When a line undergoes a major changeover — new fixture, revised torque sequence, or a different supplier lot — quality teams face a familiar tension. Throw away the old control chart and start fresh, or carry forward limits that no longer describe current variation?
We recommend a three-step approach we use with clients in Suwon and Changwon plants. First, mark the changeover date on the chart rather than deleting prior points. Second, collect a minimum of 20 subgroups under the new conditions before proposing revised limits. Third, run a side-by-side comparison chart for two weeks so supervisors can see whether the shift is a location change or a spread change.
When limits should move immediately
If a gauge was recalibrated and the bias shift exceeds half the former control limit width, waiting for 20 subgroups will produce false signals on every other point. Document the calibration event, reset the center line, and note the reason in the chart header.
When to keep old limits longer
Minor operator rotation rarely justifies immediate recalculation. Track the next 10 subgroups against existing limits. If more than two points fall outside, schedule a formal limit review with engineering.
These decisions belong in your handover notes, not in a separate email chain that the next shift will never read.