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What each boundary means
A drawing specifies the acceptable range for a part characteristic. A control chart uses a process baseline to set statistical limits for the quantity plotted, such as an individual reading or a subgroup average. A point beyond a control limit prompts investigation of a possible process change. A specification decision uses the applicable product requirement and measurement decision rule.
An in-spec point can still warrant investigation
Suppose a hypothetical individuals chart monitors a pin diameter with drawing limits of 1.95 to 2.05 mm. Its established control limits are 1.97 and 2.03 mm. A reading of 2.04 mm lies within the drawing limits and above the chart’s upper limit. The inspection decision and the process investigation are both relevant.
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A stable process can still produce parts outside specification. For a subgroup chart, compare each plotted average with its own control limits and use the individual readings for part conformance. Revisit the baseline when a documented process change warrants it.
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