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I'm trying to understand Blender constructs and manifold vs non-manifold geometry.

I do a lot of modeling for 3D printing, so really need to get this right.

I've looked at a number of tutorials on YouTube, and Googled the subject extensively, but still cannot find anyone/anywhere that explains the basics. I have tried all sorts of things, but I'm still confused.

Let me start with an example:

I've created a cube manually.

1)Set Snap-to to "edge"
2)Turn on Snap
3)Add --> Mesh --> Plane
4)Copy plane --> rotate 90 degrees on the x-axis
5)Move new plane and snap to edge of 1st plane.
6)Repeat (similar) steps 4-5 for remaining 4 sides of the cube
7)Join all into one object
8)In edit mode, select all
9)Merge vertices (by distance) and confirm only 8 vertices now exist
10)Using 3D Print addon, do "Check All"

This shows 20 non-manifold edges. If I use the addon's "Clean Up" --> "Make Manifold" option, it removes three of the six faces and still shows eight non-manifold edges.

I simply don't get it. Can ANYONE help me with my understanding or point me to a place where I can get it?

Many, many thanks in advance!

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  • $\begingroup$ Check the directions of your normals - perhaps try ‘Recalculate Outside’ before you Make Manifold. $\endgroup$
    – Rich Sedman
    Aug 18, 2020 at 21:28

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When you turn on edge-snapping, then try to snap a point to a vertex, the point will end up on the edge, not on the vertex.

Try the same procedure you gave with vertex-snapping enabled instead of edge-snapping. Everything should work as you expect.

Alternatively, when you merge points that are not in the same place, you can increase the merge distance. Or you could select a group of points you want to merge and merge to center, but your resulting cube is liable to be out of square.

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  • $\begingroup$ Hey @sobe608 - thanks for taking the time to help. It turns out that your second recommendation (increasing the merge distance) did the trick. I thought I had confirmed all 16 "extra" vertices had been removed, but I was wrong. Once I got that right, either snapping approach (vertex or edge) worked fine and rendered a manifold cube. Oddly enough, the 3D Print addon still claims that there are 6 "Bad Contig. Edges" - not sure why, whether that will be a problem with printing (I doubt it) or how to fix it. $\endgroup$
    – deep_forrest
    Aug 19, 2020 at 18:47

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