The big idea: A drawing is chosen for the job it has to do, not for how good it looks.
Four drawing types
- Isometric shows the whole object in three dimensions.
- Orthographic specifies it in true sizes.
- Assembly shows the parts together.
- Exploded shows how they go together.
Each type with the job it does. Sketch explores, isometric communicates, orthographic specifies, exploded assembles.
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| Type | What it is | Use it when you need to… |
|---|---|---|
| Isometric (3D) | Three axes at 120°, all lines drawn to the same scale, nothing foreshortened | Show the whole object to somebody who has not seen it |
| Orthographic (2D) | Front, plan and side views in true size, aligned and projected | Give a manufacturer sizes that can be worked to |
| Assembly | The parts shown in their fitted positions, often sectioned | Show how the product goes together and what touches what |
| Exploded | Parts pulled apart along their assembly axes, usually numbered | Show the ORDER of assembly, and which fixing goes where |
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Built up the way it is actually drawn — front view first, the rest projected, then the conventions that make the sheet readable.
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The order to draw it in
Choose the front view
The one that shows the most about the shape. Every other view is projected from it, so a poor choice makes the whole sheet harder to read.
Project the plan and the side
Plan directly below, side directly beside. Heights carry across the row and widths down the column, which is how a reader transfers a size without measuring.
Add hidden detail and centre lines
Dashed for an edge you could not see from that direction; chain-dotted for the axis of any hole or cylinder. Drawing a hidden edge solid describes a different part.
Dimension from a datum, and label the sheet
Each feature dimensioned once, on the view that shows it best, from one datum face. Then scale, units, material and the projection symbol.
First angle or third angle: In third angle the view appears on the side you looked FROM. In first angle it appears on the opposite side.
Get it wrong and the part is made as a mirror image. That is why every sheet carries the projection symbol.
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The statement says construct AND interpret. Reading a drawing is a separate skill, and it is the one a Paper 2 question is most likely to test, because you can be handed a drawing you have never seen.
Reading a drawing you have not seen before
Find the projection symbol and the scale first
They tell you which side each view is on and whether a 30 mm line means 30, 60 or 15 millimetres.
Match a feature across two views
Pick a hole in the front view and find it in the plan. Doing that once tells you the orientation and stops you reading the part backwards.
Read dashed lines as things inside
A pair of parallel dashed lines running the length of a view is usually a bore or a slot passing right through.
Check the drawing against what you can measure
A stated dimension always beats one you scale off the paper. Sheets get printed at the wrong size and the dimension is what a manufacturer works to.
Which drawing for which conversation: Showing a concept to a user: isometric, or a rendered 3D model. Agreeing a size with a client: orthographic.
Explaining how a part is replaced: exploded. Choosing the wrong one makes the conversation twice as long.
What an assembly drawing does for you
Shows how parts fit
Every component is drawn in its place, so anyone can see which part touches which and in what position.
Fewer assembly mistakes
Workers, or users of flat-pack products, follow the picture instead of guessing, which matters most when there are many parts.
Quick to read
A visual is understood faster than written instructions, and across languages, by clients, makers and users.
Starts the conversation
The team can point at a joint or a part and discuss changes before anything is made.
How this is tested — constructing and interpreting 2D drawings and 3D models. It comes up two ways:
Paper 1 — multiple choice
- Identify the drawing type from a description or an image.
- Read a dimension or a hidden feature from a given drawing.
Paper 2 — analysing a product
- Describe the drawings needed for a named product and justify each.
- Interpret a supplied orthographic drawing of a component.
The trap: Offering an isometric view where an orthographic drawing is needed. Isometric shows the whole object but cannot carry a reliable dimension set, and it has no sections.
A cordless drill housing is to be discussed with a client, agreed with a manufacturer, and explained to a repair technician. Describe the drawings needed for each, with a reason.
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