Unwanted speed increase near corner of print



  • While the top layer of a raft was being printed at 100% infill, everything was going great. But near the edge of the print when it had 8 or 9 back and forth lines to print, the speed increased gradually.

    I checked the g-code as well as the preview in Simplify3D - there is no speed increase in either of them. No G1 commands with any F in them except for one line after a retraction.

    Then when it got to that side of the print for the 2nd raft top layer, it did it again. I would say it reached double the speed it was supposed to be. Again, the Simplify3D preview and the actual g-code have no speed increase in it for this portion of the code either. It did not affect the quality of print - it looks like it extruded proportionally faster as well.

    Has anyone ever experienced this before? And can anyone guess what caused it?



  • I don't actually know if the speed increased - I was basing it off of the sound (an increase in pitch for each new line) and it seemed like the time between each printed line decreased even though the lines are the same length. I will start the print again when it is finished and take a video.



  • Could it be my heightmap? I made a custom heightmap that spans 0 to 100 for the X and Y dimension.

    My printbed is actually 200x200mm. Half of the actual model extends past the 0 to 100 (in the X dimension). The speed increase is happening around the X values greater than 140mm - though I'm not sure if this is a problem with anything other than this particular print.



  • In addition to the speed weirdness, there is a shift in the infill of the first raft infill, but not in the raft's perimeter on the same layer. I thought that was a separate gantry problem.

    I sliced it again, except made sure to move the model such that its end was within the bounds set by M208. It worked this time with no layer shift or weird speed thing.

    So I think the problem is that those particular lines were outside of the M208 bounds. I would have thought the firmware would have handled it differently, but I think the firmware for some reason messes up all the lines within the gcode buffer getting the position and speed wrong somehow. Is this known behavior?


  • administrators

    What should happen is that the firmware truncates the XY coordinates to fit within the M208 parameters. This is likely to result in over-extrusion. If you have the M208 limits set a little too wide so that the motor reaches a hard stop before reaching the M208 limit, then you will get motor stalls and layer shifts.



  • @jml said in Unwanted speed increase near corner of print:

    While the top layer of a raft was being printed at 100% infill, everything was going great. But near the edge of the print when it had 8 or 9 back and forth lines to print, the speed increased gradually.

    I checked the g-code as well as the preview in Simplify3D - there is no speed increase in either of them. No G1 commands with any F in them except for one line after a retraction.

    Then when it got to that side of the print for the 2nd raft top layer, it did it again. I would say it reached double the speed it was supposed to be. Again, the Simplify3D preview and the actual g-code have no speed increase in it for this portion of the code either. It did not affect the quality of print - it looks like it extruded proportionally faster as well.

    Has anyone ever experienced this before? And can anyone guess what caused it?

    It doesn't get faster - it just appears to. I suspect you have acceleration and /or instantaneous speed change set high.

    Assume for a moment that you had infinite acceleration and that you set a print speed of 50mm/sec. A 50mm long move will take 1 second. Now as you move towards the corner, the moves get shorter so take less time. A 25mm move will take 0.5 seconds. Halve it again to 12.5 mm and it'll take 0.25 seconds and so on. Each time the length shortens, the time to make it shortens too. However, in practice the only time we have infinite acceleration is when instantaneous speed change (jerk) is applied. So if you reduce the "jerk" then the print head will have to decelerate to a lower speed. Also lower acceleration will increase the time it takes to get up to full speed and slow down again. So in a nutshell, try lower jerk and/or acceleration.



  • @deckingman I know what you're saying, but that is not what is happening. It actually is increasing in speed.

    Shifting the print so that it did not exceed the M208 limit fixed it, so its not an acceleration/jerk thing.

    @dc42 the weirdness starts happening way before it reaches the M208 limit, and it definitely is not crashing at all. I will post a pic or video soon.





  • I could be wrong about the speed. The reason I think its actually speeding up is because the stepper motor noise increases in pitch. But perhaps the X motor is decreasing in pitch, and both Y motor is increasing in pitch, and since the Y motors are louder than the single X motor, I'm only hearing the increase in the pitch (even though the nozzle travel speed is constant). See the video.



  • Video here : https://www.youtube.com/watch?v=yF9YvsEuV44&feature=youtu.be

    The speed seems to slow down in the first part of the video, then in the next part, it is speeding up. You can also see how the infill is shifted.

    Again, shifting the print over a few millimeters fixed the shift and the speed artifacts.


 

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