CAD solid model programs use a database of primitive forms of solid geometry like cubes, cones, cylinders, pyramids, spheres, etc. These are combined in a variety of ways (adding or subtracting) to produce more complex models.
This gives a well defined model. All volumes, surfaces and angles are well defined and known unlike surface modelers or wire frame modelers. Since all solids are represented inside the system as combinations of primitives or parts of primitives a task like calculating the volume of a complex space present few problems. It is just a question of taking the volumes of the constituent primitives and adding or subtracting them as appropriate. So volumes and weights are well known in the beginning of the design process and lead no room for misinterpretation.
Also designers found out that building a shape up out of primitives allows them to concentrate on essentials. The whole process brings them closer to the actual manufacturing process, and is in some cases almost a literal simulation of it. The parallel between boring a hole in an object and subtracting a model of a cylinder from a model of that object is after all very close and adding a model cogwheel to a model cylinder is very like slipping a real cogwheel onto a real axle.
The solid models are representing the geometry and can be the base for engineering analysis because the geometry is well defined. This integration between CAD and Engineering is very important in Concurrent Engineering because it makes more iteration steps possible during the design. So many changes during the design process are possible. This will help to optimise the product, which will give an increasing product quality and a reduction in the product development time.
The analysing methods that are mostly used can calculate and the mechanical, kinematics and dynamic behaviour of the simulated software prototypes.
The ability to develop 3-D solid models is bridging the gaps between design, engineering, and manufacturing in a concurrent engineering environment. To save time and costs 3-D solid model files are electronically transferred to moulders, and used to cut tools directly (CAD/CAM); the same files can also be used for rendering and analysis to reduce the risk of failure.
Assembly modeling further builds on solid modeling by combining solid part models together into an overall assembly model. The individual part solid model data files describing the 3D geometry of individual components are assembled together to create an assembly describing the whole product or assembly. Components can be positioned within the product assembly using absolute coordinate placement methods or by means of mating conditions. Mating conditions are definitions of the relative position of components between each other; for example alignment of two holes or location of two part faces to one another.
Solid and assembly modeling can caputure more than just the part geometry. Tolerancing, geometric dimensioning, and notations can also be captured.
Many CAD tools then allow generation of part and assembly drawing with minimal effort.