Can't get this object to cap and shell

Hi all, I have been battling a minor issue for AN HOUR. After drawing up a complete draft of a Delta 3D printer I have been bottlenecked by not being able to cap and shell a minute detail. It must be ‘Rhino Sundays’.

Anyway, I have made a file that speaks for itself, I cannot detect any geometry errors, curve errors, surface errors, the planes are all planar… And still it keeps on jamming on me. Loft? It won’t work. Sweep 2 rails? No luck there.

What am I missing? I hope someone else knows, I’m going bald.

The goal is to create the box on the right and shell it with a 1mm thick wall. In the example box the faces that are to be open are already removed.
issue_capping_shelling.3dm (2.8 MB)

Dunno, seems to work here. You have everything grouped, so I ungrouped, selected all the surfaces on the right, Join, Cap - that resulted in a closed object. Then Shell with 1mm. Seems OK…

Edit - Ah, I see you want to remove two faces from the object. hang on…

Yeah, Shell will fail on this because it doesn’t like the triangular surfaces very much - the offsets create intersections that are too complex for it to figure out. I can sorta shell this more or less manually - but there are a few different possible results… here’s one. Someone can probably do better.

ManualShell.3dm (189.4 KB)

This is probably a cleaner implementation, but requires more manual work…

ManualShell2.3dm (107.7 KB)

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