A new line appears on the edge of a surface when I use Shell. I can't figure out why. As I'm still officially a beginner, I have no fear in shamelessly exposing my shortcomings and begging for help online. In fact, I would be really happy if someone could educate me and point out what I'm doing wrong.

Picture 1: The shape as it is before I use Shell


Picture 2: After Shell (2mm / Flip).


Thanks!

Phil

Hi Phil, looks like a bug in shell, unfortuantely shell is one of the less robust areas of the geometry library that MoI uses so it's not particularly unusual for it to need some manual cleanup work for its results. Shell for thickening an open surface also usually works best for thickening an entirely smooth piece, it can have a lot of problems with dealing with an open surface that has sharp corners on it. Usually for something with sharp corners it's better to have a solid and then use Shell to hollow out the solid rather than trying to thicken an open surface. That's because with the solid case it tends to be easier for side walls to be constructed from existing objects rather than needing them to be cooked up completely from scratch. See here for some discussion on this:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5633.3

In your particular case here my best guess is in that problem area you've circled that the offsetter has decided to extend and reintersect those edges and done a kind of bad job of that process so that the offset edges don't match up very well to the original ones.

You'll need to repair that area by deleting the weird looking surfaces there, and then constructing a new piece to fill in that area which can be done in this case by selecting the 2 edges above, running Construct > Extrude to extrude a shape downwards, snapping on to an endpoint of one of the bottom edges to get the proper extrusion height, and then joining those pieces together.

There's also some badly formed areas on the hole area on the left hand side, those will also need a couple of pieces to be deleted, rebuilt, and joined in order to make a proper final result.

- Michael

Hi Phil, I've attached a repaired version of your object here which looks like it's all set up now.

After doing the shell I deleted the mangled surfaces you circled above and joined in an extrusion there to fix up that area, and then there was also a problem in the hole in the side which was a bit more complicated because in 2 areas the edges of the offset piece were crossing each other, that required those edges to be extracted as regular curves, trim them to make them formed properly end-to-end, then untrim and retrim the surfaces and join them back in again. Then after that I built a loft for a side wall in that area and joined it in too.

I'm not entirely sure why the offsetter is trying to extend the edges in these particular areas where there were problems, it looks like they were meeting up smoothly and so shouldn't need an extension.

It tends to be tricky to extend and intersect curves that are tangent or nearly tangent to one another because the intersection is sort of more like a "zone of overlap" rather than a crisp intersection point. So some of these mechanisms that involve extension and intersection can have difficulty when the things involved are tangent or near tangent.

- Michael

Hi Michael,

Thanks a lot for your explanations and for having repaired the model. It's really appreciated!

All the best

Phil