Hi Timo, filleting is a pretty complex topic, and one of the things that makes it complex is that you may need different strategies in different kinds of situations.
So that situational nature makes things somewhat context dependent and it's a bit difficult to come up with just some simple "one size fits all" kind of rules that are always going to work in every case.
It can be easier to discuss it if you can give some example models that you'd like help with, that would help to give some context to things.
But I will see if I can give some of the most common tips here...
> Is filleting in some cases then separate process that you
> do after modeling, and you just find out how to get exact
> fillets that you are looking for?
Pretty often it comes in later during the modeling process because a lot of times NURBS models are made up of different modules that are booleaned together, and normally it's only after the pieces are booleaned together that you then do the fillets.
There is an exception to this, there is a kind of filleting called surface/surface filleting that builds a fillet surface just between 2 individual surfaces, that particular case does not need to be booleaned together first. But edge-based filleting where you select edges or faces to fillet will need to have the pieces booleaned together first.
> I have been filleting some splines before making surface
> to save work afterwards.
Yes, this can work but it only applies to certain kinds of situations where the result is sort of self contained instead of being made up of multiple booleaned-together pieces.
But if you are doing something like an extrude or revolve or sweep for example where all you want are some rounded edges on the resulting object, that can be a good situation where you can fillet the profile curves and then have rounded pieces get constructed from them (instead of filleting the edges of the resulting solid). But often times these are not very difficult situations for edge-based filleting to handle ok as well though.
One tip - edge-based filleting can be sensitive to having edges fragmented into multiple pieces instead of made up of long single edge segments. There is a Merge command that can be used to glue together edge segments that touch smoothlly into longer single edges, that can be useful in some circumstances:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#merge
Another thing that can help for filleting in general is to make sure your original profile curves are cleanly constructed and don't have things like excess fragmentation in their segments or have areas where 2 segments are kind of close to being tangent to one another but are actually off by a like 5 degrees or something, that can happen if you're just kind of "eyeballing" that things are close to being smooth instead of using tools that guarantee tangency like tangent/tangent arc construction, blending, or filleting of the curves. It can be hard on the filleter to resolve intersections in areas where the fillet surfaces are colliding together in places where things are close to being tangent but not quite and are a few degrees off.
One tool that can help to clean up messy curve structures with too much segmentation is the Rebuild command:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild
But in general it helps to make sure your initial profile curve structure is nice and clean before you even start to build surfaces from it, if there is any messy conditions in your initial curves that will tend to have a ripple effect and cause problems down the line as well. It takes a lot less time to fix these problems in the initial curve framework to start with rather than trying to fix the surfaces later on.
Also another thing that can produce fillet problem is if you have little tiny edge or surface fragments in your model. That can get caused by messy profile curves, but it can also be caused by a lack of precision when positioning 2 pieces together before you boolean them. You want to make sure to position things accurately using object snaps to make sure things are aligned properly before doing booleans. If things are a bit off when booleaning 2 pieces together you can end up with little fragmentary pieces instead of clean cuts. That's another thing to watch out for.
- Michael
