Visual and Dimensional Inspection
Visual inspection is the most-used NDE method and the cheapest first line of defense. Candidates must know the conditions that make it valid: adequate illumination (commonly around 100 fc / ~1000 lux at the surface for general work, higher for fine detail), surface preparation (clean, accessible, correct finish), viewing distance and angle, and inspector vision (near-vision acuity and color discrimination, periodically rechecked). Direct visual is unaided eye; aided visual uses magnifiers, borescopes, or remote cameras. Traps: assuming a borescope image equals direct visual resolution; forgetting that lighting and access limits define what visual can detect; confusing surface discontinuities with defects (a discontinuity is only a defect if it exceeds acceptance criteria).
Dimensional inspection covers direct measurement (calipers, micrometers, height gauges), comparison (go/no-go gauges), and GD&T verification. Understand datum reference frames, feature control frames, and that position tolerance defines a cylindrical zone at true position - a common trap is treating it as a simple ± box. Know GD&T bonus tolerance: as a feature departs from MMC toward LMC, the allowable position tolerance grows by the amount of departure.
Other concepts: flatness/straightness (form, no datum) versus perpendicularity/position (orientation/location, datum-referenced); runout (TIR); and gauge contact vs non-contact effects.
Rule of thumb: verify the datum scheme before measuring - a correct number against the wrong datum is a wrong answer. Worked example: a hole is Ø10.00 +0.10/−0.00 at MMC, with position tolerance Ø0.20 at MMC. Measured hole = Ø10.06 (0.06 departure from MMC). Bonus = 0.06, so allowable position tolerance = 0.20 + 0.06 = Ø0.26.