If the goal is symmetry you could try Pufferfish’s “Mirror Cut Mesh” component.
Here is the mesh after splitting with my Python script:
This mesh has some issues. For the top half:
2156 pairs of faces that intersect each other
For the bottom half:
3953 pairs of faces that intersect each other
212 non-manifold edges
44 duplicate faces
All of these are in the starting mesh so none were added during the split operation.
The Rhino .3dm file with the split sphere is on Google Drive:
The split sphere is on layer Pieces. The starting mesh is on layer Start Mesh. The closed polyline used to split the sphere is on layer Ordinary. My script does not use cutting planes, it uses any number of closed polylines which can be straight or curved. For your case, I used a polyline with straight lines between the corner points.
The Python script is attached. It has an Eto GUI that works on Windows 10 and may work on Mac Rhino 6 WIP but not Rhino 5 as it does not support Eto I believe. To use the script, select Split on the first line of radio buttons, Reuse on the second line of radio buttons and then press the Split Mesh button. On my machine it splits the mesh in under 10 sec.
TrimMesh2Curve.py (289.5 KB)
Regards,
Terry.
Ooooooh what! Yes you’re right, I can split it too, now. I think it is due to some changes in the parameter of my mesh in comparison to a few days ago… Awww. Thanks for the great news!
Hi Terry,
Thank you for your help and attaching your great script!!! That’s amazing!
And good to know that Rhino for mac WIP is launched:)
Thank you!
Best regards,
Mengqi
Thank you Michael,
the problem is solved:)
Thank you again Seghier!
Hello Laurent_delrieu.
I just used your coded grasshopper for splitting the mesh with a plane and thank you, it works fantastic.
I have one question, can i change the code a little so the cutting is not done by a plane but by another mesh ?
Thank you.
Ian
It could be whatever you want you just need a signed value at each vertex of the mesh.
Because I am a newby with grasshoppe… can you explain this ( signed value at each vertex of the mesh. )
How do I do that with my mesh ?
Thank you. Ian
Splitting means you have to decide If it belongs to part 1 or part 2. For a plane you are above or under. For a closed mesh you are inside or outside … Up to you to find a criteria. It depends on your cutting mesh.
If you want more help you 'll have to post an example.
here you go. the two meshes in a rhino date and your code 
thanks for helping me.
Ian
cut mesh.gh (13.3 KB) cut.3dm (4.1 MB)
Why not using “Boolean split” in Rhinoceros or Grasshopper ?
Here a way using Grasshopper and IsoSplitting, but it isn’t the way to do what you want
cut mesh.gh (2.4 MB)
The only advantages I see there is the low number of faces.
Hello, thank you so much for your method but I want to know how to close the mesh after spilling it so i can work on it please .Thank you
Hello
There are many answers as it depends on the splitting surface (a plane or more complex form) and what you want to close.
The most simple is surely to use that
Close Mesh from Pufferfish plugin
Also i have a problem of medelling my meshes , i want to deforme them to get to something similar to the image … ( by combining two sphere with cocon ) like the the precedent image i sent you. thank you
At this step it is not possible to help without more description and some material to test. Post script with some meshes. I have no 
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the difficulty that I’m facing is modelling and transforming meshes to get free forms similiar to the exemple that I sent you but I dont know how to do it on grasshopper but I can’t join the script because it’s too
bulky. Can I have your adress email so I can send you the link with we transfer please ?. Because it’s an emergency, my finall exam in February. Thank you so much for your help ``
Hello,
I am not your tutor nor it is my full time job to help on this forum.
So split your problem in little parts and put it on this forum. You 'll have to make the same thing in professional life so it is best you begin early. And also there are many people that can help on this forum at whatever time. Me it is just at 9pm to 10pm French time (example)
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