Solids vs. Surface Modeling
Computer aided design (CAD) isn’t like a car in that you can use it pretty well even if you don’t know how it works. It pays to know what happening ‘under the hood’ when
using CAD. It is important to know about surface and solids modeling because it does affect the way you model, and it is important to know if you are switching platforms. It is also very important to know about for rapid prototyping.
Surfaces and solids are the underlying math that defines the geometry of the forms
you create. There are three ways to define 3D geometry: solids, surfaces and wireframes.
Wireframes don’t play much of a role in CAD, but primarily in digital content creation
(DCC) and gaming. The easiest way to understand the difference between surface and solids modeling is to think of a water balloon; the water in the balloon would be solids modeling, while the latex skin would be surface modeling. Need more of an explanation? No problem.
Solids modeling is defining an object with geometric mass. Solids modeling programs usually create models by creating a base solid and adding or subtracting from it with subsequent features. Features such as extrudes, extrude cuts, revolves, radii, chamfers, etc. Examples of solids modeling programs are Solid works, CATIA, and Pro Engineer. It was originally developed for machine design, and is used heavily for engineering with large part assemblies, digital testing and rapid prototyping.
Surface modeling is defining an object’s exterior with an infinitesimally thin skin.
This skin is created by lofts, sweeps, and NURBS curves - i.e. sculptured surfaces with lots of curvature. The surfaces are either defined by poles or guide curves. A surface is considered a solid only when it is completely enclosed. It is used to make technical surfaces
(e.g. air plane wing) or aesthetic surfaces (e.g. car’s hood).
It was developed for the aerospace and automotive industries in the late 70s. Rhinoceros 3D and Alias Studio Tools are examples of a surface modeling programs. It is generally considered more difficult than solids modeling, but the models are more robust because the programs aren’t generally feature based. Later changes have to modify the existing geometry as opposed to just editing the original feature, which is more difficult but keeps the model from collapsing when one feature interferes with another.