're at it, have you checked that your X and Y dimensions are coming out correct?

Finally, if it really is the over-extrusion that is causing the problems, you might want to look at the gcode to see if you're getting sensible values in there. Look to see what steps-per-e value is set in the start of the gcode (M92 Ennn). Also try slicing a simple shape like a 100 x 100 x 10mm block, and a) see if it prints ok, and b) look at the gcode for each of the edges of the block, and see if the amount of plastic being fed in matches what you would expect given your filament diameter.

 

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Iluminatri, sorry for the tardy reply. I thought I had email notifications enabled for new replies. From what I can tell, Repetier sends the steps/e value to the controller's memory. I was sure to change the value in Cura and verify that Cura's g-code output had the correct value. I am using a packing density of 1.

I am printing PLA at 210-220. I was printing reasonably large objects at 100% fill so this will make an over extrusion problem readily apparent because the extra material has nowhere to go. When I said the filament gets sheared, I didn't mean that it snaps. I guess you could say it is the failure mode beyond grinding and slipping. The bolt will leave crescent shaped depressions in the filament where it was "grinding". It is different failure mechanism than grinding with the stock gear because although it has a lot more bite, once it tears the crescent in the filament, there is little to no chance of it catching again. This is much more preferable to me because it is more tolerant of little errors and let's you know when something is VERY wrong.

The pressure plate onto the bolt is pretty tight. Eventually I will move to a roller bearing pressure device but I haven't had success yet. I have not had filament get stuck in the bowden tube from deformation due to the hobbed bolt. When the tube pops out, the head has already been scraping the print for a while, indicating to me that I have compressed filament in the tube.

The bed is properly leveled and homed. I have dialed back the lower layer speed a lot (back down to the stock ~20mm/s). It is possible that with a "too fast" lower layer, I was having to bury the first layer into the bed to get it to stick. This still doesn't explain why the problem gets worse as the print goes on.

I have not verified X Y or Z calibration. I assumed because my prints are coming out square and straight then no steps are being missed but assuming is a dangerous thing. I will perform a repeatability test on the Z-axis.

After much fussing, I seem to be getting good prints at 885 steps/E. I am not fudging the filament diameter. I am torn between "if it ain't broke, don't fix it", and wanting to know what the root cause is. I will do some testing and report back.

 

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