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We recently wrote an overview on How to Prepare CAD Files for 3D Printing, going over some general considerations and steps to properly export your 3D printing files.
There are some unique considerations to keep in mind when preparing 3D print files specifically in Rhino, so for this post, we’re going to go into more detail on how to export your CAD models here.
Let’s dive in!
Units and Tolerances
Set correct tolerances before you start modelling by finding the tolerance of the program you are exporting your model to and setting Rhino tolerance to be 10 times that (i.e., printer tolerance is 0.1mm, so Rhino tolerance should be set to 0.01mm).
Rhino file tolerance can be set from the document properties window, units tab.
No matter what units you use to create your model, files exported from Rhino are almost always read as mm. Make sure your units are set to mm before exporting your file for printing.
Check Your 3D Models
For successful 3D prints, the 3D model should be watertight, i.e. if you dropped the model in a bath, no water would leak inside.
To achieve a watertight model, you need a closed polysurface. The most common reason models become unprintable is due to bad objects and naked edges.
Bad Object: Objects that violate NURBS rules or have structural problems. They often appear after joining and Boolean operationsNaked Edge: Surface edges not connected to other surface edges, which create open meshes that cannot be 3D printed
Find open polysurfaces by selecting Analyse Direction → Show Edges → Select Open Polysurfaces, and then follow the steps below:
To Prevent Naked Edges
- Select Analysis → Show Edges → Naked Edges to display naked edges in your object.
- Naked edges due to missing surfaces can easily be corrected by filling in the hole with a new surface and joining the new surfaces together. Created surfaces and existing surfaces must be within model tolerance to join correctly.
- If precision is not important, the JoinEdges command can be used to join surfaces forcibly, however it’s better to recreate the bad surface.
- Your NURBS model can be converted to mesh with the Mesh command. SelectShow Edges → Naked Edges as before. In Rhino 3.0 Service, Release 4.0: Under Bonus Tools you will find ‘FillHole’ and ‘FillHoles’ to fix naked edges in the mesh. Make sure to save your NURBS model before this operation as it is difficult or impossible to reverse.
To Prevent Bad Edges
- Use the ‘Checknewobjects’ command to verify objects as they are created. This runs in the background and instantly detects bad objects and corrects objects instantly.
- Many bad surfaces can be fixed by exploding and rejoining the model. If this doesn’t work, that means there are NURBS structural problems in the model.
- Explode detected bad objects and use the Selbad or Extract Bad Surfacescommands to delete specific bad surfaces. Use the Rebuildedges command to recreate the deleted surface.
- Using Osnap helps to prevent open curves, a common source of bad objects.
Creating and Exporting a Mesh
Rhino creates a NURBS solid model. However for 3D printing, the model must be exported as a mesh model. To achieve this, follow these steps:
- File → Save As → .STL and select a name for your STL file.
- There are simple controls and detailed controls – always select detailed controls.
- These are the specific settings recommended for 3D printing:
- Density: 0.0
- Maximum Angle: 0.0
- Maximum Aspect Ratio: 0.0
- Minimum Edge Length: 0.0
- Maximum Edge Length: 0.0
- Maximum Edge to Surface: 0.0
- Minimum Initial Grid Quads: 16
- Refine Mesh: yes
- Jagged Seams: no
- Simple Planes: yes
- Maximum Distance edge to Surface: Less than half of the printer’s resolution. For millimetre unit models try 0.1 to 0.01.
- In the STL Export Options dialog box, set the file type as Binary and click OK.
Check Your Mesh Model
Even with the recommended settings, your STL mesh may have errors that make it unsuitable for 3D printing. Here are the most common problems and solutions:
Too Many Polygons
If your mesh polygon number is higher than 1,000,000, the file will be unwieldy and difficult to print. Small items like jewelry should have around 150,000 polygons, while a 30 x 30 x 70mm object will have more like 600,000. Use the Reduce Mesh command to make your file more manageable.
Incoherent Mesh Normals
The ‘positive’ direction of your mesh faces may not be coherent throughout the model. Use the ‘Unify Mesh Normals’ tool to fix the issue.
Selecting open meshes and performing a naked edge analysis as described above will identify any holes in your mesh. This is easy to fix using the ‘FillHole’ command.
Can be caused when multiple mesh surfaces share a vertex or edge. Use the ShowEdgestool as before to find these errors. It’s recommended to view the object in wireframe mode for easier analysis; frequently the excess faces are inside the model. Use the ‘Delete Mesh Faces’ tool to remove the excess faces.