Hi Dan, it's not that you can't fillet any sweep, but fillet is a particularly sensitive function and if you have irregularities in your objects it will tend to cause problems with filleting.

The reason why fillet tends to be sensitive is that it does several pretty intensive calculations inside of it to get the final result, including calculating offset surfaces, then intersections between those surfaces, creating fillet surfaces and extending them, and also calculating the intersections between fillets and possibly filling in corner areas where they have collided.

If there are problems with any one of those steps the fillet will not be successful.

In your case there are several areas that are problematic - one of the worst is the shape of the surface in this area here:



If you zoom in a bit on that area, you can see that the surface in that area actually goes through a very sudden and violent fold in its shape:



Here's a view of the surface control points to give you some idea how chaotic the surface point arrangement is in that area:




That means that the surface normal very suddenly and violently changes direction right in that small area, that pretty much hoses the offset calculations.

It looks like that bad surface may have been created by trying to use a scaling rail that did not fully cover the whole sweep. When you use a scaling rail, if the scaling rail stops before the end of the sweep the sweep will revert to its natural non-scaling-rail shape in that area, which tends to make for this kind of sudden shift in shape. When using a scaling rail it is a good idea to actually extend the scaling rail to be longer than the sweep to make sure it covers the whole thing.


It can also be difficult to fillet things that collapse down to a single point like you have on the other side of the shape, because it is pretty easy for the surface to have little tiny bumps and folds as it kind of bunches together when it is collapsing down, and any kind of little tiny folds will mess up offsetting.


I'd actually suggest smoothing out your initial profile curve before doing the sweep, that way you should generate a nice shape right after the sweep. So for instance fillet the profile curves to put in arc or blend segments there, or unify the profile curves into a single smooth segment by turning on control points and deleting the control point where the segments touch each other.

In general it is a big time saver to try and manipulate the profile curves first when possible.

- Michael