AiVibeIndustry 4.0 & Manufacturing Operations
Process Capability (Cp and Cpk)
Process capability indices compare the natural spread of a stable process with its specification limits; Cp measures potential capability from spread alone, while Cpk also accounts for how well the process is centred between the limits.
Cp is the specification width divided by six standard deviations of the process: the upper specification limit minus the lower, divided by six sigma. Cpk is the smaller of the distances from the process mean to each specification limit, divided by three sigma, so it falls when the process drifts off centre. A Cpk of 1.0 means the nearest specification limit lies exactly three standard deviations from the mean. For Cp and Cpk, sigma is normally estimated from within-subgroup variation, such as the average range on an X-bar and R chart.
Capability studies are used to qualify new processes and equipment, to support production part approval in automotive supply chains, to decide inspection frequency and to prioritise improvement. Customers often specify minimum values; 1.33 is commonly quoted, with higher values such as 1.67 often required for new processes, but the applicable figure is set by the customer. Process performance indices Pp and Ppk use the same formulas with the overall standard deviation of all data, so they reflect longer-term behaviour.
Capability indices are meaningful only for a process in statistical control; an unstable process has no predictable capability. They also assume approximately normal data, so skewed characteristics may need non-normal methods. A capable measurement system and an adequate sample are prerequisites. The ISO 22514 series provides statistical methods for process capability and performance.
Key points
- Cp equals specification width divided by six standard deviations.
- Cpk uses the distance from the mean to the nearer limit, divided by three sigma.
- Pp and Ppk use the overall standard deviation and reflect longer-term performance.
- Valid only for stable processes with a capable measurement system.
- The ISO 22514 series covers process capability and performance methods.
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory as the OEM: edge devices plus AI agents that let people talk to PLC, CNC and robot controllers in plain language. Existing lines are retrofitted with edge devices, and a trained operator confirms every change an agent proposes.
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Related terms
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