Metrology
Metrology is the science of measurement. For this exam it is also the craft: pick the instrument, handle it so you do not lie to yourself, keep it traceable, and know what the number is worth.
Accuracy, precision, and the rest of the MSA (measurement system analysis) words
Learn these cold. They appear in II.G and they leak into every gage question.
| Term | Meaning on the floor |
|---|---|
| Accuracy | Closeness of the average of measurements to the true / reference value. A biased mic can be precise and still wrong. |
| Precision | Closeness of repeated measurements to each other. Tight cluster, maybe in the wrong place. |
| Bias | Systematic offset from the reference. One-direction error. |
| Linearity | How bias changes across the operating range of the gage. |
| Stability (drift) | How the measurement system changes over time when measuring the same standard. |
| Repeatability (EV (equipment variation)) | Same part, same appraiser, same instrument — the equipment variation. |
| Reproducibility (AV (appraiser variation)) | Different appraisers using the same instrument on the same parts — appraiser variation. |
| R&R | Repeatability and reproducibility combined. The measurement-system slice of observed variation. |
| Resolution / discrimination | Smallest increment the instrument can indicate. Rule of thumb: at least ten increments inside the tolerance (related to the 10:1 rule). |
| Uncertainty | The doubt that remains after the measurement, expressed as an interval with a confidence statement. Calibration certificates carry it. |
II.A Common gages Apply
Variable gages — they give a number
- Steel rule / scale. Coarse. Good for stock and rough layout. Typical resolution 1/64" or 0.5 mm.
- Calipers (vernier, dial, digital). OD (outside diameter), ID (inside diameter), depth. Typical resolution 0.001". Faster than a mic, less rigid, easier to cock.
- Micrometers. Outside, inside, depth. 0–1", 1–2", etc. Typical resolution 0.0001" with a vernier. Square the spindle, use ratchet or feel consistently, zero on a standard.
- Dial / digital indicators. Comparative. Need a setup and a zero. Watch cosine error if the tip is not perpendicular to the motion.
- Gage blocks. End standards. Wring them. Temperature matters. Grade (0, AS-1, etc.) sets how you use them — working vs. reference.
- CMM (coordinate measuring machine). Measures coordinates in X, Y, Z. Software fits features to datums. Powerful and easy to misuse if the part is dirty, the probe is wrong, or the alignment is fiction.
- Borescope, thermometer, temperature probe. Named in the 2025 BoK (Body of Knowledge). Visual access and thermal conditions belong in the inspector’s kit. Steel expands about 6.5 µin/in/°F (11.7 µm/m/°C). Reference temperature for dimensional work is 20 °C / 68 °F.
Attribute gages — they give yes / no
Go/no-go plug, ring, snap, flush pin, progressive plug, pin gage, radius gage, thread plug and ring. Attribute gages do not tell you how close you are. They tell you whether you are inside the limit they were built to. Never force a no-go.
Transfer gages
Telescoping gage, small-hole gage, spring caliper. They copy a size so a micrometer can read it. The skill is the feel when you lock the transfer tool.
Scales on instruments
Vernier, dial, digital. You must be able to read a vernier if they print one. Main scale plus coinciding vernier line.
II.B Special gages Remember
Know what the family is for. You will not be asked to operate an X-ray cabinet from memory.
- Electronic: oscilloscope, multimeter.
- Automatic: machine vision, ultrasonic, X-ray, laser.
- Pneumatic: air probes and rings. Fast, good for honed bores, sensitive to air supply and orifice condition.
- Force: torque wrench, load cell.
- Environment: chart recorder, data logger.
II.C Selection, handling, 10:1, correlation Apply
Select the gage from the characteristic, the tolerance, the environment, the resolution, and whether you need a direct reading, a differential (comparison to a master), or a transfer. Handle gages like they are the only thing standing between you and a false accept: clean, no drops, no stacked heat from your hand on a long measurement, storage in a case, not in the chip pan.
Correlation: two systems measuring the same parts should agree within a stated limit — bench mic vs. in-line laser, manual vs. CMM. You establish it with a study, not a hope.
II.D Surface plate work Apply
A granite or cast-iron plate is the reference plane. Keep it clean. No food. No slamming parts. Rotate wear. Periodic calibration and, on granite, lapping when it goes out. Height gage, V-blocks, angle iron, jack stands, indicators, gage blocks, sine bar, protractor, angle blocks. Align the part to the datums the drawing names, not to whatever face sits flat by accident.
II.E Specialized equipment
- Mass: balance, scale, calibration weights. Level the bench. Draft shields on analytical balances.
- Finish: stylus profilometer (Ra (average surface roughness), Rz (mean peak-to-valley roughness), etc.) and optical methods. Know that “32 finish” on a drawing is a specification, not a guess with your thumbnail. Fingernail comparators were dropped from the 2025 BoK.
- Shape / profile: mechanical comparator, roundness tester, precision spindle, profile tracer. Understand-level in 2025.
- Optical: optical comparator (shadow graph) and measuring microscope.
- Software-based: vision systems (white light / blue light), digital cameras, articulating arms, laser trackers. Know the software can lie about datums if you pick the wrong targets. X-Y-Z is the machine coordinate idea; alignment creates the part coordinate system.
- Inclination: mechanical or laser measurement of slope.
II.F Calibration Understand / Apply
Calibration is comparison of an instrument to a standard of known value, with documented uncertainty, on a schedule. A sticker and an asset number are how you control it on the floor. RFID (radio-frequency identification) and barcodes are named in the BoK.
Hierarchy of standards: international (SI (the international metric system) realizations at national labs such as NIST (National Institute of Standards and Technology)) → national → primary / reference → working standards → the shop instrument. Traceability is the unbroken, documented chain with uncertainties.
Intervals come from use, environment, stability history, and risk — not from a random annual habit if the tool is beaten daily. After adjustment or repair, you record as-found and as-left. As-found out of tolerance means you assess product that was accepted with that tool (recall / reinspect logic).
Environment: temperature (steel vs. granite vs. aluminum grow differently; 20 °C / 68 °F is the usual reference), humidity, vibration, dirt. Do not calibrate a mic on a running punch press.
II.G Measurement system analysis Understand
MSA asks whether the measurement system is a small enough slice of the tolerance that you can trust the number. A common shop study is Gage R&R: several parts, several appraisers, several trials. Results are often reported as % of tolerance or % of process variation. Rules of thumb you will see in references: under ~10% is generally considered adequate, 10–30% may be acceptable depending on application, over 30% needs work. Memorize the meanings of bias, stability, precision, accuracy, linearity, repeatability, reproducibility. The exam at Understand level will not make you run Minitab. It will make you pick the right term for a story.