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Calibration plot

edited April 2014 in Egg-Bot

I've created what I call a 'Calibration plot' that I think may be helpful to others. It uses the egg bot template and creates vertical, horizontal, and diagonal lines over the entire area and also plots circles in every other square. I found it helpful to understand how plots are realized on an egg or any other shape. Specifically on an egg how things are stretched and squashed on different sections of the egg. The lines should also intersected correctly. Also, how the pen lands on the eggs makes a difference (i.e. is it perpendicular). I think it would be an excellent first plot for anyone new to the egg bot or if you think it is not plotting correctly. I would make it available on this post, but I do not see a way to attach the file.

If you are interested, I would be happy to send you the file.




  • A good way to share the plot or plots is to post them at and tag them with "eggbot".

    See also some of the plots at  E.g., the coordinate system demo plot .
  • Hi.

    Yes, we are interested. Please upload.

  • Thanks for telling me about  

    I uploaded my file and egg photo to: ;
  • edited April 2014


    The correct link.

    Nice work!

  • edited April 2014

    When examining your "Calibration Plot" and really trying to understand how the diameter of the egg affects the shape of plotted objects I made a small program in python to plot the values.

    I have used a term I called 'Aspect Ratio" which is the length of a line segment made with 1° of travel along the pen axis divided by the length of a line segment made with 1° of egg rotation. The aspect ratio is plotted as a function of position on the egg.

    To have a circle appear to be round in the middle of the egg it has to be drawn as an ellipsis which is 1.48 times as wide as it is tall. At the end of the eggs the value changes to around 2.4

    See illustration.image

    The plot is based on a chicken egg with 42mm diameter and 58mm length.
    (To read the diameter from the plot you have to go from negative value to positive along the y-axis.) I have the pen motor shaft set 10mm lower than the egg-motor shaft and about 2mm from the middle of the egg along the length of the egg for minimum "tracking error" with this egg size.
    (Red line represent the egg shape - )
    XKCD Style Plot

    Will make a 'Calibration plot" based on the values from this plot for verification, stay tuned.


  • Nice graph and great info. When I created the simple calibration plot, I expected some of my circles to actually plot as circles instead of all ovals. After looking at your graph, I realize I need to adjust my pen arm motor. I'll play around with that and see how it affects the plot. It definitely takes some experimentation to get the plots as expected.
  • edited April 2014

    As you see from the graph in the middle of the egg the "aspect ratio" is 1.5. The circles have to be 'elongated' 150% in order to plot as round circles on the egg. See this post as well.

    Here are the text output of the plotted data;
    Pen Radius; 31.50
    Egg radius = 21.00
    Egg length = 58.00

    At egg max diameter;
    Step length pen = 0.99
    Step length egg = 0.66
    Aspect ratio = 1.50

    L=  0.01, Inkscape y value=   0.00, aspect ratio= 6.462
    L=  3.75, Inkscape y value=  50.00, aspect ratio= 3.481
    L=  7.50, Inkscape y value= 100.00, aspect ratio= 2.445
    L= 11.25, Inkscape y value= 150.00, aspect ratio= 2.036
    L= 15.00, Inkscape y value= 200.00, aspect ratio= 1.807
    L= 18.75, Inkscape y value= 250.00, aspect ratio= 1.655
    L= 22.50, Inkscape y value= 300.00, aspect ratio= 1.566
    L= 26.25, Inkscape y value= 350.00, aspect ratio= 1.523
    L= 30.00, Inkscape y value= 400.00, aspect ratio= 1.507
    L= 33.75, Inkscape y value= 450.00, aspect ratio= 1.508
    L= 37.50, Inkscape y value= 500.00, aspect ratio= 1.538
    L= 41.25, Inkscape y value= 550.00, aspect ratio= 1.615
    L= 45.00, Inkscape y value= 600.00, aspect ratio= 1.751
    L= 48.75, Inkscape y value= 650.00, aspect ratio= 1.974
    L= 52.50, Inkscape y value= 700.00, aspect ratio= 2.458
    L= 56.25, Inkscape y value= 750.00, aspect ratio= 3.843
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