Statistical Process Control Transition Rules for Skip Lot Qualification
Transitioning to skip-lot sampling demands a proven Cpk of 1.33 across consecutive lots and contract rules that force instant reversion to full inspection upon any defect.
Cpk Calculation
Meaning
Statistical capability indices measure the ability of a manufacturing process to produce output within specified tolerance limits while accounting for the centering of the distribution. The cpk calculation uses the mean and standard deviation of a process to determine how close the output is to the nearest specification limit. A higher value indicates that the process is both stable and well-centered.
Deviation Measurement
Formulae for this index compare the distance from the process average to the specification limit against three times the estimated standard deviation. This approach highlights the risk of producing scrap when the process mean shifts away from the target.
Process Centering
Adjustments to the machinery are necessary when the index falls below the minimum requirement set by the customer. While the cp index only looks at the spread of the data, the specific calculation for cpk penalizes a process that is off-center even if its variation is small. Most industrial contracts in the electronics and automotive sectors demand a value of 1.33 or higher.
Specification Tolerance
Limits defined by the engineering team dictate the width of the acceptable range for any given part. Without tight tolerances, the index loses its power to drive quality improvement.