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I am fairly new to this forum and since I am not directly from a mathmetics background I recently ran into a problem I cannot solve.

What I am trying to do is to intersect a cone at a specific angle and want to receive the minimum radius of curvature. I know how to do it for a cylinder and I also know that I will get either an ellipse, a parabola or a hyperbole for a conic section, but I cannot find a source for either the euqtion of the intersection nor the minimum radius of curvature/curvature itself.

I found some theoretical proofs for the different intersections, but unfortunately the math behind it was a bit too high for me.

Is there maybe a short and clear answer to this question (for a stupid engineer as myself)? Or can someone refer me to an other sourve where I could the infromation from? I would be more than happy to hear from you! Thanks in advance!

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  • $\begingroup$ It’s “hyperbola,” not “hyperbole,” (which is a type of figure of speech). $\endgroup$
    – amd
    Commented May 6, 2019 at 1:08

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First of all, the minimum radius of curvature for a conic section occurs at a vertex, and it has the same length as the semi-latus rectum of the conic, that is $b^2/a$ for an ellipse or hyperbola, where $a$ and $b$ are as usual the semiaxes.

The values of $a$ and $b$ depend not only on the inclination of the intersecting plane, but also on the distance of that plane from the vertex of the cone. I'll express them as a function of the distances $m$ and $n$ of the vertices (of the ellipse or hyperbola) from the vertex of the cone. If $u$ is the semi-aperture angle of the cone, we have: $$ 4a^2=m^2+n^2\mp2mn\cos2u,\quad b^2=mn\sin^2u, $$ where sign $-$ must be taken for an ellipse and sign $+$ for a hyperbola. You can find the proof (for an ellipse, but that for a hyperbola is analogous) in this answer.

It follows that the minimum radius of curvature is $$ r_{MIN}={2mn\sin^2u\over\sqrt{m^2+n^2\mp2mn\cos2u}} $$ For a parabola, you just need to take the limit of the above result for $n\to\infty$: $$ r_{MIN}=2m\sin^2u. $$

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  • $\begingroup$ Thank you very much! That sounds like the kind of answer I was looking for. Is there maybe an offcial source like a book or something that you could recommend, where I could also get that information from? $\endgroup$
    – SteveU
    Commented May 7, 2019 at 5:52
  • $\begingroup$ I just have one more question: What are m and n exactly? Are those the distances from the vertex "on the cone" itself? $\endgroup$
    – SteveU
    Commented May 7, 2019 at 6:04
  • $\begingroup$ $m$ and $n$ are the distances from the vertex $V$ of the cone to the two vertices $A$ and $B$ of the ellipse or hyperbola (endpoints of major axis). Points $A$ and $B$ do lie on the cone surface, so distances $m=AV$ and $n=BV$ are indeed "on the cone itself" (if I understand what you mean). $m$ and $n$ can then be easily related to other parameters, such as the distance of the plane from $V$ and the angle between the plane and the axis of the cone. I don't know modern books treating such matters in detail. $\endgroup$
    – Intelligenti pauca
    Commented May 7, 2019 at 10:31
  • $\begingroup$ Perfect! Thank you very much, this is going to help me a lot! $\endgroup$
    – SteveU
    Commented May 9, 2019 at 20:37

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