The model that you plan to 3D print should be completely closed and watertight**. **The easiest way to accomplish this is to be a meticulous 3D modeler - paying close attention to seams and joins in your model.

## Step-by-step guide

- Always model using OSNAPs turned on. This stands for Object Snap and allows you to move geometry using its exact geometric attributes.
- Turn the
**Near**from the OSNAP off to decrease the possibility of having an open object. We use**Near**only if it is necessary. - If possible, always build closed geometry as you work. Regularly employing the Join for surfaces and Boolean Union or Boolean difference for 3d objects.
__Failure of these commands is a fast indicator that your 3D model is not watertight! Each stl file should be 1 object in rhino.__- If you have modelled an object which has zero thickness to its surfaces, follow these steps to add required thickness:
1. Make sure all surfaces are joined into one polysurface, then run OffsetSrf

2. Set direction to offset and set wall thickness (must be at least 1/16th inch)

3. Use MergeAllCoplanarFaces to simplify surfaces

4. Final result

- List of useful commands to create closed NURBS volumes:
- Join
- Join Edge
- Boolean (split/union/intersection/difference)
- Sweep1
- Sweep2
- Blend
- Cap
- Loft
- Revolve
- ExtrudeSrf + Cap (to close it)
- ExtrudeCrv (must be closed curve)

- After you finish modeling, the first thing you should do is use
**ShowEdges**command to make sure the object is closed. The image below shows an open object vs a closed object. The open surfaces are shown with purple edges in Rhino after you use ShowEdges. Remember to choose**Naked Edges**in the list of ShowEdges command - If your 3d model was carefully built in Rhino3D using NURBS geometry, the first step in preparing a print file is to convert the model into a mesh using the “mesh” command. The main difference between a NURBS surface and a mesh is that the former is defined by curve functions rather than simple planar polygons. NURBS (non-uniform rational bsplines) volumes are usually computationally much heavier than meshes, and therefore meshes are used to create file paths for 3dprinting.
__Thus when converting a NURBS model to mesh, you must define the resolution at which to replicate the original geometry using simple planar polygons. Ideally, you would aim to use the fewest amount of polygons to complete the desired geometry.____(ie. sinuous surfaces will require much more polygons than flat straight surfaces)__

Select object and enter mesh command | Select polygon amount in mesh dialogue box | The resulting meshes from moving slider to "fewer polygons." This works well for flat surfaces and rectilinear geometry. | The resulting meshes from moving slider to "more polygons." Only needed on detailed models or to create smooth curves |
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- After your model has been converted to a mesh with the desired resolution (may take several attempts), you will run the “ShowEdges” command.

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