Process Capability (Cp & Cpk) Calculator
This calculator helps professionals calculate the Process Capability indices (Cp and Cpk) to gauge how well a process is performing relative to its specification limits. It is tailored for quality engineers, production managers, and Six Sigma practitioners.
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Data Source and Methodology
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti dal manuale "Quality Control Handbook" by Juran and Gryna.
The Formula Explained
\[ Cp = \frac{USL - LSL}{6 \cdot \sigma} \]
\[ Cpk = \min \left( \frac{USL - \mu}{3 \cdot \sigma}, \frac{\mu - LSL}{3 \cdot \sigma} \right) \]
Glossary of Variables
- Mean (\(\mu\)): The arithmetic average of a set of numbers.
- Standard Deviation (\(\sigma\)): A measure of the amount of variation or dispersion in a set of values.
- USL: Upper Specification Limit, the maximum acceptable value.
- LSL: Lower Specification Limit, the minimum acceptable value.
How It Works: A Step-by-Step Example
Suppose a process has a mean (\(\mu\)) of 50, a standard deviation (\(\sigma\)) of 2, a USL of 56, and an LSL of 44. The Cp value would be calculated as follows:
The Cpk value would be calculated using the minimum of two calculations:
Frequently Asked Questions (FAQ)
What is the Cp index?
Cp is a measure of a process's potential capability to meet specification limits.
What is the Cpk index?
Cpk is a measure of a process's actual capability to produce output within specification limits.
How do I interpret Cp and Cpk values?
A Cp or Cpk value less than 1 indicates the process is not capable of producing within specification limits.
Why is Cpk important?
Cpk considers both the process mean and variation, providing a more accurate representation of process capability.
Can Cp be greater than Cpk?
Yes, Cp can be greater than Cpk if the process mean is not centered between the specification limits.