Line types, dimensioning & scale

Line types (British Standards)

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.

1. List what to check before it goes to the workshop.

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°.

Tolerances HIGHER

No part can be made to an exact size, so drawings state the acceptable range.

• Symmetrical tolerance: 80 ± 0.25.
• Limits: the maximum and minimum written out, e.g. 82.5 / 80.5. The purpose is that a part within these limits will still fit and work.
• Common tolerance: a note in the title block such as "all tolerances ± 0.5 unless stated".

Questions may ask for the maximum and minimum limits: add and subtract the tolerance.

Worked example: limits from a tolerance HIGHER

A dimension reads 64 ± 0.2.

1. Maximum limit: 64 + 0.2 = 64.2 mm.
2. Minimum limit: 64 − 0.2 = 63.8 mm.
3. A part between these limits will still fit and work.

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. 1:5 is a reduction, so divide the real size by 5.
2. 600 ÷ 5 = 120 mm on the drawing.

Real size from an enlargement. A slot measures 40 mm on a 5:1 detail view.

1. 5:1 is an enlargement, so divide the drawn size by 5.
2. 40 ÷ 5 = 8 mm real size.

Missing size. A bracket is 150 mm long overall; the sizes shown along it are 40 mm and 65 mm.

1. Overall length minus the known parts gives the missing size.
2. 150 − 40 − 65 = 45 mm.

Diameter from a radius. A rounded end is dimensioned R12.

1. Diameter = 2 × radius.
2. 2 × 12 = Ø24.

Key terms

Quick check