Solid Modelling vs Surface Modelling

By Niraj M Wanikar, Chief Editor, CNCTimes.com, 2013-09-14 06:41:57

By Ashish Poddar

Confused ! What is there in a surface or solid, as long as my application is fulfilled ? In software, a lot of Buzz words and jargons are used, ultimately a software is just a mathematical tool, which helps us in doing our activities at efficient way. How powerful the tool is determines the increase in efficiency resulting in accurate (quality) and timely result.

Surface normally is referred as an extension of area- surface in 3 dimensional space. It is fitment of cover, which is spread on a skeleton of 2D and 3D curves.

Surfaces are classified in two ways.

a.) Application specific, Depending upon Input information of oriented curves is available, surfaces can be coons, loft, swept mesh of curves, revolutionary etc. This terminology also can be changed from computer software to software.

b.) Mathematical representation: Between given curves surfaces can be fitted by various mathematical equations, there are linear, parabolic, cubic equations. Higher the order of the equation are accurate its representation. Here care has to be taken to select optimum order equation in the sense it is always better to fit line (liner equation) between 2 points instead of putting spline (higher order equation)

Solids normally are referred to as a geometrical model, which has bounded volume. Say cube of size 10 mm consists volume of 1000 mm3. Also cube contains 6 surfaces with 12 edges. Initially when solid modeling concept has launched it used to only refer primitives (easy to defined objects like cylinder, cube, sphere, cone etc.) Presently even complex shapes (surfaces) having common edges making closed boundary is also referred as a solid. Hence now a days boundary between surface and solid is very faint, people represent surface is solid with zero thickness hence doesn’t have volume (third dimension for volume)Solids normally represented by two mathematical ways

a) Constructive solid geometry (CSG): Entire volumetric information is stored.

b) Boundary representation (B-rep): Here information of only boundaries bounding individual faces (surfaces) is stored.

c) From application point of view solids are used when information of mass properties like volume, weight, center of gravity, moment of inertia is required. Hence solids are mainly used in product design, analysis and assembly application.

Here is a simpler way to explain the difference. If you draw a circle, and extrude the surface area, the end result will be a solid cylinder, with properties such as mass, Moment of Inertia, volume etc.. This is an example of solid modeling. Now instead, if you just extrude the circumference of the circle, the result will be a hollow cylinder, without any wall thickness. This is an example of surface modeling.

In case of machining (NC code generation) one is only interested in area that can be machined from particular coordinate system, hence surface information is enough. Since machining requires say outside entry direction, some times surfaces are to be extended. In this case if we use solids, common edge condition is violated and it is better to use surfaces for modeling. Hence, most of the CAM software are predominantly surface modelers. However, considering the need of the hour, some of the High end CAM packages too, now allow solid modeling features.

With latest technology surfaces can be considered as a subset of solids. Surfaces can be considered as building blocks for solids at the same time solid can be used to extract surfaces.

In case of applications where models have to be created and used in assemblies and mechanisms or have to be analyzed for stress- strain and other properties, solid modeling is mainly used.

Surface modelers normally have more functionality for extremely complex surfacing features, allowing NURBS Geometry etc.. Comparatively, solid modeling allows lesser flexibility due to the structure in which it works.

There are a number of Solid modeling and surface modeling software available today. Many of them allow only one type of modeling, i.e either only surface or only solids. However, some high end software support hybrid modeling, thus allowing you to work with solids and surfaces in the same model.

Still confused ! A little advise- Use software from maximum utilization (Implementation) point of view. A hybrid modeler will allow you to work with a variety of applications

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