Hi Bill - it looks like you're building this sort of one little sub-patch at a time, maybe with sweeps or network?

In a case like this where you've got a somewhat irregular outline that you want the final shape to follow it tends to be better to initially build a larger more simple extended surface and then use a boolean or trimming operation to generate the irregular outline rather than trying to construct surfaces that hug the outline directly right from the start.

Doing it in this other sort of patchwork manner has ended up with surfaces that are nearly smooth to one another but not quite - all the various pieces you've got here are meeting each other at shallow angles and that kind of situation is particularly difficult for the filleter to handle.

When 2 faces meet at a crease instead of meeting smoothly it means that the fillets generated from different pieces will also not meet up smoothly where they collide into each other and that means that they have to be cut back and a corner patch put in to blend that area together. But it's difficult to form corner patches very well when the angle is very shallow, the corner patch would need to be kind of compressed or sometimes just a little tiny slivery thing and often times the fillet engine will get confused if the fillet you are trying to calculate contains those kinds of difficult-to-handle things in them.

Instead of building in this kind of patch-by-patch manner instead try to initially ignore the outside profile of the shape and instead focus more on the overall full surface form that you want to have.

So for your case here maybe you would draw in just 2 curves like this:



And then do a sweep to make one big extended surface patch. Maybe think of this more like your base stock material:



Then get your outline curve as just a 2D outline curve drawn in the Top view like this:



In the 3D view it looks like this:



Then you can use the Edit > Trim command to cut your base form with that 2D outline, slicing it into pieces like so:



Delete the portion you don't need:




Then you can thicken that surface into a solid using Shell or Extrude.


If you construct your object more in this manner, it will make a much higher quality model that is all smooth throughout its whole interior and also contains a more sort of uniform shape throughout it, since it comes from just 1 constructed surface instead of a patchwork of them constructed separately. See here for some more examples of how constructing surfaces independently tends to make for creases between them:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.18
http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.19


So something constructed out of one larger smooth surface will be much more cleanly formed and should then have a lot better shot at being filleted.


Also though if you've got some pretty tight bends in the outline you may want to leave them sharp and then make it get rounded out by filleting as well - if your object has tight bends in it, that can limit the fillet radius to have to be less than the radius of that bend because otherwise it would cause the fillet to get bunched up on itself, see here for more discussion and illustration on that:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4828.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=4773.7

- Michael