An interval is a bet on how long an instrument stays within its limits in your conditions of use. Start from a sensible default, then let the as-found history move it. Write the reason for every interval change on the register.
| Kind of instrument | Starting interval we often use | Shorten when | Lengthen when |
|---|---|---|---|
| Calipers, micrometers in daily use | 6 to 12 months | Any out-of-tolerance result; heavy use on hard or abrasive materials | Several calibrations in a row with as-found results well inside the limits |
| Bore, plug, ring and thread gauges | 6 to 12 months (by use count where heavy) | The go member trending toward its wear limit | Light use and stable wear readings |
| Height gauges and indicators | 6 to 12 months | Drift in the as-found data, even in tolerance | Stable history and a daily check on a block |
| CMM and vision systems | 12 months, with probe qualification at each start-up | Crashes, moves, repairs, temperature problems | Rarely: the interim artifact checks carry the confidence |
| Torque tools | 6 to 12 months or by cycle count | Use near the top of the range; any drop | Light use and stable results |
| Gauge blocks, masters, surface plates | 12 to 36 months | Damage, heavy use as a setting master | Lab use only and stable history |
Two rules keep intervals honest. First, any out-of-tolerance result shortens the interval until the history says otherwise. Second, a run of results that drift toward a limit, while still inside it, is a reason to shorten before it fails, not after.
Adjusting an interval: an example
Three calibrations of a height gauge, all in tolerance, against an accuracy limit of plus or minus 0.020 mm. The as-found history is the evidence for the decision; the register notes carry the reason.
| Calibration | Largest as-found error | Share of the limit | What we would decide |
|---|---|---|---|
| Year 1 | +0.004 mm | 20% | Keep 12 months |
| Year 2 | +0.009 mm | 45% | Keep 12 months, but the drift is now visible: add a monthly block check |
| Year 3 | +0.015 mm | 75% | Shorten to 6 months: on this trend the next one fails |
| Two 6-month results under 25% | +0.004 mm, +0.005 mm | 20%, 25% | Return to 12 months, keep the block check, write the reason |
Out-of-tolerance results move the interval at once. In-tolerance results move it on a pattern: one good result says little, a run of them says a lot. A written rule of thumb that the whole team uses is better than a clever method nobody follows.
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A management aid, not certification or a registrar audit. Using it does not by itself show conformity to AS9100D or any other standard, and your customers' requirements may add to it.
Clause numbers refer to AS9100D (SAE AS9100 Rev D) unless another document is named. The standards' text is not reproduced here: use your own licensed copy.
Aegis Insights are our own guidance, labeled wherever they appear: how we approach this work, not a requirement of any standard or customer.