Shelling problem

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 From:  3image
7966.1 
Hi there,

I've got this rather simple object here. This object should be made into a shell (extruding inwards). The yellow face represents the opening.

Even with only 1 mm wall thickness inwards the calculation fails. Outwards is no problem.

Why (or where) does MoI fail with such an object?

(Just a note here: No problem with Rhino v5, but I would like to understand the behavior of MoI in this case)


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 From:  Michael Gibson
7966.2 In reply to 7966.1 
Hi 3image, unfortunately the Shell command in geometry library that MoI currently uses is very weak when it comes to shelling, it can't handle anything that would produce a different topology like if 2 vertices need to combine together or branch out to more.

You can usually get an idea of what in particular is going wrong by using the Offset command. To do this, select your object (as a whole object, not as a face selection), then run Construct > Offset > Offset (instead of Shell), put in Distance = 1 and then check the "Flip" checkbox, then right-click or push Done. Then delete or hide the original object and look at the offset result and you can then see the spots that look weird where there were problems.

In your particular case here it's also a problem that there seem to be some tiny slivers on the bottom face - if you hide or delete all the other things than bottom face you can see these little sharp points coming out of it:



As a side note, the anti-aliased edge display can help to identify areas like that because multiple edges drawn on top of each other will be darkened a bit as a side effect, it's subtle though.

- Michael
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 From:  Michael Gibson
7966.3 In reply to 7966.1 
Hi 3image, I've attached an updated 3DM file where I've tuned it up by untrimming things, and then moving your diagonal cutting line over so it matches exactly with the other ends, it was off by about 0.02 units before.

But this will still not offset or shell well in MoI though because it's got one of those "diverging vertices" that I was talking about before, in this spot here one vertex on the original object matches to 2 vertices on the offsetted result, but unfortunately that is not accounted for by the geometry library that MoI currently uses:



- Michael

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 From:  3image
7966.4 
Thanks a lot, Michael. Now I understand better.
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