help for " shell"

 From:  Michael Gibson
133.11 In reply to 133.10 
> Is it normal that MoI performs differently for both objects ? After all, they are very similar in shape.

It's not normal to go into an endless loop, that's definitely a bug.

But it would be normal for it to work differently between those 2 objects, with it failing on the left one. I'll explain why.

If you do a "Separate" on that shape, and then offset each surface individually, you can see this is what happens:

And zoomed in:

You can see that those surfaces don't make a clean edge-to-edge intersection there. The surface on the left there (without the curved edge) needs to be extended to make it work properly. The extension mechanism normally works by extending the surface out and then intersecting it with all the other ones that neighbor it. But since one neighboring one is missing in this case, it prevents the extension from working along that side.

Since this is a simple case, if it was smarter it could know how far to extend it here without needing to intersect with the missing surface, but it doesn't work like that because that wouldn't work for the general case where the surfaces are all bendy and curved (it is actually difficult to extend a curvy surface by a constant amount in one direction, it is easier to extend it further and then trim it back).

So basically, having the other surface there helps make the extension-and-trim-back mechanism work properly, and extensions are frequently required when doing a shell. Does that make more sense?

And actually, even if that was able to extend properly, there is another reason why doing it with the solid is better - it creates a cleaner result.

Here you can see a box that was open on one side that was shelled. The side surface in this case is created by doing a bunch of individual lofts between surface edges. This creates several surfaces instead of a single surface, with more edges. These will tend to cause complexities with further operations such as filleting. When you do a shell with a solid box instead, it will trim that surface to make a hole in it, so you'll only have one surface there without the extra edges that are circled.

Does this all make sense? Let me know if you need more info.

- Michael