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Solid, Surface and Mesh: What's the Difference?

By Callie Morgan and John Stauffer
December 28th, 2021, 10:30 AM PST.

When learning Computer Aided Design (CAD) modeling, you will come across different methods of computer-based modeling: solid, surface and mesh. You may wonder how these modeling methods differ and which type works best for a given situation. Let’s first define each of these terms so we can break down the uses of each type of modeling.

Solid Modeling
Solid modeling is by far the most complex form of CAD modeling because solid modeling simulates an object both externally and internally. When you slice a model, you can see the parts inside, just as you would if it were a physical object. 
Solid models can be either manifold solids, or non-manifold solids. A manifold solid is completely closed on all edges, which allows the model to have a measurable volume. This can be helpful when you need to know the mass of a part, if you need to know the center of gravity, the moment of inertia, or other information that might be important to the physics of an object. 
A non-manifold solid (also called an open sheet solid in Mastercam) has a gap in its faces somewhere, meaning it is impossible to calculate volume. Imagine this by filling the object with water. If water would fall out of it, then it is a non-manifold solid. This is also referred to as not being “water-tight”. 

Applications for Solid Models

In Mastercam, solids have the unique advantage of tracking the edges that define the solid model, and edits made to the faces can be reflected in the toolpath with a simple regeneration in most cases. This makes solid models the strongest type of model to program off of, due it its flexibility. However, solid models generally work best for models that are fairly geometric, as modeling organic shapes can be quite difficult, if not impossible in some circumstances. 
If a perfect cube was generated from a solid model, it would contain only 1 solid model with 6 faces and 12 edges.

Surface Modeling
Surface modeling is the oldest form of 3D modeling, dating back to the late 70’s, well before Mastercam’s time. Surface modeling  is less intensive than solid modeling, but, unlike solid modeling, you can’t cross-section a surface model and see its internal components. 
Surfaces are defined by 3 things: A boundary, UV curves, and a surface normal. Boundaries are made from wireframe geometry (lines, arcs, or splines), and define the total area of the surface. UV curves are the curves inside of the mesh that define its surface shape. These are generally distributed along the surface in the “along” direction and the “across” direction. 
In Mastercam, if you turn off shading on a surface with ALT+S, you can see the UV curves of the surface. The surface normal determines which side of the surface is the “outside” of the surface. Because a surface has no thickness, a point drawn in the center of any given surface on either side would have the same location in space, but by default the tool vector on a surface would point the tool at the normal, or “outside” of the surface.

Applications for Surface Models
In Mastercam, surfaces are still used for programming from the model directly, but they have become less common since solid modeling has increased in popularity. Surfaces may be best used for scenarios where having the toolpath follow the U or V curves is helpful, or in situations where a set of complex solid faces can be condensed into a single surface. This is very helpful for those who only have a Mill license, but want to work with truly 3D shapes. 
If a perfect cube was generated from a surface model, it would contain 6 individual surfaces, one for each face. This would not be a manifold, or water-tight model because it is made of separate components rather than a single component. 

Mesh Modeling
Meshes are also 3D models entirely made up of linked, non-overlapping, triangles from point clouds taken from a 3D scanning device on a physical part. Since the model is created by scanning, this makes mesh a non-CAD form of computer modeling. 
Due to the nature of meshes, they don’t contain what we would consider in Mastercam a true “edge”. Mesh models can be either manifold or non-manifold just like a solid. A mesh can be used as machining geometry, but they are generally best suited for organic shapes rather than geometric shapes. This is especially true when a geometric shape contains circular data, as meshes are not capable of creating clean circles. 

Applications for Mesh Models
Meshes are great for modeling organic shapes because the size of the triangles that make up the mesh can vary, as can the included angles of each vertex. When designing a mesh entity, it is important to consider the size of the triangles being used to create the shape. Many more small triangles will give more accurate results than using larger triangles.
Meshes are used when reverse-engineering a part where a programming file for it is not available. Meshes can also be used for inspection by comparing a scanned physical part to its original CAD model, revealing any areas that are out of tolerance. Meshes are also used for 3D printing since their file type, .STL, is used by 3D printers. Starting in 2022, Mastercam can now directly edit mesh entities.
If a perfect cube was generated from a Mesh model, It could contain at least 12 individual triangles in the single mesh entity. This equates to 2 triangles to make each square face for 6 faces. However, depending on the software settings used when creating the mesh, there’s no telling how many triangles might be used to create the cube. 


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