Metrology & Measurements
From transcript: 20ME430 Metrology And Measurements (B)
Cheat sheet
Formulas
Error = measured − true ; uncertainty statement with confidence
Abbe principle: measuring line along scale line — minimize Abbe offset
Limits & fits: basic size + deviations; hole-basis H system common
Allowance = MC hole − MC shaft (clearance fit positive)
Tolerance grade IT01…IT16 — smaller number tighter
Surface Ra arithmetic average roughness; Rz peak measures
Sine bar: h = L sinθ
Gauge R&R: repeatability & reproducibility of measurement system
Error = measured − true ; Uncertainty statement with coverage
Least count (vernier) = main scale division − vernier division
Gauge R&R: % study variation from repeatability/reproducibility
Definitions
- Accuracy vs precision
- Close to true vs tight scatter
- Resolution
- Smallest discernible change
- Calibration
- Compare to standard; adjust/report correction
- Comparator
- Relative measurement vs master
- Interchangeability
- Parts assemble without selective fit
- GO/NOGO gauges
- Attribute check of limits
- CMM
- Coordinate measuring machine — 3D probing
- Form error
- Straightness, flatness, circularity, cylindricity
- Abbe principle
- Measure in line with axis to reduce cosine error
Topic-wise short notes
Linear & angular measurement
- Vernier, micrometer, height gauge, dial indicator — least count & care.
- Slip gauges (Johansson) build lengths; angle gauges / autocollimator.
- Optical flat / interferometry for flatness.
Limits, fits, surface texture
- Clearance, transition, interference fits — select by function.
- Shaft-basis vs hole-basis systems.
- Lay, waviness vs roughness — filtering cutoff.
- Statistical process control: X̄–R charts from measured data.
GD&T measurement link
- Datums define fixturing for CMM — same as drawing.
- Bonus tolerance at MMC enables functional GO gauges.
- Form vs orientation vs location controls — hierarchy of restraint.
Exam traps & quick notes
- Pick instrument for tolerance: rule-of-ten (resolution ≤ 1/10 tol).
- Surface finish Ra vs Rz — process fingerprint.
- CMM / optical for complex aero surfaces.
- Temperature 20°C standard for dimensional metrology.
- Always wring gauge blocks; clean contact surfaces.
- Choose instrument 10× better than tolerance (rule of thumb).
- GD&T datums define measurement setup — not optional art.
- Surface finish affects sealing, fatigue, friction — specify Ra and process.
Comprehensive notes
Linear & angular measurement
Vernier, micrometer, dial indicators, sine bar, slip gauges, comparators.
External: NPTEL Metrology ↗ NIST metrology resources ↗
Limits, fits, surface texture
ISO hole basis fits; geometric tolerances overview; profilometry.
External: Engineering fits chart ↗
Measurement systems
Vernier/micrometer least count; dial indicators; slip gauges; sine bar. Abbe principle. Uncertainty vs tolerance (rule of ten).
External: NPTEL ↗ Engineering Toolbox ↗
Limits, fits, texture
ISO hole-basis fits; geometric tolerances intro; Ra/Rz surface texture as process fingerprint. Gauge R&R in quality systems.
External: NPTEL ↗ Engineering Toolbox ↗
Interview Q&A for this subject
Q: Accuracy vs precision?
A: Accuracy = closeness to true value; precision = repeatability scatter.
External: ASQ / quality ↗ MIT OCW ↗