Hi Janine, I'm glad that you like MoI!

I don't think that you've exactly done anything wrong here, I'm afraid that sometimes the filleting is just overly sensitive to certain things.

In this case, somewhere along the line some very small inaccuracies have come into the model:



These are not necessarily abnormal, there are a lot of times that there will be gaps similar to this which are normal if they are within the modeling tolerance. But in this particular case these little things are giving the filleter a headache.

I've attached a reconstructed version of your model that is a little bit more accurate, and you should be able to fillet this one now.

To increase the accuracy, what I did was I went back and filleted the original curve and the revolved that profile that already had the curve corners rounded. This generates a more simple surface object than if you do an edge fillet on the already-solid model. A lot of times you do not have to worry about this, but in this case I think something went slightly off in the intersection between shapes, so having the surfaces as simple as possible helped increase the accuracy. I did a similar thing to the cross-piece, that is did a curve fillet on the original shape before extruding it.

One other quick tip - there is a quick way to fillet all these edges instead of selecting all the edges one by one. Instead, select the top surface of the in-between piece. Now drag a window select from the upper-right towards the lower-left - when you drag in this right-to-left direction the outline will be dashed, and anything that intersects the window will be selected. When you select from left to right, the outline is solid and only things completely inside are selected. Anyway, drag that window right down the middle and select all the faces of the in-between piece, and then do a fillet, this will get all those edges. When you have a face selection and you do a fillet, all the edges of the selected face will get filleted, with already-tangent edges being skipped. Sometimes it is a lot easier to grab the edges by selecting the faces than it is to grab all the edges.

Here is an image that shows this step:



I hope some of this info was useful, let me know if anything here wasn't clear.

Again, you didn't do anything wrong, you just happened to run into an area the filleter is sensitive about, which is not terribly unusual I'm afraid.

- Michael