Hi Holger, where is it that you're trying to put the fillet?

One thing that will be problematic for filleting in your case here is that you some very narrow surfaces and filleting will fail if the fillet is large enough that it would completely consume away an existing surface. So you'd probably be limited to a very small fillet radius here.

The problem area would be in spots like here:





That's a pretty skinny surface, only about 0.02 units wide - if the fillet you want would need to eat that totally away it won't work.



Another problem area can be if you have 2 surfaces that come close to being smooth to one another but are actually not entirely smooth with say a 5 to 10 degree very shallow crease between them. It looks like you have some things like that in this model as well, that would be spots like this:




As 2 surfaces get close to smooth, any fillet between them starts to become very small in size itself and when it gets too small it can make the various calculations difficult to process very well.


Sometimes it can be better to make an initial model that has all very distinct sharp edges in it and then use filleting to put in all the rounding, rather than trying to create rounded surfaces using sweep or loft (which will tend to end up with the "nearly smooth but not totally smooth" type problem). There's an example of that here, I'm not entirely sure if it applies directly to what you're trying to do but maybe:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1002.2

Do you have any photos of the keyboard that you're trying to model?

- Michael