For 3D printing, CAD and CNC, your best bet would be NURBS (Non-Uniform Rational Basis Spline) modelling
When designing a 3D object using a CAD program, the most popular modeling programs use either polygon mesh or a Non-Uniform Rational Basis Spline (NURBS) to describe the object.
On the path to creating a file for 3D printing, most CAD programs allow you to convert the file to STL format (which is converting it to a triangular polygon mesh), so you may be wondering if you should create the object with mesh from the beginning or if it is better to work in NURBS and then make the conversion.
I interviewed Dan Gustafson from NextEngine, a leading 3D scanner company, to figure out the finer points of these two major types of 3D models.
As far as computer modeling goes, NURBS will create the smoothest images. It will also create the most accurate models with even edges that are not pixilated. For engineering and mechanical applications, NURBS-based computer rendering is preferred to polygon mesh based programs. Generally, when you scan objects into a CAD program, they are initially scanned using NURB
When you are working in NURBS, you are averaging a curve between points. The points will form a rectangular mesh above the curve. To adjust the curve, you must adjust the points on the mesh. This can be a little difficult to master.
NURBS does have its limitations. Because it is a 2-dimensional rendering form, you need to create patches that you piece together in order to make a complex 3-dimensional shape.
In some cases, these patches do not fit together perfectly and seams appear. It is crucial to look carefully at your object when designing it and make sure the seams align perfectly before you convert it to mesh for the STL file.
Polygon mesh was created specifically to render 3-dimensional items on the computer.
Because of this, it is the format used by STL files. When using triangles to create 3D shapes, you create approximations of smooth edges. You will never achieve the perfect smoothness of an image initially created in NURBS, but the mesh is easier to model. You can push and pull on the mesh to move it and achieve the same results each time because it is not calculating mathematical averages of points.
When you work in NURBS and convert the file into mesh, you can choose your resolution. High resolution gives you the smoothest curves in the object you are printing. However, high resolution means you will have a large file. In some cases, the file may be too large for the 3D printer to handle.
Aside from finding the perfect balance between resolution and file size, you can use other cleaning methods to reduce your file size. For example, you need to make sure that when you crafted the object you did not create inner surfaces that will not be printed. One way this could happen is if you join two shapes together, sometimes the join surfaces remain defined, even though, when they print, they will not be separate surfaces.
Whether you initially make your object using NURBS or mesh will depend on your preference.
If you want an easier program that you will not have to convert, starting in mesh will be the best option. On the other hand, if you want a program that gives you perfect curves, you should choose one that uses NURBS (Rhino is one example that does, in fact, they have a great overview: What are NURBS?).
Ill close this post with this: The 3D Design program that you are most comfortable with, most likely, will have an option to export your NURBS or Mesh file to an STL or other 3D-printing format. Ultimately, I would heed the advice of Sherri Johnson, from CatzPaw, who I interviewed for tips and techniques: Repairing 3D Files With Meshmixer and Netfabb.
The STL file needs to be opened in a utility program that is capable of checking for issues and correcting those issues, either automatically or manually. Some slicing programs (such as Simplify3D) offer repair tools as do some of the CAD programs (SketchUp extensions). Dedicated applications that are also free, and which include the most repair tools are Netfabb and MeshMixer.
Other great resources for understanding different model formats come from 3D printing service bureaus (where I mention Sculpteo and Shapeways, to name just a couple). These firms have to handle filetypes and formats from nearly every 3D design program on the planet and often have great tips and suggestions for getting your files to print right.
Here is one from Sculpteo giving a tutorial on using Rhino 3D, specifically. You can also learn about using Meshmixer or Autodesk Inventor or Catia or Blender in this Sculpteo section: 3D Printing Tutorials: Prepare model for 3D printing.
Because so many animators and computer graphics experts come to 3D printing with experience creating characters, Shapeways offers one that really fit the bill: How to Prepare your Render/Animation Model for 3D Printing.
Stratasys Direct Manufacturing (formerly RedEye) has a great one on How to Prepare an STL File that I share near the end of my STL Files overview[/I].