A curved frame: solution one

I’ve been asked to design a rectangular frame with a geofr_sectmetric profile.

It seems to be child’s play for any parametric 3D CAD designer, but…frame

…the frame must be plugged in a rectangular widow cut out from a pipe: it must be bent around a cylindric surface…


…so the section of the frame along the straight side must be curved.

Using FreeCAD, as usual, it is indeed child’s play to draw the sketch of the section of the frame, as well as to sweep it either along the arc of circumference or along a straight segment, but… the section of the frame along the straight side must be curved to match exactly the curvature of the arched side with the above geometric (not curved) section and to follow the pipe’s shape.

Taking into account the symmetry of the final frame, it is enough to design a quarter of it, with the curved and the straight half sides matching at the corner.

As first attempt I draw the cylinder sector corresponding to the frame, then I extract the curved surface with the Face Binder tool and I extract the outer perimeter of the surface: it is the path along which the frame will be built.

I position the sketch of the section with the proper orientation, exactly at one vertex and I sweep it along the curved side of the path.

The generated curved side of the frame must be cut at 45° w.r.t. the section plane, to match it with the linear side to be designed:on the sketch of the frame section I draw a rectangle, bigger than the section shape and sharing it’s external edge, I revolve it around the shared edge for 45 degrees inwards to the frame: the generated  wedge can be used as cut tool to chamfer the rectangular ended curved frame side.

The new 45 deg sloping end face can be extracted (again with the Face Binder tool) and swept along the straight side of the path. sloping_faceThe generated half straight side will perfectly match the curved side at the corner, but will end with a 45° sloping face, which will completely intersect and overlap with the corresponding ending in its mirrored copy.

The first prototype, ready before the deadline was fine. I was nevertheless not yet satisfied of this solution: working fine but not elegant from a design point of view. In the next post, the next design solution.

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