GlossaryKind of term: Manufacturing termCritical dimension
What is a critical dimension?
A critical dimension is one the part's function depends on; the customer marks it, often with requirements on how you show it is under control.
It can be a feature as well as a dimension, and safety or assembly can be the reason as well as function: a bearing seat, a sealing face or a hole spacing to a mating part. The customer marks it on the drawing with a symbol of their own, an abbreviation such as CC or SC, or a note. The mark does not make the other dimensions optional: every requirement on the drawing applies. It does say where the function lies and often what extra measurement, recording or process control the customer expects.
What does the standard say about a critical dimension?
There is no generally applicable symbol for a critical dimension; its meaning is set in the drawing or in the customer's quality requirements. In automotive work these are called special characteristics, with requirements from IATF 16949 and customer-specific rules. ISO 129-1 has an inspection dimension shown in a frame with rounded ends; that is an instruction for inspection, not automatically a critical dimension. A dimension in a rectangular frame is a theoretically exact dimension.
How does a critical dimension work in practice?
A critical dimension mainly affects inspection and the route. It usually gets a balloon and a line in the inspection report, and often a check on every part or at a fixed interval instead of a sample. If the customer asks for demonstrated process capability, a measurement series and a report are added. Features that together set a critical dimension are made in one setup where possible, from the datum where the drawing names one, with a gauge whose uncertainty is small against the tolerance. What that costs depends on measuring frequency, gauge, quantity and reporting requirements.
What is a critical dimension often confused with?
A critical dimension is not the tightest one: a loosely toleranced dimension can be critical because a mating part depends on it. Nor is it a basic dimension: that sits in a rectangular frame without a tolerance of its own, while a critical dimension carries a tolerance you demonstrate. And an inspection dimension in a frame with rounded ends calls for inspection, but is not functionally critical by definition.
Frequently asked questions
Which dimensions are functionally critical?
Those that function, safety or assembly depend on: fits, sealing faces, bearing seats and hole spacings to a mating part. The customer should mark them. If nothing is marked, ask which dimensions belong in the inspection report before you estimate measuring time.
Do I have to measure a critical dimension on every part?
If the customer requires it, yes. Otherwise the inspection plan decides: every part, a fixed interval or a sample, depending on process control and risk. Put the choice in the quote, because measuring time differs per choice.