Reference

Inspector glossary

Working definitions. Not ISO legal text. Good enough to answer an exam item and talk on the floor.

10:1 rule: Instrument should resolve about one-tenth of the tolerance; the cal standard about one-tenth of the instrument.
5 Whys: Repeated why-questions to move from symptom toward a controllable root cause.
5S / 6S: Sort, Set in order, Shine, Standardize, Sustain, and Safety.
8D: Eight-discipline problem solving. Containment is D3; root cause is D4. They are not the same.
Ac: Acceptance number. Maximum defectives in the sample that still accept the lot. Re (rejection number) is usually Ac + 1 on a single plan.
Accuracy: Closeness of the measured average to the true or accepted reference value.
AQL: Acceptance Quality Limit. The process-average quality that a sampling plan will accept most of the time. Not a lot-by-lot guarantee.
Attribute data: Pass/fail or counted classification. Go/no-go, number of scratches, conforming vs. not.
Attribute gage: A gage that only decides accept or reject, such as a go/no-go plug.
Bias: A consistent offset from the reference value. Systematic error.
Bonus tolerance: Additional geometric tolerance available when a feature of size departs from MMC (or LMC (least material condition) if specified).
Calibration: Documented comparison of an instrument to a traceable standard, with uncertainty, on an interval. See <a href="https://www.nist.gov" rel="noopener noreferrer" target="_blank">nist.gov</a> for the top of a U.S. traceability chain.
CAPA: Corrective and preventive action. Corrective fixes a real root cause. Preventive addresses a potential issue that has not occurred.
CMM: Coordinate measuring machine. Measures part geometry in X, Y, Z relative to an alignment.
Common cause: The stable, inherent variation of a process. Do not chase it point by point.
Consumer's risk: β. Probability of accepting a lot that is actually at a rejectable quality level.
Control limit: UCL (upper control limit)/LCL (lower control limit) calculated from process data. Not the specification.
COPQ: Cost of poor quality: prevention + appraisal + internal failure + external failure.
Cp: Capability index using spec width over 6σ. Ignores centering.
Cpk: Capability index that includes centering. The lesser of the two one-sided ratios.
Critical defect: A defect likely to create an unsafe condition or prevent a primary function.
Datum: A theoretically exact plane, axis, or point derived from a datum feature.
Defect: A single flaw on a unit. c and u charts count defects.
Defective: A unit that fails. It may carry several defects. p and np charts count defectives.
Differential measurement: Compare the part to a master and read the difference (indicator + block stack).
Direct measurement: The instrument reads the size itself (micrometer on a shaft).
Discrimination: Smallest change the instrument can indicate. Resolution.
DMAIC: Define, Measure, Analyze, Improve, Control. Six Sigma project framework.
FAIR: First Article Inspection Report. Often <a href="https://www.sae.org/standards/content/as9102/" rel="noopener noreferrer" target="_blank">AS9102</a> in aerospace.
Feature control frame: The boxed GD&T (geometric dimensioning and tolerancing) callout: characteristic | tolerance | datums.
Feature of size: A feature with opposed elements that has a size dimension — hole, pin, slot width.
FIFO: First in, first out. Age-control method for material.
First-article: Inspection of the first piece(s) from a new or changed setup to prove the process can meet the print.
Flinching: Calling a borderline characteristic good to avoid conflict or delay. An inspection error.
Forming / storming / norming / performing / adjourning: Tuckman team stages. Storming is normal. Do not kill the team there.
Gage block: Precision rectangular end standard, wrung into stacks, used as a working or reference length.
Gage R&R: Study of repeatability (equipment) and reproducibility (appraisers) of a measurement system.
GD&T: Geometric dimensioning and tolerancing. Language of form, orientation, location, profile, and runout per ASME Y14.5 (the U.S. dimensioning and tolerancing standard). Official rules: <a href="https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing" rel="noopener noreferrer" target="_blank">ASME Y14.5</a>. See also the <a href="/gdt">GD&T field card</a>.
Go/no-go: Attribute pair. Go must enter / fit; no-go must not. Never force the no-go.
Histogram: Bar picture of how a single variable is distributed.
Incoming inspection: Check of purchased product before it enters your process.
ISIR: Initial Sample Inspection Report. Dimensional / capability package similar in spirit to PPAP (Production Part Approval Process) evidence.
Lay / surface finish: Specified texture. Measured with a profilometer, not a thumbnail, when the print calls a number.
Linearity: Change in bias across the range of the instrument.
LMC: Least material condition. Largest hole, smallest pin.
Lot: A collection of units from which a sample is drawn for a decision.
LTPD: Lot tolerance percent defective. A poor quality level the plan should reject most of the time. Also called RQL (rejectable quality level).
Major defect: Likely to cause failure or serious reduction in usability.
