Circles are drawing as ovals on round objects

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    I’ve been having a blast with my EggBot Pro but have run into an issue that’s a headscratcher because it doesn’t always do it.

    Every so often I find that designs printed on spherical objects come out stretched when they weren’t stretched before. For example, a circle will come out as a circle when it’s working, and when it’s not, it will be an oval.

    I’ve been switching between two types of objects – Christmas ornaments and ping pong balls. The pen motor is in the top most position. The three things I change are the horizontal position of the pen motor, the height of the pen arm, and the position of the egg coupler.

    I’m thinking that one of these things is perhaps causing the problem and that I’m not putting one (or more) of them in the right spot every time. Or it’s something else in my setup that I’m messing up.

    Any ideas what might be causing it? I’ve wasted a few things accidentally or on purpose and would love to get some sort of direction to focus my testing on.

    I did try printing the circle/square file as suggested on one of your
    wiki pages. It came out oblong/rectangular on the ping pong ball I was
    testing on. I fiddled with settings on the machine to no success.

    For what it’s worth, here’s my setup:

    Windows 10
    EggBot Pro
    Inkscape 0.91
    Eggbot Extension 2.5.0

    Thank you for your time!

    Windell Oskay

    This effect is actually due to map projection, from the different effective radii of curvature on the two axes. 

    Please see here for more, and recommended workarounds:

    Hi Windell,

    Thanks for the reply! I’m sorry I didn’t describe the problem better. I’ve read the article on Map Projection and understand about stretching designs for printing on eggs.

    It’s when it’s printing on a sphere that’s the problem. I’ve been printing on spheres successfully. The design needed no squishing in Inkscape and printed proportionally on the sphere whether it was an ornament or ping pong ball.

    But for some reason, sometimes a design that printed properly before comes out elongated.

    For example, I printed an identical design on three identical ornaments. The designs on two of the ornaments are elongated. The design on the third one is perfect. (and for the record, these are round ornaments.)

    Same with the ping pong balls. I printed a circular design on one and it came out perfect. The next time I tried it, it was elongated.

    However, I didn’t print all of these things in a row. I printed an ornament, switched to a ping pong ball, switched back again, and so forth. That’s why I was thinking one of the manual settings might be to blame for my inconsistent results.

    I guess I’m wondering whether there’s a particular setting on the machine – pen arm height, motor height, pen holder height, etc. – which might cause an occasional elongation?

    Windell Oskay

    Hmmm. All of the settings should be purely deterministic, and it should work exactly the same way every single time that you print the same design. If something is not coming out consistent, it’s hard to imagine what the issue might be.

    Do you suppose that you could try some tests to help determine under what circumstances this occurs– for example printing something simple (maybe just a circle or square) over and over on the same ornament, to see if it’s consistent? If you could get more information about what might be triggering it, I might be able to figure out how to fix it.

    (Also, for spheres, the motor height should be fully up– both motor shafts pointed directly at the center of the sphere.)

    Thanks for your patience, Windell. I did more tests as you suggested and I think I’ve found the culprit. The screw that adjusts the height of the distal arm wasn’t as tight as it could be. It was a bit deceiving because while the screw felt tight, gently twisting the arm showed that it would rotate out of alignment very easily.

    After tightening the screw further and running some more tests, it’s much improved. Either that or I did something else during my tests that helped. Regardless I seem to be up and printing again until the next user error, haha.

    Thanks again for your help. I love this machine :)


    I wonder if there might also be an element of not getting the pen arm horizontal when the pen tip touches the object.

    If the pen is not perpendicular to the sphere, the geometry will be off – and this is the sort of thing that might happen when changing between spheres of different radii.

    Windell Oskay

    Oh! Yes, that would totally do it. Glad there’s a reasonable explanation. :)

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