Sunday, 27 December 2015

Day of the living skew

Suggestions from last episode, to be tested against the standard 20mm cube:
  • Slow down the print, down to as low as 15 or even 10 mm/s.
  • Reduce max acceleration.
  • Twisting the motor wires to reduce interference (maybe near the controller, but otherwise it's the same setup as for many successful Mendel90s).
  • Maybe the gear is not solidly on the motor shaft.
  • Check if power supply voltage drops, it may not be powerful enough.
  • Check if the X gears are properly perpendicular.
  • Adjust X steps (somehow, maybe, as a last resort).
    First a sample print as baseline, with XY pots reset to the starting values, checking the power supply voltage at various steps over the black/yellow multiwires.

    Voltage at start: 11.76
    Voltage while homing: 11.69
    Voltage while heating bed & extruder: 11.00 (varying a bit at first)
    Voltage during print: 11.10 - 11.60
    Voltage when 12V is disengaged from the Melzi: 11.79

    Compared to the numbers in this post, getting so far down could be a problem indeed.

    The baseline print, as expected, came out skewed, even some of the infill is skewed:

    I halved the speed of everything in the Slic3r settings, but curiously the estimated time to print is the same, even after I made sure to reslice by removing the gcode file. It doesn't seem any slower, and there is still noticable skew, though not as bad. On the other hand, there is more coiled-rope effect:

    I do like that the Slic3r includes homing, getting to the plate, and heating at the same time. 

    Third attempt, setting the Slic3r speeds back to normal but halving the print speed in Pronterface. This does seem to have more of an effect on the speed.

    I just saw the print head get stuck on a piece of print. Interesting. Entirely possible then that I have lowered the start height too much, so it squishes out clumps early on. It also looks at time as if it's running through a mud of half-melted filament when doing infill. The result is clearly horrible:

    Going on the theory that the extruder bumping into things is bad, I raised the Z start level by .3 mm. Let's try again, at full speed. The tape in the center is getting rather abused, and I don't have any spare here. I should get some, but it would also be nice to have a way to have the prints randomly distributed.

    Starting this print, the printer got stuck while heating (maybe because I set the print speed after starting the print), so the head got up to 200C before I noticed and turned it off. Restarting the print gave good adhesion at the start, though, so the height is probably ok.

    This one came out no worse than the first few, but the coiled-rope effect is back in full force:

    The alignment of the X motor rod might actually be a little bit off, but it'll be tricky to affix something to make it less off.

    Also, at one point during this, the Slic3r settings reset themselves. Not pleasant.

    There's a failure mode with this setup that I hadn't considered: Strings or pieces of plastic landing on the Y rods, getting into the linear bearings.

    Tried reducing the max acceleration in Marlin by a factor 10 for all but Z. Had some problems getting started, the bed didn't want to heat, and the extruder temperature was set to 200 for no apparent reason. Retrying had the bed not get set until I did it manually. Why so flaky? And when it started printing, it didn't extrude anything for quite a while, missing half the first infill.

    It sounds different now, with the lower acceleration, much more relaxed. The result is actually rather nice and crisp, except for the X skew:

    Trying again with the reduced acceleration and half speed. Worked reasonably nicely, though the first few layers looked dotted, as if the thread was falling apart. Only a little X skew, and once actually skewing the other way, until the X belt fell off:(

    That's probably a sign that I should give up for tonight and go to bed.

    I notice the left corners, even when not skewing, look a little blobby, as if too much is being extruded. A retraction problem, mayhap? I haven't kept an eye on whether it actually retracts.


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