I took 2 shapes from the file provided by jbergen in this topic:
https://www.qcad.org/rsforum/viewtopic. ... 488#p43486
I then used the following method to draw the NFP of one vs the other and inversed:
QCAD/CAM .. Misc .. Draw .. No-fit Polygon
Not going into the minute details because the original shapes are polylines with bulges and NFPs are based on polygons.
But then I would expect it to be an approach based on some sort of polygon hull, see lower.
In the file:
Blue = Original shapes
Magenta = NFP
Yellow = Overlay of the other shape on the NFP contour.
Red indicates where it fails.
It all fits like a glove when you radically remove the bulge factors (Polyline .. Arc to Line Segments).
Toggle visibility of both the 'Object' and the 'Overlay' layer and the related sub layers.
A nesting test with just these shapes on one sheet with the the "Align" strategy gives me this as best solution:



Regards,
CVH