About repairing parts

Fix errors in a triangle mesh that make a part unsuitable or unusable for further printing preparation steps

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• Why repair?
• Repairing meshes in Netfabb

Why repair?

For many actions during preparation for additive manufacturing, a mesh model must exist in a known and well-defined state. If it doesn't, steps must be taken to bring the mesh into this state. Repairing means just that: One application may be perfectly fine with a mesh that another application (in this case: additive manufacturing) can't find but unacceptable; so, a mesh doesn't have to be so completely broken to require repair in the traditional sense, it just has to fail to meet the specific requirements of preparing for 3D printing. These are:

• "No holes."

  Surfaces must be closed. While a mesh may have multiple surfaces, in each of those, all the triangles must connect with another triangle at each of its edges.

  Other terms for this: Mesh must be watertight.

• "No non-manifold meshes."

  Each edge of any triangle in a mesh must only ever meet with exactly one other triangle's edge. Arrangements whose cross section would look like Y-junctions, for example, are not allowed.

• "Triangles must be oriented correctly."

  What counts as inside or outside is usually not derived from enclosed volume, but from the orientation of the triangles (more specifically, their normals) that enclose the volume. Normals of neighboring triangles must all, without exception, point either away from, or towards, the volume enclosed by those triangles, and must do so appropriate to the role of the shell they belong to, which could be an outside shell or an inside, hollowing shell.

• "No self-intersections."

  No triangles (or, by extension, entire shell surfaces) may intersect anywhere with themselves or each other, particularly so without a common edge along the intersection.

In other words, if you could not …

1. Take your mesh,
2. slice it in two anywhere and at any angle,
3. and trace each and every contour thus created with exactly one closed line each,
4. without ever crossing over other contour lines or itself,
5. and without the inside and outside changing sides along any stretch of line,

… then your mesh very likely needs repair.

Note: However, you still need to decide whether a mesh really, actually needs the repair, as in, needs to meet all the requirements listed above. For example, support structures may live quite happily as open, perforated structures without fully enclosed volumes or even wall thickness because of the way toolpaths for these meshes are generated. It is your decision and responsibility to decide upon the necessity and the selection, if any, of the appropriate repair steps. Netfabb is your tool to help you do both.

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Repairing meshes in Netfabb

Netfabb provides a range of indicators to recognize repair-relevant properties as well as automated and manual tools to address them. For instance, parts that have very obvious errors have their display color swatch marked with a warning symbol in the project tree. A larger warning symbol is displayed in the bottom right corner of the display if any part is in such need of repair.

Repair in Netfabb is covered with the repair module.

To enter and finish repair

1. Select the part, then choose Home > Repair Part.
2. Perform the required repair.
3. Click Apply Repair
4. Choose from the dialog what to do with the old part:
   • Remove old part: Creates a new part while deleting the original part.
   • Keep old part: Creates a new part and keeps the original part.
   • Keep and compare: Creates a new part and keeps the original part. It also launches a mesh comparison on these two parts.

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