There are three main types of 3D models or modeling methods integrated into CAD systems for a single software package, and each has its own strength and weakness: wireframe models/modeling, surface models/modeling, and solid models/modeling.
Although there are other types of 3D models that can be used in CAD systems for technical & engineering drawings/graphics and designs, most of them exist as a subset of the three main types of models, or they can be used individually for specific purposes.
1. Wireframe model
The Wireframe model/modeling method was the first 3D modeling method to be established. Wireframe models are often used as a starting point in 3D modeling since they eventually create a “frame” for 3D structures.
A wireframe is a three-dimensional model that only includes vertices and lines; it doesn’t contain surfaces, textures, or lighting like a 3D mesh. Instead, a wireframe model is a 3D image comprised of only “wires” that represent three-dimensional shapes.
Generally, “wireframe model/modeling” refers to any computer screen display of a model in regard to only the edges and contours of the object or artifact it represents.
Wireframes provide the most basic representation of a three-dimensional scene or object: using simple lines and curves, a wireframe model can represent a “skeleton” for building a 3D object.
Wireframe models express the contours and edges of objects by using circles, lines, and arcs orientated in 3D. The portrayal of contours and edges in the wireframe model was developed from 2D modeling practices.
The wireframe modeling method derived its name from the process of mentally or visually representing objects, sculptures, or carvings with wires as shown in Figures 1, 2, and 3 below.
Figure 1: The Wireframe model (lines, arcs, and curves in 3D) of an artefact (Source: Quora)
Figure 2: The Wireframe model of a car (Source: DepositPhotos)
Figure 3: The Wireframe model of the world (Source: PngEgg)
A wireframe model can be created in the same way a 2D CAD drawing is created. Simple geometric tools (such as circles, lines, and arcs) are drawn in 3D to express each edge where the surfaces on an object intersect.
Wireframe models don’t look as realistic or “close to reality” as some other models because they incorporate surfaces or boundaries that are often shaded.
A wireframe model can help to visualize 3D equipment which is more difficult to visualize when represented or expressed in 2D. In many cases, it is advisable not to overlook wireframe during the design of an object.
The major advantage of wireframe 3D models is that they provide a single and clear understanding of some shapes and make it possible for objects not to be represented unambiguously, especially when expressed using multi-views.
2. Surface model
CAD surface modeling/models specify limits and define an artifact or shape by using the stored information about its surface to create a realistic visual or mental picture or impression.
However, surface models do not assign any material thickness to surfaces, and the outer surfaces of objects (balls, boxes, cubes, etc.) can be shaded between or around the limits specified and defined by surface models.
Generally, surface models focus on modeling the outer/outside parts of objects, artefacts, shapes, or products, instead of the inner and mechanical parts within.
Figure 4: Surface modeling for Audi car (Source: GrabCAD)
The information (or definitions) about an artefact is stored in the CAD database. This information represents the outer or external boundaries of surfaces that could form part of the 3D model. (The process of storing the definitions of surfaces in the database is called “boundary representation” [BREP].)
To create a surface model, an entire surface doesn’t need to be created at once: only pieces or “patches” are combined into a continuous model. In summary, surfaces can be created from bits or pieces technically referred to as “patches”.
Each patch can be interpolated or approximated like a spline curve. A patch can be regarded as a “coon’s patch”—this refers to an interpolated surface that is defined by four boundary curves which have points that can be interpolated using mathematical methods.
The surface models employed in computer-aided manufacturing (CAM) need to operate at a high level of accuracy to produce really good and smooth surfaces that may require fewer control points to make their curves more fluid.
3. Solid model
The term “solid models or modeling” is a technique used by solid modelers or designers to create a representation (called a solid) of a solid object. Unlike surface and wireframe models, solid models ensure that objects are geometrically correct and all surfaces meet precisely.
Solid models not only represent the edges, vertices, and surfaces of the part, they also help determine the locations of points in space within an object or outside it.
In terms of storing information concerning the volume occupied by an object of artifact, solid models go way beyond surface models. In fact, solid models record information about the vertex and edge of the 3D wireframe modeler, the surface modeler’s surface definitions, and volume.
Because many solid modelers store the operations used to produce features, it is possible to quickly edit features of objects and easily test, define, and refine the designs that solid models represent.
Figure 5: Three technical/engineering drawing projections and their corresponding solid (shaded) model (Source: ResearchGate)
Solid models are easy to understand and highly accurate and visual if modelled accurately. Some 3D solid models are often more capable of replacing physical models.
However, 3D solid models work better with analysis packages and contain all the information about the volumes of objects which are very important in many calculations.
During the refinement process of designs, the solid model can be used as a foundation from which centroid, mass properties, moments of inertia, and weight can be estimated as many times as may be needed.
Figure 6: Difference between Wireframe model and Solid Model (Source: LearnMech)
The behavior of an object or its system can be simulated from the information stored in the solid model which can further provide useful information that may be required for other analyses.
Because solid models define entire objects, their information can be used by finite element analysis (FEA) softwares to automatically generate FEA meshes and break up complex objects into smaller objects, thereby making it easier to even calculate material properties such strain, stress, and heat transfer.
Some solid modeling softwares and FEA are structured to provide this and similar types of analysis. Other modeling softwares even permit direct model optimization based on results from FEA, and could be used to produce a new version of the model which the modeler can review further.