# Building the geometric models in CAD, Describe the approaches.This question is related with CAD/CAM Engineering.

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In computer aided design (CAD) systems the geometry of components and products is described in terms of mathematical data and equations called models. And the system of representing these models are modellers. These models are stored in computer memory and used for designing as well representing the products and components in engineering drawing and in producing their isometric views. These are also used for other design related calculations such as calculating volumes, and surface area.

In CAD systems, different types of modellers are used that use different system of geometric representing to provide different levels of details and sophistication. There are four main type of Modellers.

- Wire-frame modellers
- Surface modellers
- Solid modellers
- Feature based modellers

Wire frame modellers represent just the edges of parts and their corners (or vertices), where the lines meet. The vertices are represented as coordinates on a three dimensional axis. And lines are represented as equation of straight lines, circles and arcs.

Surface modellers are able to represent continuously curved surfaces such as, say, body of a car. Cross- and longitudinal-sections of surfaces are taken and lines of these sections are represented by polynomial equations. Alternately the surfaces may be divides in small patches and each patch represented by corners of the patches and slope of the surface at the corners.

Solid modellers provide a true three dimensional representation of the part. These type of models are created bu taking a two dimensional shapes and projecting them in the third dimension.

Feature based modellers are basically solid modellers with added facility for representing specific feature common in parts - for example holes, pockets, flanges ribs.