Hi Marco, so the part I don't quite understand is why you decided to make this part with a sweep or network instead of a fillet:



Was it something having to do with the side pieces? :



I mean something like wanting the side pieces to connect up with the larger round edge-to-edge?

The thing is you can't really make a good quality surface that is constructed directly to a boundary like that. To have a good shape that isn't pinched or skewed weirdly it needs to be a surface that is extended and trimmed like how a fillet is made.

If you put in a bunch of isocurves on that surface you can get a better sense of how that surface is "stressed":



It's trying to transition between 2 different orientations:



While at the same time it's trying to slide directly between these different length sides which is where you'll get a type of skew problem:



That's too many things it's trying to do all at the same time so it's going to have bad shaping. Like in this spot here it's going to have stuff like bumps in it but in a very small area so you would have to zoom in a lot to see them:



That's what can happen pretty easily with a "fill in areas patch by patch" method. Even though yes there is a surface that fills in that area if it's an irregular boundary your surface can be a lot messier than what it first appears. To relieve the stress the surface would need to be extended in a more natural way something more like this:



and then it would have a trimming boundary on it to cut it (created by intersecting it with the other neighboring surfaces) while the "underlying surface" remains with that regular shaping. Hope that helps explain some why that is not good.

So the thing is filleting will do all that for you so that's why you would want to use fillets to do all those areas instead.

- Michael