Process capability analysis is a set of calculations used to assess whether a system is statistically able to meet a set of specifications or requirements.
Capability analysis is summarized in indices. They can be monitored and reported over time to show how a system is changing. The main indices used are Cp and Cpk.
- Collecting data (The operating conditions that affect the measured characteristic should be defined and controlled, in other words process must be stable). More data collected, more precise will be the results.
- Verify the process is under control.
- Review individual data values on a histogram chart to verify that the distribution of values is normal.
- Analyzing results.
- Check process is in statistical control.
- Demonstrate that the process is centered within the specification limits and that the process variation predicts the process is capable of producing parts within the tolerance requirements.
- Check improvement made on a process.
- Identify improvement opportunities in the process by reducing or possibly
eliminating sources of variability.
Main possible actions:
- Reduce variability.
- Center the process. When the process spread is approximately the same as the specification spread, an adjustment to the centering of the process may bring the bulk of the product within specifications.
- Reviewing tolerances based on the inherent variability of a process.
- Change the specifications. The specification limits may be unrealistic. In some cases, specifications may be set tighter than necessary.
- Accept the losses. Some centering and reduction in variation may be possible, but the principal emphasis is on handling the scrap and rework efficiently.
- Do nothing. If the process limits match the specification, no action may be required.
A process capability analysis can determine if a process is capable of producing parts that conform to engineering specifications. While prototype parts can provide preliminary process performance information, the final process capability assessment must be based on series production.
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