Understanding (and fixing) non-manifold geometry 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 &quot;edge&quot;
2)Turn on Snap
3)Add --&gt; Mesh --&gt; Plane
4)Copy plane --&gt; 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 &quot;Check All&quot;
This shows 20 non-manifold edges.  If I use the addon's &quot;Clean Up&quot; --&gt; &quot;Make Manifold&quot; 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!
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.