Finding the inspection characteristics on a drawing

What counts as a characteristic?

• Sizes with a tolerance, such as Ø20 0/−0.05.
• Sizes without a tolerance: they take the general tolerance named in or near the title block (see reading sizes on a drawing).
• Geometric tolerances: form, orientation, location and run-out, each in its own frame (GB/T 1182, ISO 1101).
• Surface texture requirements, such as an Ra value on a surface symbol.
• Notes that set a requirement, such as “remove burrs and break sharp edges”, a material or a heat treatment.
• Requirements added by the purchase order or by a specification the drawing invokes.

Reference dimensions, usually shown in parentheses, and basic (theoretically exact) dimensions are for information or show a relationship; IAQG’s first article guidance classes both as reference characteristics, not requirements to judge. A basic dimension is not checked on its own; the tolerance zone it locates is judged through the geometric tolerance. Some customers still ask for them to be numbered and measured; follow the customer’s procedure.

Numbering the drawing (ballooning)

Give every requirement a unique number, usually written in a circle next to it on a copy of the drawing; this is the ballooned or bubbled drawing. The IAQG guidance on AS9102 does not prescribe a method or make a ballooned drawing mandatory: any technique works as long as each requirement can be traced from the drawing to its result.

Number in a fixed order, for example zone by zone or view by view, so that a second person can check that nothing was missed.

From the drawing to a characteristic list

For each number, write what is required and work out the limits. The stepped shaft from the KELIBRON lab gives this list:

The next step, choosing an instrument and the measuring points for each line, is the measurement plan; the same numbers then head the columns of the first article record.

Key and critical characteristics

Some drawings mark characteristics with designators such as key, critical, major or minor. The design authority defines what each mark means in its engineering documents; the first article standard does not.

IAQG’s overview of AS9103 quotes the 2001 AS9100 definition: a key characteristic is a feature of a material or part whose variation has a significant influence on fit, performance, service life or manufacturability. AS9103 (current edition AS9103B, 2022) sets how that variation is managed in production. For the inspector, a marked characteristic carries whatever extra requirement the customer attaches to the mark, such as variation data under AS9103.

Characteristics you cannot reach on the finished part

Some requirements cannot be measured once the part is finished, such as a bore closed off by a later assembly step. Verify them during manufacture, after the last operation that can affect them, or by a destructive test, and record where the result was taken.

Practising in KELIBRON

The part list gives each part’s drawing size and deviations, and the panel shows the limits under the reading. On the stepped shaft, measure characteristics 1–3 from the list above: characteristic 3 is made at 20.01 mm, above its 20.00 upper limit, so it is a reject. The lengths carry no tolerance in the lab and are not judged. On the keyway shaft the lab gives a tolerance only for the outside diameter, Ø35 0/−0.05; on a real drawing the keyway width and depth would be characteristics as well. On a real part, close the jaws across the round, not into the slot.