Hi chipp, thanks for your order, I hope that MoI will be useful for you!

> When I subtract this "parting line" solid from the larger widget solid, the resulting
> object is hollow. Why is this? How do I fix? (Newbie issue).

The problem is that your larger widget solid is not actually a closed solid, it has some open areas in it and does not make a watertight volume yet.

You can tell if an object is a solid or not by the "object type indicator" that is displayed in the upper-right corner of the propeties panel when you select it. When you select a solid it will show like this:



If your object is an open surface, the object type will read as either "surface" if it's just one surface, or "joined srf" if it's made up of multiple joined surfaces but not all closed up to be a solid. Yours reads as "joined srf":



This usually means that there are some "naked edges" in the model which means edges that only belong to one surface rather than joined between 2 surfaces like edges of a watertight solid need to be.

The best way to find these naked edges is to set up a script on a shortcut key as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3540.2

Once you set that up you can then push N and have the "naked edges" of your model get highlighted so you can see if there are any, and where they are. If you do that on your model you'll see these areas highlight:




These areas will need some repair work on them.

There seems to be 4 areas and 2 naked edges in each of them, it looks like the fillets here slightly overshoot each other rather than coming to a single common ending spot:






Hope this helps!

- Michael