Computer-aided design (CAD) is software in which an engineer draws a part as an exact 3D shape on a computer screen, before the part exists. The shape is called a 3D model. Many engineering students have drawn a part in AutoCAD, a well-known CAD tool, in a college lab.
A CAD model is not a picture. It stores the exact size of every edge, hole and curve. So the same model can be tested, sent to a machine, or used to draw the flat 2D drawings that a workshop reads.
Consider a mechanical engineer at a pump maker in Coimbatore who designs the casing of a small pump. She starts with a flat sketch of a circle, 120 mm across. She pulls the circle up to a height of 80 mm, which makes a cylinder. Then she cuts a 12 mm hole for the inlet pipe and sets how much the hole may differ from 12 mm. This permitted difference is an engineering tolerance. If she later changes the circle to 130 mm, the software updates the cylinder and every feature built on it.
Once the model is ready, it goes to many other products. Simulation software (CAE) tests it. CAM software turns it into a cutting program. PLM software stores each version. A 3D printer can print it, and an injection moulding mould can be made from it. An on-demand manufacturing platform reads it to quote a price. A car company's car configurator can reuse the same 3D model.
A CAD file can hold more than one part. An assembly is a model in which many parts are placed together, so the engineer can see whether a shaft really fits into its bearing, and whether two parts touch where they should not. The engineer can also produce the flat 2D drawings, with sizes and tolerances written on them, which are sent to the workshop.
CAD is sold as a licence for each user, usually paid every year. Companies pay for it because the cost of one wrong drawing is far higher than the licence. Many CAD makers give free student versions, which is why engineering students often learn one tool in college and keep using it for their whole career.
CAD is also used outside factories. Architects draw buildings in BIM software, a kind of CAD for buildings. Animators draw characters in similar 3D tools.
CAD software has started to do more of the design itself. In generative design, the engineer states the goals, such as the load a bracket must carry and the metal it is made of, and the software proposes shapes. Newer AI models can also create a shape from a written description.