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 is centered and how variable it is relative to the specification limits.

2. How Does the Cpk Formula Work?

The calculator uses the Cpk formula:

Where:

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

Explanation: The formula calculates the capability of the process by comparing the distance from the process mean to both specification limits, divided 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 the upper and lower specification limits, process mean, and standard deviation. All values must be valid (standard deviation > 0).

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.

Q2: What's the difference between Cp and Cpk?
A: Cp measures potential capability assuming the process is centered, while Cpk measures actual capability considering both centering and variation.

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

Q4: How many data points are needed for reliable Cpk calculation?
A: Typically, 30-50 data points are recommended for stable Cpk estimation.

Q5: What are the limitations of Cpk?
A: Cpk assumes normal distribution and may not be accurate for non-normal processes or small sample sizes.