Blender-Fixing Automatic Weight Errors
Why Blender Stack Exchange recommends it
Blender Stack Exchange answerers point people to it when automatic weights fail with a 'bone heat weighting failed' error, since it demonstrates the mesh cleanup that resolves it.
Recommended by 2 people across 2 different months, 2014 to 2018.
What it is
A short 2011 Blender 2.5 tutorial showing how to fix errors that appear when using automatic weights to bind a mesh to an armature. It walks through cleaning up the mesh, such as removing doubles and recalculating normals, so bone heat weighting can succeed. It is presented by the channel bugzilla2001.
What people said
Recommends A Wild RolandiXor Has Appeared · 2014-07 · Blender Stack Exchange · on “Blender assign armature to mesh: “bone heat weighting failed to find solution for one or more bones””
The OP originally posted this in their question. If it happened to you too and you were looking for any solution, I solved in this way: select mesh in edit-mode; in the bar "Mesh tools" select the voice "Remove Doubles" (that remove duplicate vertex). Here [this video] there's a video that explain the procedure. Another way to solve the problem would be add a modifier to the mesh: "Decimate" modifier.
Recommends MooKorea · 2018-03 · Blender Stack Exchange · on “My mesh freaks out when I rig it, i think because it's made of separate pieces”
Before you apply the armature, make sure your character has no modifiers. You can check this by selecting the entire character and hitting [alt]+[c] and convert to mesh from curve/meta/etc. If you're getting an error message about bone heat weighting, make sure you remove all doubles (select all verts in edit mode and "remove doubles" in tools panel) and clear the normals [ctrl]+[n] in edit mode. This is explained very well in the following video Blender-Fixing Automatic Weight Errors [this video] If none of that worked, try separating the mesh into loose parts (select the entire mesh and [ctrl]+[p] in edit mode) and apply the armature while all the loose parts are still selected.
Cites Sigfrido · 2014-11 · Blender Stack Exchange · on “How to recalculate normals?”
in version of Blender 2.72, where is the option recalculate?. Like in this video [this video] Here is a screenshot: [LINK]