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

Yes like Burr mentions it is really hard to diagnose a problem from a screenshot. If it is possible can you please post a 3DM file with hopefully a small number of parts that would let us examine the geometry and make some suggestions?

From the screenshot though it looks like possibly the problem is that the fin may not be booleaned with the other piece - most of the time before filleting you will want to boolean objects together so that they connect in a single skin with edges between them.

If you just have 2 objects that happen to be sticking through each other but not connected to one another, that could get in the way of filleting (unless they are 2 single surfaces, it is possible to fillet 2 unconnected objects in that particular case).



> My question is, can you fillet a revolved and/or lofted object onto a normal
> plane or box etc?

Yes, you can normally fillet a revolved object and a box, although since a box has sharp corners in it, it will tend to create a more complex fillet result that has "corner patches" in it.

Here's an example of a revolved object and a box, which have been combined together with Boolean union:



Now the common edges can be selected and filleted:




I'm not sure if that is the kind of revolve + box type situation that you are thinking about though.

- Michael