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B Transforming a circle onto another plane

  1. Apr 24, 2017 #1
    I am putting in a new flue for a Pot Belly stove. The ceiling through which the flue goes slopes at 12.5º. I want to cut a hole in a metal plate that will be fixed to the ceiling, and through which the flue passes.

    So, a vertical circular flue, radius R, passing through a ceiling sloped at 12.5º to the horizontal.

    How do I get the shape of the hole? I assume it is going to be an ellipse with short axis A, long axis B, where:
    A = R
    B = R/Cos (12.5º)

    My thinking is to:

    1. Generate a circle of Radius R in Photoshop.
    2. Transform (stretch) the circle horizontally by 1/Cos(12.5º). It becomes an ellipse.
    3. Print the result, lay it on top of the sheet metal, punch a few holes as markers, then cut it out.

    Is the mathematical part of the above correct?
  2. jcsd
  3. Apr 24, 2017 #2


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    That looks right to me.
  4. Apr 24, 2017 #3
    Thanks PeroK. I'm a bit long in the tooth to be sure of my maths anymore.

    Next step: try it with a cardboard cutout.
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