Non manifold edges

Never really understood the manifold /non manifold …so I was working as usual doing my basic joins / boolean unions working with all solid closed polys surfaces sometimes I will notice closed poly srf with …manifolds or so many open manifolds but I keep on works everything fine solid closed polysrf… then meshed it and out of curiosity it showed as a bad mesh… undid mesh checked for ‘bad objects’ my model was selected … I ended up finding select manifolds found where the ‘issue’ was but not sure how I created a non manifold…can anyone explain real simple what causes non manifolds and if it effects 3d printing as properties showed it as an open mesh. and now I would like to how to create one open and one with just manifold edges real simply …here’s 3 pieces if I booleon everything is fine /booleon intersect then booleon all good

Hello - a Non-manifold edge is one that is joined to more than one other edge - like a Y shape, for example. These are relatively uncommon in polysurfaces but it is possible to get them from Boolean operations if you set things up just right. The objects in your image do not look like they would create a Union with non-manifolds, to me.


It’s not all that hard actually, in the file below, just BooleanDifference the triangle shaped solid from the box shaped one.

NonManifold.3dm (372.4 KB)

can’t open the r7 file I have r6…do you think extruding faces solid can cause them or extruding faces solid then booleon them sometimes I do that when scaling 1d doesn’t get the results I need thanks

Thanks is was just a example of three intersecting objects and your right it didn’t.

Here it is in V6.
Non-Manifold_V6.3dm (368.1 KB)

This is just a deliberately provoked case of course. Usually you end up with non-manifold structures accidentally because something is just slightly out of tolerance when you run your Boolean operation and a surface that is supposed to go away in the operation doesn’t - and it gets "stuck’ inside the resulting object, leaving more than 2 surfaces sharing one common edge (the defininton of Non-Manifold).

Here is perhaps a more common example of something that looks perfectly correct so you don’t bother to check the result - until it comes back and bites you in the a$$… Try running BooleanUnion on the two boxes in the file below.

Non-Manifold_2_V6.3dm (402.5 KB)

also, creating objects that touch at their edges (same size cubes in this example) will boolean just fine:

that should not happen imo

also notice this when creating a mesh:

Hi Gijs - Yeah… so, the mesh is technically valid- you’re allowed to have meshes like this, there is nothing really wrong with the thing as a mesh, however, the MeshRepair thing attempts to call out things that users might care about, apart from strict ‘validity’… It does get messy, I agree.