Fillet not calculating

martin_borzan

I could use some advice/help on how to do this operation. Any feedback is appreciated!

I have an object that is essentially a loft between a circle and a hexagon. The hexagon has alternating straight and rounded edges. 


Now I am trying to create a fillet along the connecting edges to round over the part. The operation works just as expected for these four edges: 


But the other two edges won't compute properly and I don't know why:


The only thing these two edges have in common in contrast to the others is that their intersection with the circle (back face of the object) is tangent to a major axis (the Y axis):

Perhaps when Onshape calculates a fillet, there's some division by 0 operation going on there due to the relative position to the major axes or something? That's my best guess but I have no idea what the real reason is. Any idea how to get past this problem?

Thanks!

S1mon

Those edges may not just go tangent, sometimes you’ll get strange results where it will go from convex to slightly concave. You can use the dihedral analysis (under the protractor tool in lower right) to check what’s happening with that edge. If the edge goes to zero, a regular rolling ball fillet should work, but if it inverts, it will fail. 

You’re probably better off adding carefully placed splits to the circle and fillets in the truncated triangle sketches before lofting, or you’ll need to manually build some offset edges from the ones you’re trying to fillet and then add a loft or boundary surface. 

If you can share a public document, it would be easier to help. 

martin_borzan

Here's the Dihedral Analysis of the edges: 

They all look the same for all the edges.

You can find the body under this link:
https://cad.onshape.com/documents/236592654d131a5e2e2d0b3b/w/af9e5388a33fec23cc9dfb76/e/8d0ae55bae74e571569397f4

S1mon

Hmmm. Oddly I can get a single edge to fillet, but some of the edges fail.

I tried a few things to add nice looking fillets to the part, but in the end I manually created them by sweeping a circular section and splitting surfaces and then lofting the "fillets".

https://cad.onshape.com/documents/b131fd750fc1a38f5cc9e2fd/w/50dc81a122f70f8b0471fdc2/e/bdf3e305799afa0f33358d6b

martin_borzan

That's not quite what I was going for, although I like the approach. The look just isn't right. 

Any idea what's causing the fillet issue?

wayne_sauder

@martin_borzan
 I am not sure if this is on track, but if you select faces instead of edges you'll get a different result however the spot marked by the arrow needs help. 
 Not sure of the exact look or feel you're going for and maybe this is all wrong.  

wayne_sauder

@martin_borzan
Something else to consider onshape is not going to play well with trying to fillet one edge that comes into a set of (I think G2) curves. There are some ways to get results but I doubt you would be happy with them. 
 

martin_borzan

@wayne_sauder I am too amateur to understand that last point about G2 curves. Any chance you could explain it in layman's terms? I've only been using Onshape for about 2 weeks or so now...

wayne_sauder

@martin_borzan
 I'm sorry, I believe I used the wrong term, there are a number of fellows on this forum that understand curvature-related topics better than I do. If you want an interesting, deep, and challenging study sometime, google the subject of surface or curve continuity. 
 
 Basically, you have multiple arcs but the curve of each one matches the next one perfectly, in essence making a complete circle. When you try to fillet the edges you are asking the software to change the radius at those points which would require the removal of material (not at all what you are looking for and onshape knows that), so instead the software is trying to fade the fillet into nothing at that end. It should be able to do this however there are 2 spots where the loft is behaving oddly, I did not spend the time to figure out why but I suspect it has something to do with the direction the arcs are drawn. (clockwise vs counterclockwise, loft can be picky about such things at times). Hope that makes some sense and helps a bit.