Tools to represent an idea
Before making a product, designers must show the idea to others and check it. They use several tools:
- Freehand sketch: quick pencil drawing to explore ideas. Fast and cheap, but not exact.
- Technical drawing: exact views with dimensions and standards (front, top and side views).
- Physical model or prototype: card, foam or a 3D print to hold and test.
- Digital model (CAD): an exact 3D model on a computer, also called a digital mock-up.
- Simulation: the computer tests the model, for example how it bends under a load.
CAD stands for computer-aided design. It lets us change a design quickly, share it, check sizes and send it straight to a machine (CAM: computer-aided manufacture).
2D CAD and vector drawing
2D CAD draws flat shapes made of lines, arcs and circles. These are vector shapes: they are stored as maths (start point, end point, radius), not as pixels, so they stay sharp at any zoom. 2D CAD is used for floor plans, laser-cutting files and technical drawings.
A sketch in a 3D program is also 2D, drawn on a flat plane. Two kinds of rules make it exact:
- Constraints (geometric rules): horizontal, vertical, parallel, perpendicular, equal, tangent, coincident.
- Dimensions (sizes): lengths, diameters and angles, usually in millimetres.
When every line is fixed by constraints and dimensions, the sketch is fully defined and cannot be dragged out of shape by mistake.
Building 3D solids: extrude, revolve, cut and more
- Extrude: pull a 2D sketch straight out to give it thickness (a plate, a block).
- Revolve: spin a profile around an axis (a cup, a bottle, a wheel).
- Cut / pocket / hole: remove material using a sketch.
- Fillet rounds an edge; chamfer bevels it.
- Pattern repeats a feature in a row or circle; mirror copies it across a plane.
- Sweep moves a profile along a path; loft blends between two profiles.
Parametric modelling and the feature tree
Most CAD programs are parametric: every feature is stored, in order, in a feature tree (history). If you change a dimension in the first sketch, all later features update. Good practice: name features, fully define sketches and build from the main shape to small details.
Other ways to model
Surface and mesh / sculpt modelling shape free-form objects such as car bodies, characters and jewellery. Block-based tools build shapes from simple solids and are good for beginners.
Assemblies and working together
An assembly is a file that brings several parts together. Mates (assembly constraints) say how parts touch: concentric (centred on the same axis), coincident (faces together), distance, angle. Mates let you check that parts fit and move correctly, and find clashes (parts overlapping).
A bill of materials (BOM) lists every part and how many are needed.
In collaborative CAD, a team works on the same model online. Version history records who changed what, and each person can work on a different part of the assembly at the same time, like a shared document.
Simulation, output and uses of CAD
Simulation tests a design before it is made: stress under a load, airflow, heat, or motion of a mechanism. It saves time and material.
Outputs:
- 2D technical drawings with views and dimensions.
- Renders: realistic pictures for clients and marketing.
- STL / 3MF files for 3D printing; DXF / SVG for laser cutting; tool paths for CNC machines (CAM).
Who uses CAD: product design, mechanical and civil engineering, architecture, fashion and jewellery, furniture, film and game art, medical implants.
Advantages: exact, easy to edit and copy, easy to share, can be tested and sent straight to machines. Limits: needs a computer and training, software can be costly, and an on-screen model can hide problems you would notice by holding a real prototype.
Try it
Measure a real object, such as a phone stand or a pencil holder, with a ruler. Make a freehand sketch with the sizes, then build it in a free CAD tool using sketch → extrude → cut. Compare your model's volume with the 3D: volume = area of sketch × thickness.
Key formulas and definitions
- Volume of an extruded part = area of the sketch × extrude depth
- Area with a round hole = W × H − π × (d/2)²
- Mass = density × volume (PLA ≈ 1.24 g/cm³)
- 1 cm³ = 1000 mm³
- Scale: drawing size = real size × scale (1:2 means half size)
Worked examples
1. Which CAD tool would you use to make a drinking cup from a side profile?
Revolve. Spinning the side profile around a vertical axis makes a round cup.
2. A plate sketch is 60 mm × 40 mm and is extruded 20 mm. Find its volume in cm³.
V = 60 × 40 × 20 = 48 000 mm³ = 48 cm³.
3. A 12 mm hole is cut through that 20 mm plate. Find the new volume.
Hole area = π × 6² ≈ 113.1 mm². Hole volume = 113.1 × 20 ≈ 2262 mm³. New volume ≈ 48 000 − 2262 = 45 738 mm³ ≈ 45.7 cm³.
4. Find the mass of the 45.7 cm³ part if printed solid in PLA (1.24 g/cm³).
m = 1.24 × 45.7 ≈ 56.7 g.
5. A part is 150 mm long. How long is it on a 1:5 drawing?
150 ÷ 5 = 30 mm on the drawing.
6. Explain why a fully defined sketch is better than a loose one.
Every line is fixed by constraints and dimensions, so it cannot be dragged out of shape by mistake, and changing one dimension updates the shape in a predictable way.
Common mistakes
- Thinking a freehand sketch is useless once CAD starts. It is the fastest way to explore and explain ideas.
- Leaving sketches under-defined, so later edits break or distort the model.
- Mixing units: designing in centimetres but printing in millimetres makes a part 10 times too small.
- Confusing a part file with an assembly. A part is one solid; an assembly joins several parts with mates.