Butterfly shape, technical drawing

I am wondering about the shape of the butterfly. Using the method described in What software do you recommend? - #3 by zbynek.winkler I was able to get one block to Freecad. It looks like a mesh so the best I could do it try to sketch over it with the parametric sketcher and try to align the lines as best as I could.

I have some questions:

  • Did I get the shape right?
  • Is there a specific reason why the 6mm radius is not tangent to the sides? It is a quarter circle but the angle of the lines is not 90 degrees. AFAIU it cannot be manufactured exactly as specified by a moving routing bit of non-zero diameter.


Overview of the shape:

Detail of the non-tangent radius:

Detail of the second radius that is too small to be seen on the overview:

I am also looking at the positions of the butterflies (looking at the wall part facing outside), however I am not able to find any obvious pattern.

The 3 butterflies on the top and bottom side seem to be 194.9mm apart. On the long vertical side the spacing is 208.9mm within each pair and the pairs appear to be 391mm apart. The top and bottom most butterflies are 195.9mm and 195.5mm from the top and bottom edges.

I think I am missing the design intent and measuring the wrong things.

As for my motivation - I’d like try to model the parts using cadquery. To get a feel what that might look like see

def butterfly():
    p = cq.Workplane("XY")
    p = p.moveTo(0, 56).hLine(78 / 2).radiusArc((78 / 2 + 6, 56 - 6), 6).lineTo(25, 0)
    p = p.mirrorX().mirrorY()
    s = cq.Sketch().face(p.wires().val())
    to_fillet = cq.selectors.SumSelector(
        cq.NearestToPointSelector((25, 0)), cq.NearestToPointSelector((-25, 0))
    s = s.vertices(to_fillet).fillet(6)
    return s


So I guess the vertical spacing follows the following pattern

  • distance from the top and bottom of the closest butterfly is the same
    • 195.5mm
  • distance between pairs is double the distance from the edge
    • 2*195.5 = 391mm
  • that leaves the distance within pairs to be 209mm to get the total for M size wall of 2400mm
    • 8*195.5 + 4*209 = 2400mm