1. What is the Cpk Calculation Equation?

The Cpk (Process Capability Index) equation measures how well a process can produce output within specification limits. It considers both the centering of the process and its spread relative to the specification limits.

2. How Does the Calculator Work?

The calculator uses the Cpk equation:

Where:

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

Explanation: The equation 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 meeting specifications and identifies opportunities for improvement.

4. Using the Calculator

Tips: Enter all values in consistent units. Standard deviation must be greater than zero. Higher Cpk values indicate better process capability.

5. Frequently Asked Questions (FAQ)

Q1: What is a good Cpk value?
A: Generally, Cpk ≥ 1.33 is considered acceptable, Cpk ≥ 1.67 is good, and Cpk ≥ 2.00 is excellent.

Q2: How is Cpk different from Cp?
A: Cp measures process potential without considering centering, while Cpk considers both spread and centering relative to 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 is used when assessing process capability for normally distributed data with two-sided specifications.

Q5: What are the limitations of Cpk?
A: Cpk assumes normal distribution and may not be accurate for non-normal processes or when data is autocorrelated.