Sphere problem


#1

I can’t build a hollow sphere doing boolean difference of two spheres!! It’s easier than draw the contour!!


(Wim Dekeyser) #2

For a boolean operation to work, you need objects that intersect.
There are no ‘solids’ in Rhino - and therefore also no ‘hollow’ objects.
What are you trying to achieve?


#3

Rhino does not have a concept of “solid” or “void” -spaces, A “solid” in Rhino is just a set of surfaces enclosing a space.
So by the same token your sphere inside a sphere is just that, two closed surfaces one inside the other. But not a “hollow sphere”.

Max.

Edit: oeps, wim was sneller… :unamused:


#4

thx, both. I was trying to build a “bubble with thickness”. It is for an architecture competition. I had it modelled in Autocad and i was trying to render in 3Ds but it doesn’t save the archive.Then, I tried to import to Rhino but it gone wrong… Then i tried to model in Rhino… Here we have the road that bring me to the forum :wink:


(Brian James) #5

You might be able to do what you’re after using NonmanifoldMerge followed by CreateRegions. Then delete the interior sphere and you’ll have a solid shell.


(P Krishnendu89) #6

Hello Brian James,

I am very new to Rhino 3D and my problem is very similar to what you have suggested.

I want to model a half hollow solid sphere. I tried using NonmanifoldMerge command after creating two spheres. Please note that, I used trim command to create two half spheres. After NonmanifoldMerge command, the two half spheres are merged. But, I am not able to use create regions as you have suggested in your earlier post. I need to finally model a solid model of half sphere with 0.10 m wall thickness. It will be of great help if you could guide me with the modelling.
Thank you.
Regards,
Krishnendu


#7

For a half-sphere, it’s quite simple.

  1. Create the outer sphere with the correct diameter, use world 0 as its center
  2. Create the inner sphere with radius smaller than the outer sphere by the wall thickness, same center
  3. In Top view, draw a plane surface (rectangular) bigger than the sphere (actual size doesn’t really matter, the plane surface should pass through the sphere in the middle and extend beyond it)
  4. Select the 3 objects and call “CreateSolid”
  5. You will be left with 4 solids. Delete the half of the sphere you don’t want and the two inner volumes.

There are at least a dozen other ways of doing this - here is another

  1. Create the outer sphere with the correct diameter, use world 0 as its center
  2. In Top view, draw a plane surface (rectangular) bigger than the sphere (actual size doesn’t really matter, the plane surface should pass through the sphere in the middle and extend beyond it)
  3. Use Trim to trim off the bottom of the sphere, delete the trimming plane
  4. Use OffsetSrf with the wall thickness desired and solid=Yes.

–Mitch


(P Krishnendu89) #8

Thank you for your quick response Mitch.
I could create a hollow solid semi-sphere.


(P Krishnendu89) #9

Dear Mitch,

I could create a hollow sphere with desired thickness but I am unable to save in .gdf file format (suitable to work with Nemoh-open source software). It says “Wamit file format does not support trims and untrimmed surface information will be written”. I had to mesh the model in Rhino and save as .gdf file compatible file for Nemoh. Is there any solution? It will be great if you could help me in this issue.

Thank you.
Regards,
Krishnendu


(Pascal Golay) #10

Hi Krishnendu - if you ShrinkTrimmedSrf your half-sphere-solid, does that work better?

-Pascal


#11

Well, yes, you can make a hollow hemisphere out of untrimmed surfaces. The easiest way is to start in 2D.

  1. In Front viewport, draw an Arc with the _Arc tool, center at 0, outer radius, and 90 degrees counterclockwise up from the X axis. (if you have ortho on this is easier)

  2. use _Offset to offset the arc to the inside the desired wall thickness.

  3. Connect the two arcs along the X-Axis with a straight line and then _Join all 3.

  4. Use the Revolve command and draw a vertical line from 0 upward with ortho on - the length doesn’t matter. Choose the FullCircle option on the command line in the Revolve command. Done. You will have a solid, and the surfaces will all be untrimmed.

–Mitch


(P Krishnendu89) #12

Hello, Thanks for your reply. The solution suggested didn’t work.
Regards,
Krishnendu


(P Krishnendu89) #13

Hello,

Thank you again.

I could model without using trim command as you suggested. Error with trim command is eliminated. Still not able to save as .gdf file. A file is getting created when I save but with a file size of 0 KB. What could be the problem now?!


(P Krishnendu89) #14


(Pascal Golay) #15

Hi Krishnendu -

see the attached files - are these what you need?

RevolveHemisphere.zip (45.5 KB)
A couple of things:

Wamit does not understand rational surfaces so true arcs and spheres will be distorted. In the example, I’ve rebuit the arcs as degree 5, six point curves (Rebuild command) which makes a very close approximation of the arcs and I used the ‘Deformable’ option in Revolve with 32 points.

As far as I can see Wamit does not pay attention to solids - it comes back in as surfaces.

Try exporting my object from the Rhino file to gdf - does that work?

-Pascal


(P Krishnendu89) #16

Hi,
When I mesh the hemisphere model attached here, it is again showing the same error. I have only used mesh tool in the model attached.

“As far as I can see Wamit does not pay attention to solids - it comes back in as surfaces.”- so are you trying to say I needn’t model as hollow model? Please help me understand.

Thank you again.