Why Won't This Fillet?

 From:  Michael Gibson
4249.4 In reply to 4249.1 
Hi OSTexo - the difficult situation you've got there is trying to apply a second fillet that tries to hug the contour of the previously placed fillet at the same radius.

That kind of creates a situation where the fillet is collapsing down on itself, it's kind of degenerate situation.

I'm talking about the fillet surface that is going to be attempted to get constructed between these 2 faces:

You can kind of get an idea of what is difficult by starting with a smaller radius and making it progressively larger.

Here's with radius = 1:

Here's with radius = 2:

Here's with radius = 2.5:

So notice as you approach radius = 3 the edge on this side is basically collapsing down:

That kind of collapsing creates complications - particularly in a case like you've got here where the surrounding surfaces are a bit swoopy and not just at an exact 90 degree angle to one another. The previous fillet kind of has an upwards swoop to it which you can see if you look at it in the Right-side view:

Because of that swoop there will be some differences in spacing and alignment between the fillet pieces with kind of one side not quite collapsing down to the same point as the other side within the same fillet surface piece, until you go to a bit higher radius, that's why something like that can start to work again at a higher radius.

When you do the whole object, those areas where there is a collision between 2 fillet pieces of the same radius get done with a special corner-patching technique instead of it trying to actually build a regular tubular kind of fillet surface between them, that's why you can see different results when you do a fillet all in one go instead of in separate stages. It can be difficult to a fillet of the same radius that collide into each other in multiple separate passes instead of all at once because of this.

I'll see if I can come up with some tips for you on how to finish the construction.

- Michael