Hi Martin,

re:
> Is there a way to retain the part that is cut out and discarded from an internal shell action?
> I'm thinking that Keeping that part might be useful for cores in casting work.

If you want to generate a solid for the inner void, that should be possible either by using the Offset command instead of Shell, or you can also convert a result from Shell with an inner void and convert it into 2 solids by selecting it and then use Edit > Separate to break it into individual surfaces followed by Edit > Join.

Join does not try to generate inner void cavities.

To go back the other way if you have 2 solids and you want one to be a void cavity, you can use Boolean difference to make that. Select the outer solid as the base object and the inner one as the cutting object.

- Michael