MBD: Model-based definition. Product definition lives in the 3D model rather than only on a 2D print.
Mean: Arithmetic average. Sum divided by n.
Median: Middle value of an ordered set.
Minor defect: Unlikely to reduce usability much; typically workmanship departure.
MMC: Maximum material condition. Smallest hole, largest pin.
Mode: Most frequent value.
MRB: Material Review Board. Authority that dispositions nonconforming product.
MSA: Measurement system analysis. Evaluation of whether the measuring process is trustworthy.
NDT: Nondestructive testing. The part remains usable: PT (liquid penetrant testing), MT (magnetic particle testing), UT (ultrasonic testing), ET (eddy current testing), RT (radiographic testing), VT (visual testing).
NIST: U.S. National Institute of Standards and Technology. Common top of a domestic traceability chain. Site: <a href="https://www.nist.gov" rel="noopener noreferrer" target="_blank">nist.gov</a>.
Nonconforming: A unit that fails one or more specifications. Distinct from a defect count on that unit.
Normal distribution: Symmetric bell curve. Mean = median = mode. Empirical 68-95-99.7 rule.
np chart: Attribute chart of the count of nonconforming units in a constant sample size.
OC curve: Operating characteristic curve. Shows probability of lot acceptance versus lot quality for a given plan.
p chart: Attribute chart of the fraction nonconforming. Sample size may vary.
Pareto chart: Bars of categories in descending frequency, often with a cumulative line.
PDCA / PDSA: Plan-Do-Check/Study-Act. Iterative improvement cycle.
Poka-yoke: Error-proofing. A device or method that makes the wrong action hard or impossible.
Population: Every unit of interest. Sample statistics use n−1; population uses N.
Position tolerance: How far an axis or centerplane may depart from true position, often with bonus at MMC (maximum material condition).
PPAP: Production Part Approval Process. Customer package: control plan, dimensional results, capability, and more.
Precision: Closeness of repeats to each other, regardless of truth.
Producer's risk: α. Probability of rejecting a lot that is actually at AQL (acceptance quality limit).
Profile: GD&T (geometric dimensioning and tolerancing) control of a line or surface versus a true profile.
Quarantine: Segregated hold area for unidentified or nonconforming material.
Random sample: Every unit in the lot had an equal chance of selection.
Range: Maximum minus minimum.
Record vs document: A document says what should happen. A record proves what happened.
Repeatability: Variation with the same appraiser, same instrument, same part. Equipment variation.
Reproducibility: Variation between appraisers using the same instrument. Appraiser variation.
Resolution: Smallest readable increment of an instrument.
RFS: Regardless of feature size. No bonus tolerance.
Rockwell: Hardness family using minor then major load and measuring depth. HRC (Rockwell C hardness), HRB (Rockwell B hardness), etc.
Runout: Composite control as a part rotates about a datum axis. Circular or total.
Sample size n: How many units you actually inspect from the lot.
SDS: Safety Data Sheet. Hazard and handling information for a chemical. See <a href="https://www.osha.gov/hazcom" rel="noopener noreferrer" target="_blank">OSHA (Occupational Safety and Health Administration) HazCom</a>.
Sine bar: Angle-setting tool. Rise = length × sin(angle).
Special cause: Variation from an assignable event. The control chart's reason for existing.
Specification limit: LSL (lower specification limit)/USL (upper specification limit) from the drawing or spec. Voice of the customer.
Stability: Drift of a measurement system over time on the same standard.
Standard deviation: Square root of variance. Typical spread of a data set.
Surface plate: Precision reference plane, usually granite, for layout and height measurement.
Traceability: Unbroken documented chain from a measurement to a national or international standard, with uncertainties. In the U.S. that chain often ends at <a href="https://www.nist.gov" rel="noopener noreferrer" target="_blank">NIST (National Institute of Standards and Technology)</a>.
Transfer gage: Tool that copies a size so another instrument can read it. Telescoping gage, small-hole gage.
TUR: Test Uncertainty Ratio. Some labs use 4:1 instead of 10:1. If the question says 10:1, use 10:1.
u chart: Defects per unit when opportunity area can vary.
Uncertainty: Quantified doubt about a measurement result.
Variable data: Measured on a continuous scale. Inches, volts, grams.
Variable gage: Instrument that reports a number: mic, caliper, CMM (coordinate measuring machine), indicator.
Variance: Average squared deviation from the mean.
Vernier: Auxiliary scale that interpolates between main-scale divisions.
Waiver: Customer or MRB (material review board) permission to use product that is not fully to spec.
White-out: Not allowed on quality records. Single line through, initial, date.
Wringing: Sliding gauge blocks together so the air film leaves and they cling. Do not leave them wrung for days.
X-bar and R: Variables control chart pair for subgroup averages and ranges.
Z1.4: ANSI (American National Standards Institute)/ASQ (American Society for Quality) sampling-table standard for attributes. Descended from MIL-STD-105 (the old military sampling tables).