1. What is Cp and Cpk?

Cp and Cpk are statistical measures of process capability. Cp measures the potential capability of a process assuming it's centered, while Cpk measures the actual capability considering the process mean's deviation from center.

2. How Does the Calculator Work?

The calculator uses the following formulas:

Where:

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

Explanation: Cp compares the specification width to the process variation (6σ), while Cpk considers both variation and centering.

3. Importance of Process Capability

Details: Cp and Cpk are critical for quality control, helping determine if a process can consistently produce within specifications and how centered the process is.

4. Using the Calculator

Tips: Enter all specification limits, the process mean, and standard deviation. Standard deviation must be > 0. Values can be positive or negative depending on your measurement scale.

5. Frequently Asked Questions (FAQ)

Q1: What's a good Cp/Cpk value?
A: Generally, Cp/Cpk ≥ 1.33 indicates capable process. For Six Sigma, ≥ 2.0 is typically required.

Q2: What's the difference between Cp and Cpk?
A: Cp measures potential capability if centered, while Cpk measures actual capability considering centering.

Q3: Can Cp be greater than Cpk?
A: Yes, always. Cp ≥ Cpk. They're equal only when the process is perfectly centered.

Q4: What if my Cpk is negative?
A: Negative Cpk means the process mean is outside specification limits, indicating very poor capability.

Q5: How many data points should I use?
A: For reliable σ estimation, use at least 25-30 data points from a stable process.