Measurement, Tolerances and Technical Drawing
Candidates must read and interpret engineering drawings and measure accurately. Understand orthographic projection (first-angle and third-angle), the difference between visible, hidden and centre lines, section views, and the purpose of dimensioning and tolerancing. Know the meaning of nominal size, limits, tolerance (the permitted variation), clearance and interference fits, and geometric tolerances such as flatness, parallelism and perpendicularity.
Measurement tools: steel rule, vernier caliper (to 0.02 mm typically), micrometer (to 0.01 mm), dial indicator, and gauges. Know how to read a vernier scale (main scale plus vernier coincidence) and a micrometer (barrel plus thimble). Understand the difference between accuracy (closeness to true value) and precision (repeatability), and the effect of temperature and parallax error.
Traps: reading the vernier's zero instead of the coinciding line; mixing up first-angle (object between observer and plane, view placed opposite) and third-angle projection; forgetting that a tolerance is a total range (e.g. 20.00 ± 0.05 means 0.10 mm total); and confusing surface finish values with dimensional tolerance.
Rule of thumb: always record the unit and the tolerance, and measure at the same temperature as the reference. Worked example: a shaft dimension 25.00 ± 0.02 mm with a hole 25.05 ± 0.02 mm gives a minimum clearance of 25.03 − 25.02 = 0.01 mm, confirming a clearance fit.