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GLOSSARY · DIMENSIONAL QUALITY

GD&T, PMI, FAI and PPAP glossary, with examples.

The terms that come up every week between the customer’s model, the drawing, the CMM and the report. Each with a short definition, an example and the reference standard.

Contents

Standards are cited by number only. Always check the official text before applying them.

Product definition

PMI

Product and Manufacturing Information

Manufacturing information carried inside the 3D model: dimensions, tolerances, GD&T symbols, datums, finishes and notes. When the model is the master document, PMI replaces the drawing.

Example. A customer sends only the 3D model, with no drawing: every tolerance lives in its PMI. A supplier who cannot read it cannot quote or measure the part.

Reference: ASME Y14.41

Semantic PMI

vs graphical PMI

PMI a program can read as data: it knows that “Ø10 +0.2/0” is a diameter with its nominal and limits, and which face it applies to. Graphical PMI is just drawn lines and text: visible, but not machine-interpretable.

Example. From semantic PMI the characteristic list is built without typing. Graphical PMI has to be read note by note, like a drawing. That is why it pays to ask customers for STEP AP242 or JT with semantic PMI.

Reference: NIST MBE PMI Validation and Conformance Testing

MBD

Model-Based Definition

A way of working in which the 3D model with its PMI is the official definition of the part. The drawing disappears or becomes a simplified reference sheet.

Example. The costliest MBD mistake is measuring only what appears on a simplified drawing and missing a surface profile that exists only in the model.

Reference: ASME Y14.47

STEP AP242

ISO 10303-242

Neutral CAD exchange format that can carry exact geometry and semantic PMI. It is the best input for automatic characteristic extraction.

Example. A STEP AP214, or an AP242 exported without PMI, carries the geometry but no tolerances: ask the customer for an export with PMI.

Reference: ISO 10303-242

GD&T and tolerances

GD&T

geometric dimensioning and tolerancing

A symbol language that defines the allowable form, orientation, location and profile of a part relative to a datum reference frame.

Example. “Position Ø0.2 Ⓜ A B C” on a hole states the zone its axis must stay in, which datums it is measured from, and that the zone grows if the hole comes out larger.

Reference: ASME Y14.5 · ISO 1101

ASME Y14.5 and ISO GPS

two standards, two readings

The two main systems for interpreting GD&T. They use similar symbols, but the meaning is not always the same, for example for size and the envelope requirement.

Example. Check which standard the drawing invokes before measuring: the same size dimension can require perfect form at maximum material under ASME but not under ISO unless the envelope modifier Ⓔ is shown.

Reference: ASME Y14.5 · ISO 8015

Datum

datum reference frame

A face, axis or point of the part from which tolerances are measured. Three ordered datums (A, B, C) fix the part in space the way the real assembly would.

Example. A 0.5 profile to A|B|C measured with a best fit is not the same result: if the report does not state how the part was aligned, the customer cannot repeat the measurement.

Reference: ASME Y14.5 · ISO 5459

Maximum material and bonus

MMC Ⓜ · LMC Ⓛ · bonus tolerance

Ⓜ means the geometric tolerance applies at maximum material condition (smallest hole, largest shaft). The further the part moves from that condition, the more tolerance it gains: the bonus.

Example. Hole Ø10 +0.2/0 with position Ø0.2 Ⓜ. Measured Ø10.15: available tolerance becomes 0.2 + 0.15 = Ø0.35. A measured position of 0.28 passes; without applying Ⓜ a good part would be rejected.

Reference: ASME Y14.5 · ISO 2692

Position

true position

The zone within which the axis or median plane of a feature must lie relative to its theoretically exact location, defined from the datums.

Example. A hole pattern dimensioned from the datums can be controlled; chain-dimensioned hole to hole, the errors add up.

Reference: ASME Y14.5 · ISO 1101

Profile of a surface

surface profile

A uniform zone around the theoretical surface within which the whole real surface must lie. It controls form and, with datums, orientation and location too.

Example. It is the tolerance most often missed when it exists only in the 3D model: it usually covers whole functional areas, such as a clip outline or a sealing face.

Reference: ASME Y14.5 · ISO 1101

Basic dimension

TED, theoretically exact dimension

A boxed dimension that gives the exact theoretical location or shape. It has no tolerance of its own: the tolerance comes from the related GD&T symbol.

Example. A basic 25 is not measured as 25 ± something: you measure the hole’s position relative to that exact 25, within the position zone.

