The big idea: What a CAD model knows decides what you can ask it.
A surface model holds only the skin. A solid model holds the whole volume. A virtual model adds behaviour — motion, loads, sometimes a user.
Each kind with what it knows, what that lets you do, and what it cannot answer.
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| Surface | Solid | Virtual | |
|---|---|---|---|
| Holds | A skin with no thickness and no inside | The whole volume — material and air | Geometry plus behaviour and materials |
| Gives you | Free, complex curved form; a render | Mass, volume, centre of gravity, sections | Motion, collisions, simulated loads, many variants |
| Cannot give | Any mass or section — nothing is inside | How it behaves in use; it is geometry | Weight in the hand, grip, fit in a real room |
| Use it to | Sculpt a shell that is still changing | Produce a part that will be cut or printed | Check the arm clears the housing through its swing |
Parametric modelling is why CAD is worth the time: A parametric solid model is built from dimensions that are still editable. Change the grip circumference and every dependent feature updates, so a variant costs minutes rather than a rebuild.
That is the advantage to write about — not that CAD looks neat.
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Advantages
- Variants cost minutes, so many options can be tried
- Gives mass, volume and centre of gravity before anything is made
- Detects interference between parts that would only show at assembly
- Exports straight to printing and to machining
- Shared instantly with a manufacturer anywhere
Limitations
- Nothing on a screen has weight in the hand
- Everything fits perfectly; real parts have tolerances and burrs
- A model is only as good as the material data behind it
- Software and training are expensive, and skill takes months
- A beautiful model persuades people before it has been tested
The limitation examiners want: CAD cannot tell you what a product feels like to use.
Weight, balance, grip, fatigue and fit in a cluttered real room all need something you can hold. Say that, and say what you would build instead.
How this is tested — constructing and interpreting surface, solid and virtual models. It comes up two ways:
Paper 1 — multiple choice
- Identify which model type a stated task needs.
- Pick what a surface model cannot provide.
Paper 2 — analysing a product
- Justify the CAD model type for a named design task.
- Explain the advantages and limitations of CAD for a product.
The trap: Writing that CAD is "faster and more accurate" and stopping. The marks are for what a specific model type gives you that the alternative does not.
A team designing a cordless drill housing must style a curved shell, quote a mass to the client, and check that the trigger mechanism clears the shell through its travel. Justify the CAD model type for each task.
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