Precision Measurement and Inspection
GD&T Principles and Feature Control Frames
The language of GD&T
A feature control frame reads left to right: geometric characteristic symbol, tolerance zone (diameter symbol if cylindrical), any material condition modifier, then datum references in order of precedence. Know the families: form (flatness, straightness, circularity, cylindricity), orientation (perpendicularity, parallelism, angularity), location (position, concentricity, symmetry), profile of a line and of a surface, and runout (circular and total). Form tolerances are self-referencing and need no datum; orientation and location tolerances require datums.
The datum reference frame is established by primary, secondary, and tertiary datums, and the order changes the result - the primary datum constrains three degrees of freedom. Basic dimensions (boxed) locate features theoretically exactly and are used with position and profile tolerances.
Material condition modifiers
Maximum Material Condition (MMC) is the size limit that leaves the most material (smallest hole, largest pin). Least Material Condition (LMC) is the opposite. A tolerance at MMC gains bonus tolerance as the feature departs from MMC; regardless of feature size (RFS) gains none. Virtual condition is the combined limit of size and geometric tolerance.
Traps
Questions test whether you know bonus tolerance exists only with an MMC or LMC modifier, that a diametral position tolerance applies to the feature axis, and that datum precedence matters. Do not add bonus for RFS callouts.
Rule of thumb and worked example
Bonus = |actual size − MMC|. Worked example: hole Ø0.250 ±0.003 with position Ø0.010 MMC. Actual hole Ø0.252 → bonus 0.002 → total allowable position tolerance Ø0.012.
Precision Measurement and Inspection
Instruments and their limits
Know the common hand tools and what they are for: outside micrometers (0.001 in or 0.0001 in resolution), vernier and dial calipers (general reference, not tight tolerance), dial and test indicators (runout, flatness, alignment), bore gauges and pin/plug gauges (hole size), depth micrometers and depth gauges, height gauges on a surface plate, gauge blocks for calibration, thread wires and thread ring/plug gauges, and production instruments such as air gauges and CMMs.
Understand accuracy versus resolution, and the 10:1 rule: gauge discrimination should be about one-tenth of the part tolerance. Understand calibration, zeroing, and measuring force (micrometers have a friction thimble or ratchet). Standard reference temperature is 68 °F (20 °C); thermal expansion of aluminum and steel will move a reading. Watch for Abbe error, parallax on dials, cosine error on indicators, and worn or dropped mics. Gauge repeatability and reproducibility (GR&R) tells you whether the measurement system can even see the tolerance.
Traps
Common distractors: measuring a tight-tolerance bore with a caliper, checking a thread with a caliper instead of wires or gauges, forgetting to zero, reading a vernier in the wrong direction, or measuring a hot part straight from the machine.
Rule of thumb and worked example
Read the barrel first, then add the thimble. Worked example: a 0–1 in micrometer with 0.025 in per revolution - barrel shows 0.275 past the zero line and the thimble reads 12 → 0.275 + 0.012 = 0.287 in.