Creating a rounded cap with continuity to a square base

Hey guys,

I am creating a pyramid/cone structure but need to have the tip rounded . I currently have half a sphere at the tip with my desired radius and a square plane as the base. You can see from the photo/file below that the continuity between the line from the base and the sphere don’t match. I tried to use BlendCrv but couldn’t get a smooth curve from bottom to top. Any advice?

microneedle.3dm (47.4 KB)

Hello - are you allowed to radius the vertical edges?


Yes, the edges don’t have to be sharp, if that’s what you’re referring to.

Sharper than .0023? And, how much leeway do you have for the transition? How long a distance do you get?


If you are looking for a more gradual transition from circular to square you could experiment with intersecting a flat plane with your sphere at the right spot to provide tangency with the sides or edges. Then use 2 rail sweep of a line using the square base and your circle that results from the intersection. If 2-rail doesn’t cooperate with the full circle and square (I’m not in a place to test it right now) try exploding the square into its individual sides and divide the circle into 4 corresponding quadrants and try the 2-rail on each of the 4 sides.

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rounding-a-cone.3dm (1.4 MB)
Hello NolanKim
This is something I wrestled with a while ago. Hope it helps.

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First, I strongly recommend you to use units that are good for the size of your object. For example, the base of your pyramid is just 0,25x0,25 mm, and the rounded tip has an extemely tiny radius of 0,0023 mm, which is way too small for the tolerance you have (0,001 mm). If you truly need this small size, then it’s better to build the object 10 times bigger and once you complete it, use Scale 3-D to make it 10 times smaller to fit its target size.

Back on the modeling approach. I purposely made this video a bit slow to make it easier to see what I’m doing. I do this kind of shape for the first time, so there were a few mistakes, but overall it will give a better idea how to create the rounded pyramid. But keep in mind that this technique will result into 4 walls that are NOT flat. They are slightly rounded towards the tip, as you can see from the video, because they were supposed to smoothly blend with the rounded tip.

Note that at the 6:20 minute of the video I created a closed curve. There is an alternative way to create the 4 main walls by splitting that closed curve into 4 pieces via the straight lines that go towards the 4 corners of the base, then use “Loft” to make a lofted surface between each piece of the split curve and each flat edge of the base. It’s up to you.

Rounded pyramid.3dm (104.4 KB)


I don’t have a specific leeway, but I’m just looking for a slightly smoother transition. The distance from base to top is 0.4mm.

Cool, how did you go about doing this?

Thanks for this. I will take a look soon. Yes, the small dimensions I have are necessary, so I’ll try your scaling approach and see if that helps.

The tiny size of the object also causes some visual bugs due to the rendering mesh settings (unless you force the polygons to be extremely small via your custom render mesh), but the NURBS geometry itself is of good quality.

It was a while ago but it went something like this:

  1. explode the pyramid.
  2. use the line from isocurve tool to set a depth for where the curved surfaces will fade into the corners.
  3. create lines from the mid points of the base to the pointy top (no idea what the technical term is).
  4. Cplane set curve to surface and pick one of the four triangular surfaces
  5. split the surface edges where step 2. meets them then use blend curve (tangent) to get the roundness and trim.
  6. repeat for all four sides.
  7. return the Cplane to the world view and trim the lines created in step three.
    Things get a bit hazy now.
  8. blend curve again and blend two of those trimed lines (opposites) in step 7
  9. line from midpoint tool. Creat a short line at the start of step 8 curve and sweep it along the curve.
  10. split this narrow surface by the rail curve and hide one half.
  11. network surface tool (tangent all round) and create the first half of the rounded top.
  12. repeat step 11 for the other half
    The red lines on the linework layer should help. I may have missed a step but this is roughly how it went.