What CAD is
Computer-aided design, usually shortened to CAD, means using software to create, edit and document the shape of a product or structure. Instead of drawing by hand on paper, the designer builds a digital description of the geometry. That description can be viewed from any angle, measured, checked against other parts, used to produce drawings, and often passed on to analysis or manufacturing tools.
The word “design” in the name is slightly generous. CAD helps you describe and refine a design, but the decisions about what to make and why still belong to the engineer. The software is a tool for expressing and checking those decisions.
2D drafting and 3D modelling
CAD tools broadly fall into two styles, and many programs offer both.
2D drafting works with lines, arcs, text and dimensions on a flat sheet, much like a digital drawing board. It is still widely used for layouts, schematics, floor plans and simple parts, because it is quick and the output maps directly onto a conventional drawing.
3D modelling builds a model of the object in three dimensions. There are a few common ways to represent it:
- Solid models describe a part as a closed volume, so the software can calculate properties such as volume and, given a material, mass.
- Surface models describe only the outer skin of a shape, which is useful for smooth, complex forms.
- Wireframe models show edges only and are mostly of historical or limited use.
For most mechanical work today, solid modelling is the main approach. A 3D model removes much of the guesswork of reading several flat views, and it lets you check that parts fit together before anything is made.
Parametric models
Many CAD systems are described as parametric. In a parametric model, shapes are controlled by dimensions and relationships rather than by fixed positions alone. If a plate is defined as 100 mm long with a hole centred on it, and you later change the length, the hole can stay centred because the relationship is stored with the model.
A typical parametric workflow looks like this:
- Draw a 2D sketch on a plane.
- Add dimensions and geometric constraints, such as “these two lines are parallel” or “this circle is concentric with that arc”.
- Turn the sketch into a 3D feature by extruding, revolving or cutting it.
- Add more features, such as holes, fillets and patterns.
- Record the sequence of features in a history, often shown as a feature tree.
The advantage is flexibility: changing a parameter regenerates the model. The cost is that the model has an internal logic, and a careless structure can break when edited. Habits that avoid this are covered in the article on CAD modelling habits.
Some systems use other methods, such as direct modelling, where you push and pull the geometry without a stored history. Each approach has trade-offs, and which suits you depends on the work.
Parts, assemblies and drawings
CAD data is usually organised in layers of increasing complexity.
- Parts are individual components, each with its own geometry and material.
- Assemblies place parts together using mates or constraints, so you can check fit, clearances and movement.
- Drawings are 2D documents generated from the model, with views, dimensions, tolerances and notes for manufacture or inspection.
Because drawings can be generated from the model, they stay consistent with it, provided the model is kept up to date. Dimensions and tolerances still need engineering judgment. A model shows nominal geometry, and real parts always vary, which is why tolerances and measurement uncertainty matter.
Where CAD fits in the workflow
CAD is one tool among many in the engineering design process. It is most heavily used during detailed design, but it appears at other points too.
- Concept stage: rough models help explore layouts and proportions, and quick sketches can come before any software is opened.
- Analysis: the geometry can often be exported or linked to engineering simulation tools for stress, heat or flow studies.
- Prototyping and manufacture: models can feed 3D printing, machining and other processes, depending on the tools available.
- Documentation: drawings and bills of materials are generated or supported by the model.
- Maintenance: models and drawings help people service and modify the product later.
Since CAD files change many times during a project, managing them carefully matters. They are often binary files that behave differently from text, and the topic of binary file versioning explains how teams keep track of them.
Choosing and learning a CAD package
There are many CAD programs, from free and open tools to large commercial suites, and they differ in focus. Some are aimed at mechanical parts, others at architecture, electronics layout or industrial styling. Rather than choosing by reputation, consider:
- What kind of product you design.
- What your team, clients or suppliers already use, since file exchange can be awkward.
- What file formats you need to import and export.
- Cost and licensing terms, which vary and change.
- The availability of training material and a user community.
The concepts transfer well between packages: sketches, constraints, features, assemblies and drawings appear almost everywhere under slightly different names. Learning one tool well makes the next easier.
Limits to keep in mind
A good-looking model is not a good design. CAD will happily let you draw a part that cannot be manufactured, assembled or inspected, or that fails under load. Checks of manufacturability, strength and safety still rely on engineering knowledge, calculation, testing and, where required, review by qualified people. Standards for drawings and documentation differ between countries and industries, so follow those that apply to you.