Drawing & spatial awareness · National 5
Line types, dimensioning & scale
8 cards · 6 quiz · 0 exam
Graphic Communication · National 5 · mrkellydt.co.uk
What you're learning in this topic
Learning intention WALT
We are learning about line types, dimensioning & scale, specifically to:
know
- British Standards errors on an orthographic drawing
be able to
- name and draw outline, projection, dimension, hidden detail, centre, cutting plane and fold lines
- apply chain, parallel, radial, diameter, angular, square, A/F and A/C dimensioning
- use and calculate with scales
Success criteria WILF
- I can name and draw outline, projection, dimension, hidden detail, centre, cutting plane and fold lines
- I can apply chain, parallel, radial, diameter, angular, square, A/F and A/C dimensioning
- I can spot British Standards errors on an orthographic drawing
- I can use and calculate with scales
Line types (British Standards)
| Line | What it looks like | Used for |
|---|---|---|
| Outline | Continuous thick | Visible edges |
| Projection and dimension lines | Continuous thin | Extension lines and dimension lines |
| Hidden detail | Thin dashes | Edges that can't be seen |
| Centre line | Thin chain (long dash, short dash) | Centres of circles, axes of symmetry |
| Cutting plane | Thin chain, thick at the ends and corners, with arrows and letters (A–A) | Where a section is taken and which way to look |
| Fold line | Thin chain with two short dashes | Folds on a surface development (net) |
Dimensioning rules
- All sizes in mm; don't write "mm" after each one.
- Extension lines start with a small gap from the object and run just past the dimension line.
- Dimension lines are thin, with filled arrowheads touching the extension lines.
- Numbers sit above the dimension line, readable from the bottom or the right of the sheet.
- Don't let dimension lines cross; place smaller dimensions nearer the object.
- Show each size once, in the view that shows that feature most clearly.
- Don't use outlines or centre lines as dimension lines.
Design challenge: production drawing check
The brief. A pupil's production drawing of a bracket has errors.
- List what to check before it goes to the workshop.
What a good answer includes
- Third-angle symbol, with the views related.
- Correct line types, including hidden detail in every view.
- Dimensions in mm, each shown once, not crossing.
- Ø and R used correctly.
- A scale big enough to read.
Types of dimension
- Chain: dimensions follow on from each other, end to end. Shows the size from one feature to the next, but errors can add up.
- Parallel (datum): all sizes measured from one datum edge, stacked in parallel. Avoids accumulated error.
- Radius: R10. The leader points from the arc towards its centre.
- Diameter: Ø20, for circles.
- Angular: in degrees, with an arc-shaped dimension line.
- Square: □20 means the feature is square in section.
- Across flats (A/F) and across corners (A/C): for hexagons.
- Chamfer: e.g. 4 × 45°.
Scale
Scale compares the drawing size with the real size.
- 1:1 full size. 1:2, 1:5, 1:10 reductions (the drawing is smaller). 2:1, 5:1 enlargements, for small parts or detail views.
- To find a drawn size: real size ÷ scale (1:5 → divide by 5). To find a real size from an enlargement of 5:1, divide the drawn size by 5.
- Choose a scale big enough to show detail and allow clear dimensions; examiners criticised assignment drawings that were too small to read.
Worked examples: scale and sizes
Drawn size from a scale. A shelf is 600 mm long, drawn at 1:5.
- 1:5 is a reduction, so divide the real size by 5.
- 600 ÷ 5 = 120 mm on the drawing.
Try it: a 450 mm part drawn at 1:10.
450 ÷ 10 = 45 mm.
Real size from an enlargement. A slot measures 40 mm on a 5:1 detail view.
- 5:1 is an enlargement, so divide the drawn size by 5.
- 40 ÷ 5 = 8 mm real size.
Try it: a hole measures 30 mm on a 2:1 drawing.
30 ÷ 2 = 15 mm.
Missing size. A bracket is 150 mm long overall; the sizes shown along it are 40 mm and 65 mm.
- Overall length minus the known parts gives the missing size.
- 150 − 40 − 65 = 45 mm.
Try it: overall 210 mm, with parts of 70 mm and 85 mm shown.
210 − 70 − 85 = 55 mm.
Diameter from a radius. A rounded end is dimensioned R12.
- Diameter = 2 × radius.
- 2 × 12 = Ø24.
Try it: R7.5.
2 × 7.5 = Ø15.
Key terms
| Centre line | Thin chain line: long dash, short dash. |
|---|---|
| Cutting plane line | Thin chain, thick at the ends, with arrows and letters. |
| Fold line | Thin chain line with two short dashes, used on surface developments. |
| Chain dimensioning | Dimensions end to end; shows feature-to-feature sizes but errors accumulate. |
| Parallel (datum) dimensioning | All sizes from one datum; avoids accumulated error. |
| □20 on a dimension | The feature is square in section. |
| A/F | Across flats: the size between opposite flat sides of a hexagon. |
| Scale 5:1 | The drawing is five times bigger than the real object. |
Quick check
1. Which line type shows the centre of a circle?
- Thin dashed
- Continuous thin
- Continuous thick
- Thin chain
Show answer
D: Thin chain. Centre lines are thin chain.
2. Dimensions placed end to end are called…
- Chain
- Parallel
- Datum
- Angular
Show answer
A: Chain. Chain dimensioning.
3. Which avoids the build-up of errors?
- Chain dimensioning
- Parallel (datum) dimensioning
- Running leader lines
- Hidden-detail dimensioning
Show answer
B: Parallel (datum) dimensioning. All sizes from one datum.
4. On a drawing at 1:5, a 40 mm line represents…
- 8 mm
- 40 mm
- 200 mm
- 45 mm
Show answer
C: 200 mm. Real size = 40 × 5.
5. The symbol □ before a dimension means…
- Cubed
- Approximate
- Area
- Square section
Show answer
D: Square section. Do not write "cubed".
6. Dimension figures should be readable from…
- Any direction around the drawing
- The bottom or right of the sheet
- The top or left of the sheet
- The left-hand side of the sheet only
Show answer
B: The bottom or right of the sheet. BS convention.
Printable worksheet
An I do, we do, you do worksheet for this topic, with a separate answer sheet.