Topic 2 of 4 · Form 1
Marking out and measuring
Coat with marking blue (or chalk on rough surfaces) so scribed lines show clearly.
Notes for this topic 1 file, in order
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Tools
- Steel rule and tape: measuring lengths.
- Scriber: marking lines on metal.
- Engineer's try square: marking and checking right angles.
- Odd-leg calipers (jennies): lines parallel to an edge; finding the centre of a bar.
- Dividers: circles and arcs; stepping off distances.
- Centre punch: marks the centre of holes for drilling. Dot punch: marks lines so they stay visible.
- Surface plate, scribing block (surface gauge) and vee blocks: accurate marking from a flat surface and on round bars.
- Vernier calipers (to 0.1 mm or 0.02 mm) and micrometer (to 0.01 mm): precise measurement.
Preparing the surface
Coat with marking blue (or chalk on rough surfaces) so scribed lines show clearly.
Datum
Measure from a single reference edge or line (datum) to avoid adding up errors.
Worked example: Read a micrometer showing 7.5 mm on the sleeve and 23 on the thimble (0.01 mm divisions).
7.5 + 0.23 = 7.73 mm
Exam tip: Always mark the centre of a hole with a centre punch – otherwise the drill wanders.
Check yourself
- Which tool marks lines parallel to an edge?
- Why is marking blue used?
- What is a datum?
Show answers
- Odd-leg calipers (jennies).
- So that scribed lines show up clearly.
- A reference edge or line from which all measurements are taken.
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