Meaning
Statistical trends showing a reduction in process capability over time indicate that the relationship between the process spread and specification limits is worsening. Cpk degradation occurs when tool wear or environmental shifts cause the distribution of parts to drift away from the center or widen toward the tolerance bounds. It identifies the loss of precision that leads to increased scrap rates.
Index Decay
Calculating the capability index at regular intervals reveals how much of the tolerance window remains as a safety buffer. A value that drops from one point six to one point zero indicates that the process is moving toward the edge of the specification. This downward trend signals an urgent need for maintenance or calibration.
Tolerance Encroachment
Parts produced during a period of declining capability are more likely to fail inspection if the mean shifts. Variations in raw material hardness or ambient shop temperatures accelerate the widening of the distribution curve. This expansion reduces the margin for error in subsequent assembly steps.
Center Drift
Monitoring the process mean helps distinguish between a widening spread and a simple offset in the tool path. A drift in the mean is often corrected by a software offset. Frequent adjustments indicate that the process is inherently unstable.
