Measuring, marking out and inspection
This domain tests the correct selection and reading of measuring tools, accurate marking out from a datum, and inspection against a tolerance. Expect vernier calipers, micrometers, steel rules, try squares, protractors, dividers, scribes, punches and surface plates, plus concepts of accuracy, precision and zero error.
Core rules
- Work from a datum face or edge and mark all dimensions from that datum to avoid accumulated error.
- Clean the workpiece and instrument before measuring. Check the zero reading and record any zero error.
- Choose the instrument to the tolerance: a steel rule for rough work, a vernier caliper for outside, inside and depth dimensions, and a micrometer for precise outside dimensions.
- Avoid parallax by viewing the scale squarely. Do not overtighten a caliper or micrometer; use consistent measuring force.
- Inspection includes checking squareness with a try square, flatness on a surface plate, clearance with feeler gauges, and acceptance with go/no-go gauges.
Worked example
A metric vernier caliper has a least count of 0.02 mm. The main scale reads 12 mm before the vernier zero, and the 18th vernier line coincides with a main-scale line. Vernier contribution = 18 x 0.02 = 0.36 mm. Reading = 12.00 + 0.36 = 12.36 mm. If the caliper had a zero error of +0.03 mm, the actual dimension would be 12.36 - 0.03 = 12.33 mm. The reasoning matters: identify least count, read the main scale, add the vernier fraction, then correct for zero error.
Traps that cost marks
- Reading the main scale after the vernier zero instead of before it.
- Forgetting to apply zero error.
- Using a caliper as a wrench or scraper.
- Measuring over dirt, burrs or paint.
- Marking dimensions in a chain from different edges rather than from one datum.