2.1 Linear Measurement and the Steel Rule

Linear measurement is measuring length, width, height, thickness and diameter. In engineering we use the metric system: the base unit is the metre (m), and most shop measurements are in millimetres (mm).

Engineer's steel rule

• Made of spring steel or stainless steel.
• Common sizes: 150 mm, 300 mm, 600 mm.
• Graduations in mm and 0.5 mm (and inches on the other side).
• Accuracy: about ±0.5 mm.

Correct use

• Keep the rule's edge on the line of measurement, not at an angle.
• Do not measure from the worn end; start from the 10 mm mark and subtract 10.
• Keep your eye exactly above the reading. Looking from a side gives a wrong reading called parallax error.

Care

• Do not use the rule as a screwdriver or scraper.
• Keep it away from cutting tools; apply a thin film of oil to prevent rust.