Guide
What is a critical dimension symbol, and how do you mark CTQ characteristics on a drawing?
A critical dimension symbol is a mark placed next to a dimension, feature control frame or note to tell the supplier that this characteristic is critical to quality (CTQ) and must be inspected and controlled more tightly than the rest of the print. No drawing standard defines one universal symbol, so the mark only works when the drawing also carries a legend that says what each symbol means and what the supplier has to do about it.
By Saola Manufacturing engineering team · Published · Updated
Every dimension on a drawing must be met, but they are not equally likely to cause trouble. A sealing bore, a bearing seat or the thread on a gas fitting can fail an assembly or a safety test; a chamfer on a clearance edge almost never will. Flagging the few characteristics that matter lets a supplier spend its inspection time, gauging budget and capability effort where it counts, instead of checking forty features with the same caliper at the same interval. This guide covers what the standards do and do not say, how to choose which features to flag, how to write the legend and purchase-order wording, and the mistakes that make the flags meaningless.
On this page
Does ASME Y14.5 or ISO define a critical dimension symbol?
No. Neither ASME Y14.5 nor the ISO GPS standards define a symbol for criticality; the classification schemes come from industry quality standards and from each customer’s own drawing practice.
ASME Y14.5 defines how a requirement is stated, not how important it is. The nearest thing it offers is the statistical tolerance symbol, the letters ST in a frame, which tells the reader that a tolerance was assigned on a statistical basis and that the drawing must carry a note requiring statistically toleranced features to be produced with statistical process controls (or, where both are shown, to the tighter arithmetic limits). That is a real obligation, but it is a tolerancing method, not a general “this one matters” flag, and it should not be borrowed for that purpose.
The classification itself lives in quality-system standards. The automotive standard IATF 16949 requires a supplier to identify special characteristics, to mark them with the customer’s symbols or an equivalent, and to carry them through the drawings, the risk analysis, the control plan and the operator instructions. Aerospace quality systems use the term key characteristic for a feature whose variation significantly affects fit, form, performance, service life or producibility, and SAE AS9103 sets out how the variation of each one should be managed. Each automaker and airframer then publishes its own symbol set. Inverted triangles, diamonds and shields containing letters such as CC or SC are all common, and the same shape can mean different things to two customers.
The practical consequence for an industrial buyer outside those two sectors is simple: you can use any clear mark you like, but a supplier cannot be expected to guess what it means. The legend, not the shape, is the requirement.
Which critical dimension symbol should you use, and what should each class mean?
Use two or three classes at most, each with a short letter code inside a simple shape, and define every class in a drawing note that names the inspection or control it triggers.
Two flagged classes plus “unmarked” cover almost every industrial part. The top class is for characteristics whose failure creates a safety, legal or regulatory problem: a pressure boundary, a gas-tight thread, a feature that a listed product test depends on. The second class is for fit and function: the bore a bearing presses into, the face a gasket seals against, a hole pattern that has to line up with a mating part. Everything else stays unmarked and is verified by first article and normal lot inspection. A letter code inside the shape matters because shapes alone get lost in scanned PDFs and photocopies, while “CC” or “KC” survives almost any reproduction.
| Class | Typical drawing mark | Typical supplier obligation |
|---|---|---|
| Critical (safety, legal, regulatory) | Letters CC in a shape defined by the legend | Listed on the control plan; checked every part or by an error-proofing device; any change notified before it is made |
| Significant or key (fit, function) | Letters SC or KC in a different shape | Listed on the control plan; capability study at launch and charted in production; increased check frequency |
| Standard (unmarked) | No mark | Measured at first article; verified by the lot sampling plan |
Which characteristics deserve a CTQ flag?
Flag a characteristic only when a nonconformance would cause a safety, regulatory or assembly failure that your own incoming inspection would not reliably catch. If you cannot name the failure, do not flag the feature.
Work through the part with five questions, and flag a feature only when at least one answer is a clear yes:
- Safety or compliance. Does the feature hold pressure, carry a load path, limit a gas or water flow, or appear in a product listing or regulation?
- Interface. Does it mate with a part you do not make, such as a purchased bearing, seal, fastener or a customer’s housing?
