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why does the loop-cut-and-slide operation ignore certain faces but happily cut others?the model is symmetrical so it is a bit strange to pick 3 faces that won't get cut. I tried applying it to see if it was just a graphics glitch causing the purple line to not display over everything, but it did end up cutting only what the line was selecting, instead of the whole mesh.

space model with bad cutting

as requested,here's a paste of the blend: http://www.pasteall.org/blend/27821

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First you should fix the mesh. This might be part of the problem.

  1. Remove the double geometry, W -> Remove Doubles

  2. Select all geometry, A

  3. Recalculate the normals, Ctrl N

Does the problem persist? If so you might have to do some manual cleanup. The problem might be that you have ngons in your mesh.

  1. To see if you bad geometry go into vertices mode and Shift Ctrl Alt M to select all non-manifold geometry. If something was selected near at the place where the edge loop does not work that also indicates a problem, if not there still might be a problem

  2. Also to select all n-gons you can do Select -> Select Faces by Sides. Set the number to 4 and the type to Greater Than If you have n-gons they will now be selected.

  3. Go into vertices mode, in face mode it is harder to stop ngons

  4. If you see that there are random vertices along the edges you should delete them, x -> dissolve vertices.

An example of a random vertex along an edge

enter image description here

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  • $\begingroup$ what does non-manifold mean and how do I fix it? when selecting non-manifold geometry it highlighted edges that are necessary for the model's definition. $\endgroup$ – ZCoder Mar 23 '14 at 23:29
  • $\begingroup$ @ZCoder See blender.stackexchange.com/q/7910/599. It's possible you have duplicate edges/faces or something.. $\endgroup$ – gandalf3 Mar 23 '14 at 23:35
  • $\begingroup$ I've been struggling with this issue after extruding a face. Thanks $\endgroup$ – Burak Karasoy Apr 8 '17 at 13:44
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You have some internal faces (which result in non-manifold geometry):

enter image description here

If you delete them, loop cut works again:

enter image description here

The reason those faces are considered non-manifold is because they have no thickness (they are 2D).

Assuming those faces are desired for a cockpit, try creating a second mesh inside the first (with the normals facing the opposite way) to create a manifold cockpit. The solidify modifier is good for this.

enter image description here

Think of the gaps between the meshes where the normals are facing away as solid, and the faces more as interfaces between solids and air.

Example .blend

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  • $\begingroup$ so if I copy and paste that mesh I can detach it from the canopy/body mesh and it will still look the same? $\endgroup$ – ZCoder Mar 24 '14 at 0:17
  • $\begingroup$ @ZCoder I'm not sure what you mean.. Which mesh? $\endgroup$ – gandalf3 Mar 24 '14 at 0:31
  • $\begingroup$ right now the body is all one piece(except the trailfx holders and ball-turrets). the canopy is transparent so something needs to be under it to keep the insides from being visible, hence the cockpit. I was curious if I could just rip/detach that mesh from the main mesh but keep the locations,etc $\endgroup$ – ZCoder Mar 24 '14 at 1:40
  • $\begingroup$ You could, (loop cut would work), but you would still have a non-manifold mesh (it could not exist in the real world). $\endgroup$ – gandalf3 Mar 24 '14 at 6:42
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Loop cuts don't work on:

1-Duplicated vertices or faces. (To correct that use "remove doubles" or "merge")

2-Triangles or Polygons with more than 4 vertices. (Loop cuts work only in quads).

3- Edges that do not share the same vertices.

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