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Cannot fillet pointy object

james_harton433james_harton433 Member Posts: 4
Hi folks.

I'm making an RC aeroplane, and I made this fuselage shape by lofting a bunch of surfaces between splines on different planes. It worked well.  My problem is that I can't fillet the sharp edges - I think because the nose and tail come to a point (which is one of the reasons I want to fillet it).  Screenshots attached. The model is here.

I've tried a bunch of combinations of doing multiple fillets, different options, etc, but I can't make it work.  Any suggestions?


  • Options
    S1monS1mon Member Posts: 2,449 PRO
    edited March 2023
    Surfaces in Onshape are based on NURBS (like just about all other mechanical CAD). They inherently want to be 4-sided. Think of them like a rectangular wire mesh or sheet of rubber. Now imagine what happens when you have that 4-sided surface and you force one edge to be a point - or in this case two edges. It's possible, mathematically, but it is considered degenerate. Anything which is uneven in the middle of the surface will be very nasty at those  points.

    It's possible to build these surfaces other ways so that they don't have the same weirdness at the points. You can use the fill surface tool and select two profiles at a time - it will try to fit a 4-sided surface to those profiles and then trim the surface.

    The other issue is that a bunch of fillets all coming together can be very hard to solve for even in the best situation. 

    What you can do is build a blocker at the two end points - extrude a small cylinder. As long as your fillets don't all overlap, you should be able to add them. Then you can delete face on the cylinders, and fill in the hole.

    This guide for Solidworks can take you through some of the tough issues with filleting, and since Solidworks and Onshape are based on the same kernel - Parasolid - the solutions are broadly similar.

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    S1monS1mon Member Posts: 2,449 PRO
    Ok. Now I've spent some time with your model. I would say that more of your issue is coming from the curves which you're using to generate the surfaces. You're using the standard degree-3 splines which are hard to keep fair. I would do this all with Bézier curves. 

    The first curve in Sketch 1 is not too bad, but later ones are generated in ways which twist the angles between the surfaces, and then you're creating the fills off the edges of the previous fills instead of the sketches. The sketches are likely to be smoother. Using the edge of the fill is bad because it's already been rebuilt in an automatic way which is a little like taking a photocopy of a photocopy - it degrades the mathematical quality of the profiles going into the next fill.

    Sketch 1:

    However right after you add fill 6, one of the edges of fill 1 goes crazy - I believe the expression is yeets off to infinity...

    It's also really weird how at the front, the bottom two surfaces are almost horizontal:

    And then at the rear, those same two surfaces have a very sharp dihedral, and their relationship to the next faces are concave instead of convex.

    When an edge flips from concave to convex, filleting can get weird.

    The front and rear are also messy even though you did use fills. You might be able to clean up the curves which run along the length of the fuselage, and then use some of the techniques from the Dimonte stuff I linked to above to get this to fillet and then use some sort of fill at the front and rear.

    However, the whole fuselage could be rebuilt to be much smoother and more aerodynamic, but it would take a very different approach. Is there some reason that it's being built with these facets? 
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