- Stack-up driver. Is it one of the few contributors that dominates an assembly tolerance stack? A worst-case or statistical stack-up analysis often shows that a handful of dimensions carry most of the variation.
- Hidden after assembly. Will the feature be impossible to inspect once the part is assembled, coated or installed?
- Field history. Has this feature, or the same feature on a similar part, caused returns before?
Tolerance alone is not a reason. A tight tolerance on a non-functional diameter is a cost problem to fix on the drawing, as the guide to the cost of tight tolerances explains, not a characteristic to flag. And a flagged feature should carry a tolerance the process can actually hold: if the capability is not there, the flag only guarantees a sorting operation.
Keep the count small. Flags should be the exception on a drawing; once a large share of the print is flagged, the supplier can no longer tell what matters, and the flags lose their value. A useful test is whether you would be willing to pay for a capability study and a gauge study on every flagged feature. If not, some of them are not critical.
What does a CTQ mark oblige the supplier to do?
On its own, nothing. The drawing note and the purchase order have to state what the mark triggers: check frequency, capability target, gauge qualification, change control and the reaction when a check fails.
Suppliers that work in automotive or aerospace will assume their customers’ usual rules, which may be more or less than you want. Suppliers that do not will treat the symbol as decoration. Spell out each obligation once, in a purchase specification the drawing points to, so the flags on every drawing inherit the same rules:
- Control plan. Every flagged characteristic appears as its own row, with the gauge, sample size, frequency and reaction plan.
- Capability. A launch study and an ongoing index, with the target stated. The common automotive convention, per the AIAG PPAP manual, treats an initial-study index above 1.67 as meeting acceptance and 1.33 to 1.67 as possibly acceptable after review; the Cpk vs Ppk guide covers which index to ask for and how much data it needs.
- Gauge qualification. A gauge repeatability and reproducibility study on the instrument used for each flagged feature, following the AIAG Measurement Systems Analysis manual. A capability number from a gauge that cannot resolve the tolerance is meaningless.
- Change control. No change to machine, fixture, tooling source, material source, finish or sub-supplier that affects a flagged feature without written notice and, where you require it, a new first article.
- Records. Flagged results kept as variable data, not pass or fail ticks, and sent with each lot or on request.
Unflagged features still have to meet the drawing. They are covered by the first article inspection and by the lot sampling plan; the flags decide where extra effort goes, not which requirements are optional.
How do you write the legend note and the RFQ wording?
Put a classification legend in the general notes of every drawing that uses the marks, reference a purchase specification for the detailed rules, and list the flagged characteristics by balloon number in the RFQ.
A legend that works on a machined or cast part reads something like this:
CHARACTERISTIC CLASSIFICATION: CC IN DIAMOND = CRITICAL CHARACTERISTIC (SAFETY / REGULATORY). SC IN CIRCLE = SIGNIFICANT CHARACTERISTIC (FIT / FUNCTION). UNMARKED CHARACTERISTICS: VERIFY AT FIRST ARTICLE AND PER LOT SAMPLING PLAN. CONTROL REQUIREMENTS FOR EACH CLASS PER PURCHASE SPECIFICATION QS-100, CURRENT REVISION.
Place the symbol immediately next to the dimension or feature control frame it applies to, on the same side every time, and never on a basic dimension: a basic dimension has no tolerance, so the characteristic is the position or profile result in the frame. When a note carries the requirement, such as a thread depth or a leak-test limit, put the symbol next to the note number. If you balloon the drawing yourself, carry the class into the balloon list as well, so the ballooned inspection report shows the class on each line.
In the RFQ, ask the supplier to confirm three things in the quote rather than after the order: that each flagged characteristic can be held on the intended process, which gauge will be used for it, and whether the requested capability study and records are included. Useful wording:
- “Characteristics marked CC or SC are listed below by balloon number. Confirm the process, gauge and check frequency for each in your quotation.”
- “Provide a capability study for each SC characteristic from the first production run, with raw data in production order.”
- “Notify the buyer in writing before any process, tooling, material or sub-supplier change affecting a CC or SC characteristic.”
If you send the RFQ as a 3D model plus a PDF, make sure the marks exist on the PDF. Symbols added as annotations in CAD often do not survive export, and a supplier quoting from the model alone will never see them. The guide to preparing a machining RFQ covers the rest of the package.
