Non-Manifold Edges

There was a question on the Rhino group on LinkedIn about booleans failing due to "Non-Manifold" edges and just what that meant. There are two cases that fall under that heading, and in both cases if your model has them it means you've created a set of surfaces that could not in theory be turned into a real part(not by any conventional means like machining, rapid prototyping processes have their own rules.)

The first case, I'll explain using the simple case of 2D lines. Imagine you've got two lines that form a 90 degree corner, and a third that bisects the angle, so you have three lines meeting at one point. Rhino doesn't let you "Join" those 3 curves into one.

While such geometry is "illegal" for manufacturing purposes, it is actually not uncommon to to work with if you use FEA, so V5 has a tool called NonManifoldMerge that will let you create a polysurface like that pictured, it automatically splits and joins up all the edges of all the surfaces where they intersect.

Now in the second case, imagine if you have something like a sphere with a smaller sphere completely inside it, not intersecting at all, and you try to "boolean" the smaller one from the bigger one, if it would let you do it that would also be a non-manifold shape.

Rhino View Basics

There's been an issue come up from time to time on the newsgroup with apparent confusion over how the standard viewport views in Rhino work, that the Right or Left views seem to show the "wrong" side.


The above illustrates how Rhino defines the standard view directions, and is familiar to anyone who's had to take a drafting class. If your first introduction to "drafting" is Rhino, then perhaps some explanation is in order.

The confusion comes from the fact that with something like a car or a plane, the "Left" view shows what is commonly referred to as the "Right" side, it's referenced from the point of view of someone sitting in it. That's not how Rhino works, to put it most simply it's using more general-purpose principles.

You can always make up your own views that show you the projection you want with the label you want, but this is indeed the view convention used by all CAD users in all fields. I did not get different instructions on how to make drawings from the one car designer in the faculty of my Industrial Design school, or my introductory Architecture course, or my Engineering drafting class.