Measuring and Marking Out
What the domain tests
Measuring and Marking Out tests precision: reading steel rules, vernier calipers, micrometers and squares; establishing datums; and transferring dimensions accurately with scribers, dividers and punches.
Core rules
- Work from a datum, not from several unrelated edges. Mark all dimensions from the same reference where the drawing allows.
- A vernier caliper combines main-scale and vernier readings. For a 0.02 mm caliper, each vernier division represents 0.02 mm; add the number of the coinciding line to the main reading.
- A micrometer reads main sleeve millimetres plus thimble divisions. Check zero error before use and apply it with the correct sign.
- Use a scriber on prepared metal, a centre punch before drilling, and dividers for arcs and circles. Keep the point sharp and the workpiece steady.
- Allow for tolerance and finishing allowance; do not mark the finished size if material must remain for filing.
Worked example
A 0.02 mm vernier caliper shows 12 mm on the main scale and the 7th vernier line coincides with a main-scale line. Reading = 12 + (7 × 0.02) = 12.14 mm. If the caliper had a zero error of +0.03 mm, the true size would be 12.14 − 0.03 = 12.11 mm.
Traps
Parallax, a loose caliper jaw, and reading the wrong vernier line cost marks. Do not mark from a rough cast surface without a datum; do not centre-punch after drilling. Remember that a steel rule is not a precision comparator.