SolidWorks 2011 Parts Bible - Matt Lombard [262]
Exploring hybrid modeling
Modeling software has long divided itself along Solid/Surface lines with products such as Rhino (strictly surface modeling) and early versions of SolidWorks (strictly solid modeling). However, modelers are now increasing functionality in both methods and allowing them to interact. This hybrid modeling is a combination of solid and surface modeling. Surface modeling is now better understood by people who are doing general modeling, so combining the techniques is also more popular.
Surface modeling tends to be slower than solid modeling because you model each face individually, and then manually trim and knit. Cutting a hole in a surface model is much more involved than cutting a hole in a solid. To cut a hole in a surface model, you first trim a hole on one side, then the other side, then make the face of the hole, and then knit it together as a single, enclosed volume.
Solid modeling is essentially highly automated surface modeling; however, as any software user knows, automation almost always comes at the expense of flexibility, and this situation is no different. Surface modeling puts the compromised power back into your hands.
Solid modeling strengths are predisposed to a type of part with square ends or a flat bottom because solids are creating all sides of an object at once, and capping off a solid feature with a domed shape is difficult. For example, think about an extrusion: regardless of the shape of the sketch, you have two flat ends. Even lofts and sweeps typically end up with one or two flat ends because the section sketches are often planar. Surfaces enable you to create one side at a time. Another way of looking at it is that using surfaces requires you to build parts in sections.
You will find times when, even with prismatic modeling, surfacing functions are extremely useful. I do not propose that you dive into pure surface modeling just to benefit from a few of the advantages, but I do recommend that you consider using surface techniques to help define your solids. This hybrid approach is sensible and opens up a whole new world of capabilities. I have heard people say after taking a SolidWorks surfacing class that they would never look at the software in the same way again.
Understanding Non Uniform Rational B Spline
NURBS stands for Non Uniform Rational B Spline. NURBS is the technology that most modern mechanical design modelers use to create 3D geometry. NURBS surfaces are defined by curves called isoparameter lines in perpendicular directions (referred to as U and V directions), which form a mesh. The fact that perpendicular directions are used means that the surfaces have a tendency to be four-sided. Of course exceptions exist, such as three-sided or even two-sided patches. Geometry of this kind is referred to as degenerate because one or more of the sides has been reduced to zero length. Degenerate geometry is often, but not always, the source of geometrical errors in SolidWorks and other CAD packages.
Figure 20.2 shows some surfaces with the mesh displayed on them. You can create the mesh with the Face Curves sketch tool.
FIGURE 20.2
Meshes created with the Face Curves sketch tool
An example of a competitive system to NURBS surface modeling is point mesh data. This comes from systems such as 3DSMax, which create a set of points that are joined together in triangular facets, and can be represented in SolidWorks as an STL (stereolithography) or VRML (virtual reality markup language) file. When displayed in SolidWorks, this data looks very faceted or tessellated into small, flat triangles, but when viewed in software that is meant to work with these kinds of meshes, it looks smooth. Many advantages come with this type of data, especially when it comes to applying colors and motion. However, the main disadvantage is that the geometrical accuracy is not very good. Point mesh data is typically used by 3D graphic artists, animators, and game developers.
By using a SolidWorks add-in such as ScanTo3D (available only in SolidWorks Premium), it is