Make Your Own Tools: 2 pick Arches



Some students have been scratching their heads about how to get their parametric arches working.  The arches need to be defined by start and endpoints, and the height will flow from some formula driven by the length between start and end.  Here are 3 families showing 2 ways to get a parabolic arch and 1 way to get an arc segment.

Make Your Own Tools: 2 pick Arches

Download the families from here.

For further reference on a little bit of math that I’m using, check out this resource.


  1. Hi Zach,

    I've been looking for this for a while and, after you made it, it actually looks "simple"!...
    There seems to be an issue with the family though. When you flex it (change the height proportion), the arc falls out of plane (no longer vertical).
    Do we need to remake the 3pointArc family and change the adaptive points to vertical in placement also?

  2. Hi Nelson,
    I can't see the out of plane issue that you are describing. With the arc arch, there are certain length to height ratios that break, like if you set the height to half the length (this will create an impossible bit of geometry). This seems like a different issue than what you are describimg. Can you describe the problem more?

  3. Hi Zach,
    I believe I may have figured what the issue was.
    If you open the individual "archBy2points_arc.rfa" family and change the height proportion (e.g. length / 4), the arc falls out of plane. If you open "arches.rfa", edit the family and then edit the formula, the arc stays in plane.
    In the video, when you inserted the height parameter, the line fell out of plane and you needed to align it back.
    I believe that we cannot place the height dimension from the point in the middle but we need to do it from the horizontal line in its plane (complicated description?). In that way, the top of the radius line gets automatically locked and the arc should stay in plane.
    I can't really make a video with the computer that I have now but, if needed, I can post something on youtube soon.

  4. Hi Nelson,
    Thanks for looking more into this. I still can't reproduce with my file, but I have definitely seen this in the past. Generally, I like to use the method shown with the archBy2points_parabola_Frame.rfa family, using points offset from other points. I find it more reliable, if a bit more work to set up initially.

  5. Hi,
    Here's what happens when:
    1. I edit the archBy2points_arc family from the arches file.
    2. I open the family directly from the disk (the one that you made available for download)

  6. Hi Nelson,
    Yep, you're right, I think I dimensioned to a point rather than a line, and my local file seems to be a little different than the one I uploaded before. I just updated the files, and I think the arc is more robust now.


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