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# Re: Why can't I draw this?

Valued Contributor

Ryan: (The real problem for the CAD software is to determine the vector normal for the edges of the 3d solid. Since you have two edges that share a common edge the vectors point in opposite directions for the same edge. You can't have two vecotrs define the surface normal in a surface definition. That is what causes the problem.)

I have ... two edges that share a common edge the vectors point in opposite directions for the same edge

and I am still able to do solid.

So why it is alowed?

Sean,

In your sample vectors of two surfaces in the contact location are same and are opposite, just like mine.

So its is the programing.

Case closed.

I have to go back to my work, I am not on the payroll or benefit plan.

milanw

Milan Wendl, MEngSc, P.Eng.

# Re: Why can't I draw this?

Phenom

@milanw- Sorry I can't open the part. Must be ST8.

Anyway, I hoping the following text from the Parasolid XT Reference document helps clarify things.

"Restrictions on entity relationships for manifold body types Solid, sheet, and wire bodies are best regarded as special cases of the topological model; for convenience we call them the manifold body types (although as stated above, a general body may also be manifold).  In particular, bodies of these manifold types must obey the following constraints:

• An acorn body must consist of a single void region with a single shell consisting of a single vertex.
• A wire body must consist of a single void region, with one or more shells, consisting of one or more wireframe edges and zero or more vertices (and no faces). Every vertex in the body must be used by exactly one or two of the edges (so, in particular, there are no acorn vertices).  So each connected component will be either: closed, where every vertex has exactly two edges; or open, where all but two vertices have exactly two edges each, and the  A wire is called open if all its components are open, and closed if all its components are closed.
• Solid and sheet bodies must each contain at least one face; they may not contain any wireframe edges or acorn vertices.
• A solid body must consist of at least two regions; at least one of its regions must be solid. Every face in a solid body must have a solid region on its negative side and a void region on its positive side (in other words, every face forms part of the boundary of the solid, and the face normals always point away from the solid).
• Every edge in a solid body must have exactly two fins, which will have opposite senses. Every vertex in a solid body must either belong to a single isolated loop, or belong to one or more edges; in the latter case, the faces which use those edges must form a single edgewise-connected set (when considering only connections via the edges which meet at the vertex)."

# Re: Why can't I draw this?

Community Manager

All,

I am not sure all the techy math explanations help. Let me summarize here:

1. Most modern CAD systems use Parasolid -- the best solid modeling kernel around. Used by Solid Edge, NX, and SolidWorks to name a few.

2. The "non-manifold" condition is not allowed in Parasolid when solid modeling. Therefore it is not allowed in Solid Edge (or NX or SolidWorks or...)

3. Parasolid is written and maintained by the brightest set of PHD mathematicians on the planet. So whatever the math reason, it is unlikely to change.

4. So we all accept it and move on.

Dan Staples
Director, Solid Edge Product Development
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# Re: Why can't I draw this?

Gears Esteemed Contributor

Can we get an Amen?

Bruce Shand
SE2019 MP8 - Insight - Win10 - K4200

# Re: Why can't I draw this?

Legend
Don't forget, you can make two entities into two separate bodies and avoid the non-manifold error. This would work for two tangent cylinders, but not the tangent hole. in that case, a .001in. (,02mm) space between the edges is all you need to make it work.
Using SE since V12, 2002
ST10 certified