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UCL and LCL come from process data
The upper control limit and lower control limit belong to a selected control chart. Calculate them from suitable baseline data for that chart. They help identify unusual variation that needs investigation.
NIST explains that a control chart tracks a quality characteristic over time against a centre line and control limits. A point within the limits is not enough to prove control: a nonrandom pattern can also signal a problem.
Select the chart for the data. Individual measurements, subgroup averages, and proportions of nonconforming units need different calculations. Do not apply one mean-plus-or-minus formula to every chart.
USL and LSL come from the product requirement
The upper and lower specification limits come from the drawing, customer specification, or other approved requirement. They are acceptance boundaries for the characteristic being measured. Some requirements have only one limit.
Changing the process average does not change the drawing. If output no longer fits the specification, correct the process or follow the formal deviation procedure. Do not redraw the specification limits around current output.
Example: a stable process can miss the requirement
Consider an illustrative shaft requirement of 10.00 mm ±0.10 mm. The LSL is 9.90 mm and the USL is 10.10 mm. These values define the permitted diameter.
Suppose a suitable individuals chart has a centre line of 10.08 mm and established limits of 9.99 mm and 10.17 mm. A measured value of 10.14 mm is inside those control limits but above the USL. The part is out of specification even though this point alone does not trigger the control-limit rule.
Now suppose another stable baseline has limits of 9.97 mm and 10.03 mm. A new reading of 10.06 mm meets the product specification but is above the UCL. Investigate the process signal. Product conformity and process stability answer different questions.
These numbers illustrate the distinction. They are not control limits calculated from a supplied factory dataset.
Respond to the signal without losing traceability
- Verify the measurement, gauge status, unit, and data entry.
- Identify when the change began and which lots or machines may be affected.
- Check recent changes in material, setup, tool condition, operator method, or environment.
- Apply the control-plan reaction and record the owner and containment decision.
- Confirm the action with new evidence before release or closure.
Do not move a control limit to hide an unwanted point. Investigate first. When a real process change justifies a new baseline, document the reason and keep the previous chart available.
Keep chart signals linked to the relevant lot and action record. This prevents a screenshot in chat from becoming the only record of why a shipment was held.
Check stability before making capability claims
Capability compares process output with specification limits. A capability number from an unstable process can give a false sense of certainty. First review stability and the measurement system, then use a method that fits the data and its assumptions.
An AQL inspection answers a different question: whether to accept a lot under an agreed sampling plan. It does not replace ongoing process monitoring. Use control charts to manage the process and the agreed inspection plan to support the lot decision.
Key takeaways
- UCL/LCL describe process behaviour; USL/LSL define product requirements.
- Inside control limits does not mean inside specification.
- A conforming measurement can still signal a process change.
- Investigate signals and document any new baseline.
Sources and related guides
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