The “shell” tool is often dependent on the tolerance parameter (sometimes you need to increase the value for the emptying operation to work well), why not add a “tolerance” option to the command? (It would be avoided every time to enter in the panel options Rhino’s).
Hi Davide - please check the edge tolerances and vertex tolerances (in the `What`` command) after shelling at a loose tolerance - are they larger than the initial tolerance? If so, that would indicate this is a bad way to work…
-Pascal