Orthographic projection
Graphic Communication · Higher · mrkellydt.co.uk
What you're learning in this topic
Learning intention WALT
We are learning about orthographic projection, specifically to:
know
- and arrange elevation, plan and end elevation in third-angle projection
- true lengths and true shapes
- the centres of internal and external radii (tangency)
understand
- auxiliary views
be able to
- show hidden detail correctly in every view
- work out missing sizes from an orthographic drawing
Success criteria WILF
- I can identify and arrange elevation, plan and end elevation in third-angle projection
- I can show hidden detail correctly in every view
- I can work out missing sizes from an orthographic drawing
- I can identify true lengths and true shapes
- I can explain and recognise auxiliary views
- I can locate the centres of internal and external radii (tangency)
Third-angle projection
Orthographic projection shows an object as separate flat (2D) views, each looking straight at one face. SQA uses third-angle projection.
- Elevation: the main view, looking at the front.
- Plan: looking down from above. In third angle it is drawn above the elevation.
- End elevation: looking from the side. In third angle the view from the left is drawn on the left of the elevation, the view from the right on the right.
The views are related: they line up with each other, with feint projection lines carrying sizes between them. Heights match between elevation and end elevation; lengths match between elevation and plan.
The third-angle symbol (a cone frustum and two circles, circles on the right) goes in the title block to show which system was used.
Hidden detail
Edges you can't see from that viewpoint, such as holes or internal cut-outs, are drawn as thin dashed lines. Every view must show the hidden detail that exists in it: in the assignment, missing hidden detail costs the component mark.
Reading sizes from a drawing
Exam questions often ask you to work out a missing size. Use the dimensions that are shown and add or subtract: overall length minus the known parts, or a radius doubled for a diameter. Check which view shows the size you need: heights come from elevations, depths from the plan or end elevation.
True length and true shape
A surface that slopes away from you looks shorter than it really is in a normal view (it is foreshortened). Its true shape can be found by projecting at 90° to the sloping surface, so you look straight at it. Exam questions ask you to pick the true shape from several options: imagine looking square-on to the cut face.
Auxiliary views HIGHER
An auxiliary view is an extra view projected at 90° to an inclined surface, showing that surface's true shape. An auxiliary plan, for example, is projected from the elevation at right angles to the sloping face. It is used when features on a sloping face (such as holes) would otherwise appear distorted.
Tangency HIGHER
Where an arc blends smoothly into a straight line or another arc, the join is a tangent point. The arc's centre is always exactly one radius away from each line it touches, on the inside of the curve.
- External radius (a rounded outside corner): the centre sits inside the material. A corner 15 from the datum with R2.5 has its centre at 15 − 2.5 = 12.5.
- Internal radius (a fillet in an inside corner): the centre sits in the open space beside the material. A step face 4 from the datum with an R1.5 fillet has its centre at 4 + 1.5 = 5.5.
Sketch the corner and ask "which side of the line is the curve's inside?" before adding or subtracting. Almost every candidate got the 2024 tangency questions wrong, so practise this.
Worked example: centre of an external radius HIGHER
A corner is 30 mm from the datum and is rounded with R4.
- For an external radius the arc's centre sits inside the material, one radius in from the edge.
- 30 − 4 = 26 mm from the datum.
Try it: a corner 22 mm from the datum with R3.5.
22 − 3.5 = 18.5 mm.
Key terms
| Third-angle projection: where is the plan? | Above the elevation. |
|---|---|
| Elevation | The main front view of an object. |
| End elevation | The side view, drawn on the side it is viewed from in third angle. |
| Related views | Views lined up so sizes project between them. |
| Hidden detail | Edges that can't be seen, drawn as thin dashed lines. |
| True shape | The actual shape of a sloping surface, seen by looking at 90° to it. |
| Auxiliary view | An extra view projected at 90° to an inclined face to show its true shape. |
| Tangency | Where an arc blends into a line; the arc centre is one radius from each line, on the inside of the curve. |
Quick check
1. In third-angle projection, the plan is drawn…
- Above the elevation
- Below the elevation
- To the left of the elevation
- On a separate sheet
Show answer
2. Hidden detail is shown with…
- Thick continuous lines
- Thin dashed lines
- Chain lines
- Wavy lines
Show answer
3. The third-angle symbol shows a cone frustum with its circles…
- On the left
- Above
- On the right
- Below
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4. To find the true shape of a sloping cut you look…
- From the front elevation
- Looking down from directly above
- From the end elevation
- At 90° to the cut surface
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5. An auxiliary view is used to…
- Show the true shape of an inclined surface
- Show hidden detail inside the part clearly
- Show how all of the parts fit together
- Show the part in full colour for the client
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6. An outside corner 15 from the datum is rounded R2. Its centre is…
- 15
- 13
- 17
- 30
Show answer
Printable worksheet
An I do, we do, you do worksheet for this topic, with a separate answer sheet.