Login
You're viewing the y.diskcat.com public feed.
  • Koz Rosskoz@chaos.social
    Oct 17, 2026, 9:08 PM

    One common thing I end up doing in #FreeCAD is fixing up (frankly) poorly-designed models I find online. Since these are usually STLs, it leads to some annoying moments, like 'finding the centre of a circle that's now actually a bunch of vaguely-circular line segments now'.

    Decided to showcase an easy technique in part design to find the centre of such a 'circle'. First, mark three points on the perimeter - doesn't matter where.

    #CAD

    Partial screencap of FreeCAD's sketcher. There is a circle, made of lots of line segments, surrounded by a bunch of grey faces. On the circle's perimeter, three purple dots are marked in different places.
    💬 1🔄 0⭐ 0

Replies

  • Koz Rosskoz@chaos.social
    Oct 17, 2026, 9:09 PM

    Next, connect these dots using chords. Set them to construction geometry.

    2/?

    Partial screencap of FreeCAD's sketcher. There is a circle, made of lots of line segments, surrounded by a bunch of grey faces. On the circle's perimeter, three purple dots are marked in different places, and these are connected by two chords, drawn in dashed blue lines.
    💬 1🔄 0⭐ 0
  • Koz Rosskoz@chaos.social
    Oct 17, 2026, 9:10 PM

    Then, put points in the middle of each chord. FreeCAD can do this automagically by doing a symmetry constraint on the point using the endpoints.

    3/?

    Partial screencap of FreeCAD's sketcher. There is a circle, made of lots of line segments, surrounded by a bunch of grey faces. On the circle's perimeter, three purple dots are marked in different places, and these are connected by two chords, drawn in dashed blue lines. Each chord has an additional point on it, in green, exactly in the middle.
    💬 1🔄 0⭐ 0
  • Koz Rosskoz@chaos.social
    Oct 17, 2026, 9:11 PM

    Then, construct a perpendicular line from each of the centre points on each chord. No need to worry how far they extend for now.

    4/?

    Partial screencap of FreeCAD's sketcher. There is a circle, made of lots of line segments, surrounded by a bunch of grey faces. On the circle's perimeter, three purple dots are marked in different places, and these are connected by two chords, drawn in dashed blue lines. Each chord has an additional point on it, in green, exactly in the middle. From each of these green points, black lines extend perpendicular to each chord. The lines intersect, then extend past each other.
    💬 1🔄 0⭐ 0
  • 💬 1🔄 0⭐ 0
  • tilltill@chaos.social
    Oct 18, 2026, 12:00 AM

    @koz FreeCAD Sketcher also has a "Circle by 3 points" tool, that does it even more automagically :)

    Screenshot of FreeCAD Sketcher toolbar icons
    💬 2🔄 0⭐ 0
  • 💬 0🔄 0⭐ 0
  • StarkRG@myside-yourside.net
    Oct 18, 2026, 3:46 AM

    @till @koz You still need to decide whether to inscribe the circle (making the circle tangent to the of the edges) or circumscribe it (putting all the points on the circle's circumference), depending on whether it's supposed to fit into something or have something for into it (respectively).

    💬 0🔄 0⭐ 0
  • Koz Rosskoz@chaos.social
    Oct 18, 2026, 4:04 AM

    @StarkRG @till Yes, but I think for what I wanted to do (increase the hole's diameter), the circle-from-three-points would work.

    💬 0🔄 0⭐ 0