Hello, 

I have noticed that the Fillet Feature within Part Design in both 3D Experience CATIA as well as CATIA V5 has a weird functionality. 

On a further investigation I have noticed that the real problem is the "wireframe" of circles that are being extruded within both CATIA V5 as well as within 3D Experience CATIA.

To validate this I have recreated the same simple part within the same XY Plane, but both vertically and horizontally. When the sketch is created horizontally the seamline that connects the wireframe of the extruded circle will not be within the problematic area of the fillet, therefore both CATIA and 3D Experience CATIA will be able to apply the fillet.

This problem does not occur if the part is being saved as a STP file and then filleted directly on the STP file since the STP file will no longer the weird topology that extruded circles have in CATIA.

The case study part takes roughly around 5 minutes to model from scratch, I have also tested this both with projected 3D elements as well as new sketches and the functionality was the same.

Within CATIA V5 the error is the following: Internal error: operation cannot be performed 

While in 3D Experience CATIA it will created some weird 3D model topology with some reverse normals

Within SolidWorks this problem does not occur, as Solidworks does not add that additional wireframe line when extruding circles.

I am really curios if anyone has encounter this problem before, I have been using CATIA since 2008, but I haven't noticed this until now.

Here is an in depth video regarding this problem as well as the case study part that highlights this problem.

 #partPart Designsketchfilletextrude 

https://youtu.be/k7wUxFowSSE?si=pKR0r6wNwOctCz8E&t=508