1. What is the Cpk Formula?

The Cpk (Process Capability Index) formula measures how well a process meets specification limits. It considers both the process center and variability, providing a dimensionless index of process capability.

2. How Does the Calculator Work?

The calculator uses the Cpk formula:

Where:

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

Explanation: The formula calculates the minimum distance from the process mean to either specification limit, normalized by three standard deviations.

3. Importance of Cpk Calculation

Details: Cpk is crucial for quality control and process improvement. It helps determine if a process is capable of producing within specification limits and identifies opportunities for improvement.

4. Using the Calculator

Tips: Enter all specification limits, process mean, and standard deviation. Ensure standard deviation is greater than zero. All values should use consistent units.

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 only considers process variability, while Cpk considers both variability and how centered the process is within specifications.

Q3: What if Cpk is negative?
A: A negative Cpk indicates the process mean is outside the specification limits.

Q4: When should Cpk be used?
A: Cpk should be used when the process is stable and normally distributed, and when both specification limits are relevant.

Q5: What are the limitations of Cpk?
A: Cpk assumes normal distribution and may not be accurate for non-normal processes. It also requires the process to be in statistical control.