1. What is Cpk?

Cpk (Process Capability Index) is a statistical measure of a process's ability to produce output within specification limits. It indicates how well a process meets specifications, considering both the process variability and how centered the process is between the specification limits.

2. How Does the Calculator Work?

The calculator uses the Cpk formula:

Where:

• \( USL \) — Upper Specification Limit
• \( LSL \) — Lower Specification Limit
• \( \mu \) — Process Mean
• \( \sigma \) — Standard Deviation

Explanation: The formula calculates how many standard deviations the process mean is from each specification limit and takes the minimum value, representing the "worst-case" capability.

3. Importance of Cpk Calculation

Details: Cpk is crucial in quality control and Six Sigma methodologies to assess process capability, identify improvement opportunities, and ensure products meet customer requirements. Higher Cpk values indicate better process capability.

4. Using the Calculator

Tips: Enter all values in consistent units. Standard deviation must be greater than zero. The result is dimensionless and represents the process capability index.

5. Frequently Asked Questions (FAQ)

Q1: What is a good Cpk value?
A: Generally, Cpk ≥ 1.33 indicates a capable process. Cpk ≥ 2.0 is considered excellent (Six Sigma quality).

Q2: How is Cpk different from Cp?
A: Cp measures potential capability assuming the process is centered, while Cpk measures actual capability considering both variability and centering.

Q3: Can Cpk be negative?
A: Yes, negative Cpk indicates the process mean is outside the specification limits.

Q4: What if my process has only one specification limit?
A: For one-sided specifications, use the appropriate part of the formula (either (USL-μ)/3σ or (μ-LSL)/3σ).

Q5: How often should Cpk be calculated?
A: Regular monitoring is recommended, typically as part of statistical process control, with frequency depending on process stability and criticality.