tips

Do you have prints that don’t print?A pronterface that looks wrongterface? Well, you might have a bad mesh.

I ran into this when attempting to slice a whistle from Thingiverse: Slic3r would give me the ominous “Non-Manifold mesh” message and the printer would spaz out.

In order to undertand what’s going on, there should be a little information on how tools like slic3r work. Slic3r and other G-code generators tend to work on the values of face “normals” — the direction the face, well, “faces.” Let’s take our whistle for example: As downloaded from Thingiverse, it looks like this:

Bad STL file from Thingiverse

The tool I use is ModuleWorks STLView (also for android!) which allows us to show the face normals (green lines) and faces which face the wrong way (gold surfaces). Slic3r will produce some very, very strange GCode for this model:

Ugly GCode as a result of bad mesh

The tool here is the RepRap GCode Simlulator from opencode.eu.The source is included, and depends on openGL. This lets us see what the actual path for each layer of the model looks like.

So what do we do? First: don’t panic. Most of the horrible problems with ugly looking meshes can be fixed — all it takes is some time with the NetFabb cloud service. NetFabb produces a commercial 3D printer, but also runs a cloud service at http://cloud.netfabb.com/ which allows you to upload STL meshes and download the fixed version.

Here’s what the model and resulting GCode look like afterwards:

Fixed, good model with all its normals pointing out

Good GCode as a result of a clean mesh