@wim, that was impressing! Would you share the gh file?
Anyway, I guess it’d be a bit trickier if placing a geometry at a fixated position along the edge of the opening in the sphere, which constrains the placement of the moving groove.
Nice animation. I’ll get there too one of these days. Eager to learn more.
// Rolf
Sure. It’s -very- file specific though. But if you know up front that you will have a few cycles with trial-and-error in the design process it might be worth-while to use a bit of time to set it up in Grasshopper. This example shows that you can take in an existing model and pull it apart in GH before putting things back together - you don’t necessarily have to start all over in GH. Be careful with shrinking geometry in Rhino though. The initial sphere shouldn’t be shrunk and for the cutter I noticed that Rhino over-sizes the length of the fillets quite a bit. So those could be shrunk after filleting (but it wasn’t necessary in this particular case).
Parametric Cut-internalized.gh (215.2 KB)
I used a GH definition that Daniel Piker posted in March (thanks, @DanielPiker!) . It is included in the gh file that I attached here but here’s the link to the original URL as well. You just have to see the elephant 
Animated gif encoding within grasshopper - Grasshopper
No problem, I may still learn from the example.
Many thanks!
BTW, the elephant probably broke its legs. 
// Rolf