KC Key characteristic (KC)
Key characteristic (KC) · Inspección y ensayos · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “KC: hole position ⌖ · sample every 10 parts” states a key characteristic (kc) requirement.
What controls
Classification identifying a characteristic that is important to product function or process stability, but is not classified as safety-critical. As with critical characteristics, it does not modify the drawing tolerance: what it introduces is a heightened control regime, typically statistical monitoring with minimum capability indices and a higher measurement frequency than an ordinary dimension.
How to verify
Variables measurement (a numerical value, not go/no-go) with an instrument of sufficient resolution to feed the statistical analysis, taking subgroups per the control plan. Results are kept on a control chart and accompanied by capability indices when the plan requires it. The measurement system should be validated, because part of the variation seen on the chart may come from the instrument itself.
Common mistakes when interpreting it
- Waiting for an out-of-tolerance part before reacting, when the key characteristic is meant to give warning beforehand.
- Checking it with a go/no-go gauge: without a numerical value, statistical monitoring is impossible.
- Confusing it with a critical characteristic: the control regime and the consequences of a deviation are not the same.
How to get it in the workshop / if it comes out of tolerance
These are the dimensions worth always keeping on a control chart: they give warning before bad parts appear. When the process drifts toward a limit, act even though nothing is out of tolerance yet: that is precisely the point of flagging them. If the control plan asks you to measure a key characteristic every hour, that frequency is as mandatory as the tolerance itself.
Sustained drift toward a limit: act on the process (tool wear compensation, work offset adjustment) before parts go out of tolerance. Increasing spread: look for sources of variation (workholding, coolant, material changes) rather than recentering the mean. If capability falls below the required minimum, the process must be improved or the tolerance renegotiated; more inspection is not enough.
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