Which mistakes make critical characteristic flags useless?
The common failures are flagging too much, flagging without a legend, and flagging features that the drawing or the process cannot support. Each one turns the flag into noise.
- Everything is critical. A print with flags on most dimensions gives the supplier no priority at all, and the capability and gauge studies multiply until nobody does them properly.
- Symbols with no legend. A triangle inherited from a previous employer’s template means nothing to a new supplier. Every drawing that uses a mark must define it.
- Flags that do not flow down. On a cast and machined part, a flagged machined bore often depends on a cast datum or wall. If the casting drawing and the machining drawing are separate, as discussed in casting vs machining drawings, the related cast features need their own flag or a note tying them to the machined one.
- Flagging a feature that is measured by the wrong method. A flagged geometric tolerance needs a stated datum setup; a flagged diameter checked with a handheld caliper against a tight band is not controlled. Check the GD&T symbol guide for what each control costs to inspect.
- Changing the flags without a revision. Adding or removing a flag changes the supplier’s obligations, so it needs a drawing or specification revision, not an email.
- No reaction plan. A flagged feature that is checked often but has no stated response to a failed check only produces better documentation of bad parts.
How does Saola treat dimensions you mark as critical?
When a drawing marks a dimension as a process-capability control point, Saola watches it through the run rather than only at first article, and checks it more often than the rest of the print. Name the class rules, studies and records you want in the RFQ so they are confirmed in the quote.
That practice, and the documents that ship, are set out on the quality and inspection page: a first-article inspection report covering every toleranced dimension on your drawing, and a dimensional inspection record with sample measurements from each production lot. The same page says Saola will build its inspection around control dimensions and cosmetic standards you already use, and states plainly what cannot be supplied yet: a signed ISO 9001 certificate, because certification is in progress, a third-party quality audit report, and a published PPM or on-time-delivery record. A formal capability study is not on that document list, so if your flags call for one, put it in the RFQ.
For turned features, the CNC turning page notes that every dimension called out to a tight tolerance adds inspection time, which is the same argument for flagging only what matters. For bores and datum faces machined on castings, the CNC milling page notes that every GD&T callout is measured and logged. To have a drawing’s critical characteristics reviewed before tooling, send it for a quote with the classification legend and the list of flagged balloon numbers.
Frequently asked
Is CTQ the same as a critical characteristic?
Nearly. CTQ comes from Six Sigma practice and describes any characteristic the customer cares about; special characteristic (automotive) and key characteristic (aerospace) are the formal terms in those sectors’ quality standards, and many customers split them further into classes such as critical and significant. On a drawing, what matters is that your legend defines the term you use and the obligation attached to it.
Can I use my customer’s symbols on my supplier’s drawing?
Yes, and in automotive work IATF 16949 expects the customer’s special characteristic designations to be carried down the supply chain or mapped to equivalents. Include the customer’s legend or a cross-reference table on the drawing, because your supplier may never see the customer’s own standard.
Should the statistical tolerance symbol be used to mark critical dimensions?
No. Per ASME Y14.5 the statistical tolerance symbol means the tolerance was calculated statistically and that the feature must be produced with statistical process controls, or to the tighter arithmetic limits where both are shown. It is a valid tool for stack-ups, but it is not a general importance flag, and using it that way creates an obligation you may not intend.
Does a flagged dimension need 100 percent inspection?
Not necessarily. Every-part checking suits features that fail suddenly, such as a tapped hole or a missed operation; features that drift slowly with tool wear are better controlled by charting a small sample at a set interval. State which you want in the purchase specification, or ask the supplier to propose it in its control plan.
Who decides which characteristics are critical, the designer or the supplier?
The design owner, because only the designer knows the function, the mating parts and the field risk. A supplier can and should propose additional process characteristics from its own risk analysis, but it cannot remove or downgrade a flag without the design owner’s written agreement.
Sources
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018
- IATF 16949:2016, about the standard, International Automotive Task Force, 2016
- SAE AS9103B, Variation Management of Key Characteristics, SAE International, 2022
- Production Part Approval Process (PPAP), 4th edition, Automotive Industry Action Group, 2006
- Measurement Systems Analysis (MSA), 4th edition, Automotive Industry Action Group, 2010