Fillet brain freeze

 From:  Michael Gibson
3651.3 In reply to 3651.1 
(file received through e-mail)

Hi Tree, well one area where fillets can get confused is if you have difficult corner conditions where different fillet pieces are colliding into each other.

So for instance in your case here this is a probably a pretty difficult corner area:

That's sort of an "anti-tangent", you've got 2 edges that touch each other in a sharp corner with exactly opposite tangents. That means the fillet pieces will have some shared surface area in that spot and surface/surface intersections are pretty difficult to calculate in spots where there is shared surface area between pieces rather than a clear distinct line of intersection.

One quick note on fillet selections - you can also select faces to fillet and that will be the same as picking all the edges that belong to that face, so in some cases like this it can be easier from a selection standpoint to grab the face instead.

What fillet radius were you hoping to get there? I tried 0.01 units and it seemed to produce a result.

Some larger radius values such as 0.05 seem to produce a result but with some trimming problems, like for example here is the result at 0.05 units, the fillet surfaces were created but note that it had a problem figuring out that particular corner spot and did not get a fully closed outline there to trim with:

It would probably still be possible to salvage that shape and get the fillet in there by breaking those things into some individual surfaceds, drawing in some lines or something to get a proper trimming outline to use to trim the main parts away, and then trim the fillet pieces with some planes or something and then join the results together. Let me know if you'd like for me to do that, but it is a fairly involved and detailed process so it will take quite a bit of explaining.

There is an object repair tutorial that goes over some of those "low level" surface trimming and repairing operations available here:

Also one reason why you may be getting some weird results with pieces shooting off is there is actually an extra little tiny edge in one spot that you are probably not getting selected, it's here:

If you want to do a lot of filleting like this, you may want to get ViaCAD as a supplement to use along with MoI, ViaCAD uses the ACIS geometry kernel which has had a lot more time spent in the area of fillet development (which is a highly complex process) than MoI's geometry kernel. It seems to be able to handle this corner case better.

- Michael