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How to change the "arc" of an ellipse?

kay_bursigkay_bursig Member Posts: 12 ✭
in Drawings
Hallo.
When sketching an ellipse you can change the witdh and hight. But I need a defined "arc" so to speak. Can I define 2 points on the ellipse and squeeze/stretch them and keep the ellipse proportional?

Thank you very much.

Comments

  • S1monS1mon Member Posts: 3,443 PRO
    Not sure I understand what you're trying to do. A picture would probably be worth the proverbial 1000 words. 

    That said, would a conic section help?
    https://cad.onshape.com/help/Content/conic.htm

    Or perhaps you need a full ellipse and you're just trying to control it with points on the ellipse, which you can do:

    Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.

  • glen_dewsburyglen_dewsbury Member Posts: 1,120 PRO
    Are you meaning something like this?

    https://cad.onshape.com/documents/cb2bcd844a298bdf747100bc/w/98472c696ada5bf510cb4476/e/397321d3379cb36e4008c606
  • kay_bursigkay_bursig Member Posts: 12 ✭
    edited February 2022
    Yes you both got it. When I add 1 point to the "arc" I can change the look of the whole ellipse. If I add a second point it won't allow me to change the value. Instead of changing the "arc" it changes the position of the point.

    What I need is: the middle point with 2.4 is correct, the left point (where it says 1.933) has to be 2.3 at that X coordinate



  • S1monS1mon Member Posts: 3,443 PRO
    @kay_bursig

    Those points are missing any x dimensions. The center point is also not fully constrained. Try working with "Show constraints" turned on. It may help you understand what's going on.


    Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.

  • kay_bursigkay_bursig Member Posts: 12 ✭
    Ok, I tried it wth constraints. If I change the x value from your "2" to more, the whole ellipse grows. Let`s say instead of your x=2 I take 4.5 and instead of my 1.667 we take 2.3 :p ... I dont get it to work look like in my sketch.
  • eric_pestyeric_pesty Member Posts: 2,282 PRO
    @kay_bursig
    That wouldn't be an ellipse anymore!

    If you want a shape like this maybe use a spline instead (and mirror it if needed)?
  • S1monS1mon Member Posts: 3,443 PRO
    @kay_bursig

    Your sketch implies something which can be done with a conic or splines, but not a true ellipse. I'm still not sure what you're trying to do, but is this closer? Unfortunately, Rho is required in Onshape. It will not solve for Rho, so I had to iterate to get something closer to the proportions you have in your sketch.




    Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.

  • kay_bursigkay_bursig Member Posts: 12 ✭
    Aha, so there's the problem :)  I'm autodidact, so these basics I missed. I want to 3D print an adapter, thats what I need it for.

    I tried it with the conic and mirrored it. That works.

    Thank you guys very much!
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