Just a blind guess, but since this part of the block is sloped, the void-cutting would result in very thin, sharp par of geometry still left visible, and doesn't handle those things very well

I think I see why you are trying to use a swept blend; I think that's what's generating the error. While the fillet shape is indeed changing between the 2 profiles, the swept blend changes between the profiles linearly, whereas the actual model condition does not. You'd have better luck with this logic if it was adapted to the Conceptual Modeling Environment (adaptive components / masses).

Edit:
In image 7, FINAL PRODUCT, you can see the bottom edge of the void is sometimes inside the shape, sometimes outside. My guess is that the change between the profiles is related to a sine, cosine, or some other hyperbolic function.

In the traditional family editor, I would "fake it" in the following manner: Create the top from the inside line of the radius you're trying to remove (just the flat part). Run the radius as a series of solid sweeps and swept blends around the perimeter. Use a blend between THAT outer perimeter and your bottom shape.

You have to do math or drafting to manually create your 15deg slope, but you were already doing that with the blend. Once you have the shape itself correct, you might find it more prudent to abandon the desire to join the geometry, as this might start introducing problems.

See pictures. They should be self-explanatory. No swept blend necessary.

Hey,

Thank you so much for your incredibly straight forward AND effective solution.

Apologies for not mentioning in my original post, but I need to create the same fillet edges to the base as I require to the top of the solid geometry. I recreated the extrusion shown in your Fig. 2 but with a radius to the bottom angled corner. The radius of this was lesser than that of the top radius and thus would great an error message when sweeping the fillet void using the solid edges as my path.

I was able to get the void fillet (as shown in your Fig. 4) to work when I made the bottom radius of the extrusion (Fig. 2) equal to that of the top radius (65mm), both of which were greater than the radius of my void sweep fillet (50mm).

If I have interpreted this correctly, it is not possible to sweep a profile with a radius greater than that of any of the radii which occur along the path of the sweep? Or possible, more simply, some radii are simply too tight for the profile to sweep, which causes the error, and that increasing the problematic radius/radii will result in the sweep working?

Once again thank you greatly for your time and knowledge - it is greatly appreciated!

Regards,

D

As long as the sweep is not self-intersecting, otherwise revit will complain.

There is another way of doing this - by adding instead of subtracting. I don’t see any other way of doing the angled part other than shown above though.

Thanks for everything! Greatly appreciated!

Revit honestly needs fillet and chamfer tool like inventor or autocad. be done in seconds.