Hi blindfoldjump, Fillet is a pretty complex topic and so it may be a bit difficult to cover everything about it without writing a book or something. But I can try to give you some tips.

> 1. Would really appreciate if someone could briefly explain what the actual requirements are for creating a fillet/chamfer?

Some of the most common things that will cause fillet problems are complex corner junctures with many edges radiating out from a single endpoint, trying to use a fillet radius that does not fit within the available space without causing entire faces or edges from being totally consumed, and trying to use a fillet radius that would cause the generated fillet to fold back over itself and become self intersecting when going around a corner with the corner radius tighter than the fillet radius. Another thing that can be problematic is having surfaces that come close to being smooth to one another but actually have some shallow creasing between them like say 5 or 10 degrees deviation from each other rather than being fully smooth or more distinctly sharp. the problem is that any time you have sharply meeting things it means the fillets along those edges will not naturally meet up with each other like they do on smoothly meeting things, and then that will mean the fillet engine has to try to extend and intersect fillet pieces with each other at those junctures which is a delicate and complex calculation.


> 2. I attached a model where I cant get fillet to work. I tried to Join/Separate/Boolean union etc etc.
> before filleting/chamfering - nothing seem to help. If someone would have a look at it and see what Im
> doing wrong would be great.

Well you would want to use Edit > Join on it first to glue the pieces together so you have joined shared edges before attempting filleting on it. The regular way to do fillets is to do it on edges that are joined edges between 2 surfaces. It's also possible to do it on individual surfaces that are not joined, but with that mode you can only do it between 2 surfaces at a time and that type of surface/surface filleting does not try to build corner junctures between fillet pieces like the edge based one will do. But sometimes the surface/surface filleting mode can give you some fillet surfaces to work with where the edge based one is unable to work properly due to failed corner juncture handling.

But even after you use join on this example you've got numerous things in the geometry that are not good for filleting, like it's highly segmented resulting in pretty complex corner junctures especially with some sharply meeting pieces in those same junctures, like for example this type of thing is difficult here:



It would be better if the neighboring pieces here were all just one single large surface rather than built as a patchwork of smaller pieces:



In fact all these surrounding pieces would be better for filleting if they were just one large surface rather than separate fragments:




The thing that's bad about fragmented pieces is that filleting sharp things has a natural side effect of opening up holes, like for example if you want to fillet this edge here:




Then this is what the fillet engine is going to try and make from that:




So note the fillet generates a hole there - if the surrounding areas were part of one big surface the fillet will be extended and get intersected with that big surface and it will be able to automatically fill in the hole, but with the surfaces all being smaller fragments it becomes tremendously more difficult for the fillet engine to figure out how to get that area filled in.


> 3. I have read here on the forum that some people use additional software just for creating fillets.
> Is there any one in particular that works well together with MoI?

ViaCAD is probably your best bet for this, it has good filleting and also is quite inexpensive at $99:
http://www.punchcad.com/p-27-viacad-2d3d-v9.aspx

But it's probably also going to have problems on something structured like this made with a patchwork of smaller fragments and sharp pieces in it, those are some of the types of things that are just difficult to fillet well.

- Michael