Fillet problem, please help


I have to construct a rather complex stair. One part where I am struggling to find the most efficient approach is this edge fillet:

basically I would like to be able to create a fillet that would begin and end exactly between the yellow lines… I tried the fillet command but I was unsuccessful to snap to the endpoints…
I will attach the element in case someone wants to try.

fillet problem.3dm (489.5 KB)

thank you


Hi Andreas.

Here’s one way. Split/trim the top surface in the desired location. Create blend curves between the top surfaces and use these to trim back the side surfaces. After this a Sweep2 with tangency to top surfaces.

fillet problem_blend.3dm (576.5 KB)

Hi Andreas - FilletEdge with the DistanceBetweenRails rail type and ~2>2.2 radius gets you pretty close but not spot on. Mark’s way may be the one, especially if it doe not have to be an exact arc fillet.


You can also create the trim curve by using an arc with tan/tan/radius. The blend curve was faster but would be the same steps to get to the sweep2

Thank you very much guys, it is really appreciated !



Rhino is a great SW , but the support of the community is unparalleled !

… the next fillet problem…

cannot filet this edge…

fillet problem2.3dm (1.2 MB)

This one is a little messy due to the curvature in both directions, but in essence the same method with one extra step.

Split bottom surfaces in desired locations
Create blend curves between bottom surfaces
duplicate bottom edges of the side walls and trim back with blend curves
ExtendSrf on side walls, make sure you go a little bit past the blend curves
trim sidewalls with previously duplicated edge and blend curve
Sweep2 to close it back up.

fillet problem2.3dm (1.3 MB)

Thanks Mark for trying to help me !
Unfortunately I did not completely understood your technique and despite your description failed to reproduce it.
The problem with these stairs is that the basic construction lines which I received are flawed, so at some parts it is impossible to generate a clean solution, because the master curves were done in 2D without to much thinking about how everything might work in 3D. So right now I try to solve these parts as best as I can, so they at least look “acceptable”.
I have to blend the upcoming stair strings into the stair landing. In order to create a transition which works for the all stair elements I thought I´d try to create some curves first.

Here is my current problem:

fillet problem3.3dm (148.2 KB)

I am trying to construct a curve like the dashed one, which would I.) blend tangentially between A and B and II.) follow the green curve in the top view… How do you do that? I tried a I know …


…here are some more elements of the stair, just to give you an Idea what I try to accomplish

fillet problem4.3dm (2.6 MB)

Hi Andreas - try:

  1. Make a line as an extension of the angled surface’s edge (white, below)
  2. Expplode the planar blend curve.
  3. Change the arc portion to degree 3
  4. Set the points along the angled lines, as near vertically as possible (they seem not to quite line up) from their planar locations:


thanks Pascal,

… you mean the rebuild command, right?

Hi Walther -no, ChangeDegree - that does not change the shape of the curve if you go up in degree. Here we just want four points (degree 3) rather than 3 (degree 2) without changing the 3d shape of curve at all.


Oh, did not know this one … thanks!

thanks again for your help guys, I just delivered the project… ! Can I buy you a beer? :grinning: :beer:



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