Best way to "crown or curve" a flat top curve that have fillet corners

Looking for a simple way to add a slight curve to a flat top with fillet corners. The points of the fillet end should blend into the crowned top with the new curve. I tried a few things like control points and curve blends… I’m just stumbling around and really dont know the “proper” way to get this done.

Basically, these are radius 10 fillets… next I want to blend in a small crown.

Hope this makes some sense!

Hello - you can use Fillet - with some trial and error to pick a good radius, or BlendCrv, and adjust the base points back along the white fillets.


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I was trying to figure out BlendCrv before I posted this, but it just seemed to not do what I wanted (or more then likely, not using it correctly).

I can get it to work by Joining, pulling up center point and then adjusting the dimpled middle point but its all done by manually dragging. I was hoping there was something where I can say “move 3 inches” and perhaps select the mid points of the radius and get a nice clean curve.

Basically I need to design this frame based on height, width and corner radius. So I start with a box.


I see workflow, below i used ArcBlend and move the the endpoint on the fillet till it was at the fillet midpt while still in the arcblend command. Just select the point and slide it.

Hope everything else in the image makes sense.

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If you really must have rational geometry, an exact radius of 10 mm, use overbuilding. But the curvature graph looks terrible. Better use a blend curve like Pascal suggested.

Crown.3dm (32.3 KB)


Was in a rush to get out and forgot to add File, its not the correct dims but it gives a nice Curvature Graph.

Arc Blend Crown.3dm (279.5 KB)

Thanks @Lagom for pointing that out will check the file you have done too, always good to see other workflows.

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Thanks so much @pascal, @Lagom and @mattgaydon!! Great information… I’ll have to digest this a bit, haha. These examples looks like what need though.

I’ll have to figure out how to use BlendCrv. I’m just 6 weeks into learning Rhino. I have a good handle for most of the basic tools from the Online Rhino classes, but not the more advanced ones and techniques.

@mattgaydon I tried doing what you explained. I was able to follow the steps, but in the end, there’s still a flat top, just not as wide, right? The goal is not to have any flat line (or at least minimally small). Looking at @Lagom example now. I think that may be what I’m after if I can figure it out!

Edit: Looking at the 3dm, its actually not as flat as it looked. Hmm… Have to play more because mine came out “flatter”.

ArcBlend from mid/top to mid/radius.

Just as a reference, this is a 6ft x 6ft box with 10" radius.

So between Matt’s idea and using BlendCrv instead ok ArcBlend, I think this behaves a bit better. Adjusting the continuity increases the curve as expected.

I think this is what I’m after. Does this appear like the best practice for my topic?

Also, not sure if this is a bug or not, but I have to toggle the circles (radio buttons) and then back to curvature. The Blend line is mostly straight until I start changing the options around.

A useful tool. Always look at the curvature graph. Unless you need rational geometry, you want it to be smooth as seen in the upper of the two examples. A degree 5 or 7 curve will be inherently smooth and you can later adjust the shape by moving the CPs and using the MatchCurve tool.

Again, unless you must absolutely have rational geometry (radius 10 or whatever), a single BlendCurve will always yield superior results and simpler surfaces later.

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In this instance i would just use a tangent to three line circle and then trim everything as needed. You can pick the command then select the two fillets as your first two inputs, and “eyeball” your third input the way you want it.

OR option two you could offset a horizontal line upward from the one you have, at the desired distance and then use the three tangent circle command between that horizontal line and the two fillets.

That’s what i would do. Simple, easy, clean.



Based on all the feedback (thank you everyone), this is my final (least steps) solution. Use one of the Tangent Circle options. Either works the same if you have symmetry. Just have to trim up the extra lines. Provides a great curvature analysis as well.

Thanks for all the feedback!!

Exactly how i would have done it! The cleaner your curves the better your surfaces.

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Just be aware that when you “piece together” such curves from arcs and splines, you have only G1 continuity, which may, depending on the product/client, cause issues later.