Hi guys, I am trying to find the volume of a simple half sphere that has been shelled.  I am using this site as a guide:  http://www.engatech.com/nltip080401.asp

I am having trouble with the "combine" feature.  I am trying to combine then subtract just like the guide shows, but I get a "zero thicknes geometry" message.  I am hoping to be able to change the size and/or shape and allow auto updating of the volume.  Thank you very much for any advice!