Thing-O-Matic: Dodecahedra vs Print Speed

A 3×3 array of dodecahedra printed at 50 mm/s with 100 mm/s moves:

Dodecahedra - 50 mm per sec

Dodecahedra - 50 mm per sec

You can clearly see the axis oscillation near the left edges.

What’s nice: the total lack of threads between the parts: snap ’em off the platform and they’re done!

After they built halfway up the top facets, I dropped a ball bearing in each one. They rattled around something fierce, but didn’t quite hop out.

Building a single dodecahedron at 20 mm/s showed that the oscillation problem really is due to the speed. More accurately, the problem is the abrupt change in velocity as the axes change direction without any deceleration / acceleration in the middle.

Here, the single line near the edge matches up with the internal fill, so it’s not an oscillation:

Dodecahedron - 20 mm per sec

Dodecahedron - 20 mm per sec

The small ripples come from a mechanical resonance in the geared stepper mount pumped by its full-step drive at 1.28 rev/min. I’m using a failed MBI stepper driver board that can only do full stepping, so trying 1/2 stepping won’t happen until I build a 4-axis space transformer for those tiny Pololu stepper boards.

As you’ve probably noticed, I’ve gone back to Kapton tape on the build platform, rather than the ABS I’d been using. AFAICT, the Kapton didn’t work well on my earlier attempts because I didn’t have good control over the first-layer thickness and was probably printing too fast for conditions.

The Z-min switch solves the layer thickness problem and printing at 10 to 15 mm/s for the first layer glues the thread in place. So far, so good!

  1. #1 by equinoxefr on 2011-05-29 - 08:39

    Hi,

    Did you read this article http://www.neufeld.newton.ks.us/electronics/?p=1313 ? It’s about using SDCARD instead of USB to print with more accuracy.

    Regards
    Pierre

    • #2 by Ed on 2011-05-29 - 19:37

      I’ve done several comparisons of USB vs. SD Card printing and found absolutely no difference whatsoever.

      From what I can tell, Linux does a better job of USB/serial I/O than Windows, but that’s entirely circumstantial evidence. So far, so good…

  2. #3 by John Rehwinkel on 2011-05-29 - 11:22

    I just got some of those tiny Pololu boards too. I’d like to heatsink them, but the surrounding components project above the driver chips. Time to treat a heatsink to some Quality Shop Time, once I figure out how I want to secure them to the milled heatsink.

    • #4 by Ed on 2011-05-29 - 19:55

      the surrounding components project above the driver chips

      I’m thinking of a C-shaped heatsink with a peg for the chip and a plate on the bottom, with a screw compressing the plate against a TO-220 silicone gasket against the PCB. The heatsink comes in from the long end of the PCB, so as to leave room for header pins through the holes. It seems awfully complicated, though, and I don’t like it at all.

      The total ON resistance is about 0.8 ohm, so it’s nearly 2 W at 1.5 A. That definitely needs a heatsink…

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