Reference: ASME Y14.5 · ISO 1101

General tolerance

title-block tolerance

The tolerance that applies to every dimension without its own. It is usually stated in the title block by citing a standard or a customer table.

Example. A 40 dimension without tolerance on a drawing that cites “ISO 2768-m” takes the medium-class limits for that size. With no title block or note, ask the customer before measuring or quoting.

Reference: ISO 2768 · ISO 22081

ISO fit (H7, g6…)

fit tolerance

A code that sets the clearance or interference between a hole and a shaft. The letter gives the tolerance position and the number its width: H7 is a tight-tolerance hole starting exactly at nominal.

Example. A Ø20 H7 allows a few hundredths above nominal and nothing below. It needs reaming or boring and a plug gauge or CMM to check: it adds cost even though it takes two characters on the drawing.

Reference: ISO 286

Surface roughness (Ra, Rz)

surface finish

A measure of the fine irregularities of a surface. Ra is the mean deviation; Rz the peak-to-valley height. It is shown with a surface-finish symbol on the face.

Example. A general Ra 1.6 is common in machining. A local Ra 0.4 on a seal seat usually means an extra finishing pass or grinding, plus a profilometer check.

Reference: ISO 21920 · ISO 1302

Part approval

PPAP

Production Part Approval Process

The automotive approval process for production parts. The supplier shows, with documents and real parts, that its process meets every customer requirement. It includes the dimensional results and the control plan, among other elements.

Example. Each new engineering level can trigger a new PPAP: the characteristic list must be redone and whatever changed measured again.

Reference: AIAG PPAP

FAI

first article inspection

A complete, documented check of the first part made with the production process: every drawing characteristic, with its requirement, result and measurement method. In aerospace it follows the AS9102 structure (EN 9102 in Europe).

Example. A change of drawing, process or manufacturing site, or a long production gap, usually triggers a new full or delta FAI, depending on the customer’s requirements.

Reference: AS9102 · EN 9102

PPF and initial sample report

VDA production process and product approval

The German counterpart of PPAP, defined by the VDA. The initial sample report (Erstmusterprüfbericht, EMPB) records the dimensional results of the sample parts.

Example. A supplier working for both American and German OEMs prepares the same measurement in two formats, PPAP and PPF. The characteristic list is the same.

Reference: VDA Band 2

Characteristic list and balloons

ballooned drawing

A numbered list of everything to be verified on the part, with nominal and limits. Each number appears in a balloon on the drawing or model, so the measurement and the report refer to the same feature.

Example. Balloon 7 must be number 7 on the drawing, in the Excel and in the CMM program. If numbering differs between documents, the customer rejects the report.

Reference: AS9102 · AIAG PPAP

Special characteristic

critical or significant

A characteristic affecting safety, regulations or function, flagged by the customer with its own symbol. It requires tighter control: proven capability, reinforced or 100% inspection.

Example. The same dimension can be normal for one customer and special for another. The characteristic list must keep the customer’s symbol, not replace it with an internal one.

Reference: IATF 16949 · AIAG APQP

Cpk and Ppk

capability indices

Indices comparing process variation with the tolerance, taking centring into account. Cpk uses short-term variation; Ppk, all observed variation. 1.33 and 1.67 are common targets.

Example. Five parts cannot prove a Cpk: the index swings too much from sample to sample. You need at least a few dozen measurements from a stable process.

Reference: AIAG SPC · ISO 22514

Measurement

Coordinate measuring machine

CMM

A machine that probes the part at points in space and computes sizes, forms and positions relative to the datums. It needs a program stating what to measure, how to align the part and how to evaluate.

Example. Two different programs on the same part can give different results if they align differently. That is why the report must state the alignment used.

Reference: ISO 10360

QIF

Quality Information Framework

An open format for exchanging quality information: characteristics, measurement plans and results. It lets the characteristic list reach the CMM without retyping.

Example. With the list in QIF, the CMM programmer starts from the same characteristics and numbers as the ballooned drawing instead of rebuilding them by hand.

Reference: ISO 23952 (QIF 3.0)

MSA

measurement system analysis

A study that checks whether an instrument and its method measure with enough repeatability and reproducibility for the tolerance being controlled (gauge R&R).

Example. A digital caliper with 0.01 resolution cannot control a ±0.01 tolerance: the instrument itself uses up the whole tolerance.

Reference: AIAG MSA · VDA Band 5

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