1. What Are Cp And Cpk?

Cp and Cpk are statistical measures used in process capability analysis to determine how well a process meets specification limits. Cp measures potential capability while Cpk measures actual capability considering process centering.

2. How Do The Formulas Work?

The calculator uses the following formulas:

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: Cp compares the process spread to the specification range, while Cpk considers both spread and centering of the process relative to specifications.

3. Importance Of Process Capability Indices

Details: These indices are crucial for quality control, helping manufacturers determine if processes can consistently produce output within specification limits and identify areas for improvement.

4. Using The Calculator

Tips: Enter all four required values. Standard deviation must be greater than zero. The calculator will compute both Cp and Cpk indices.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between Cp and Cpk?
A: Cp measures potential capability assuming the process is centered, while Cpk measures actual capability considering both spread and centering.

Q2: What are acceptable values for Cp and Cpk?
A: Generally, values ≥1.33 indicate capable processes. Values ≥1.67 are considered excellent. Values <1.0 indicate incapable processes.

Q3: Can Cp be greater than Cpk?
A: Yes, Cp is always ≥ Cpk. When the process is perfectly centered, Cp = Cpk.

Q4: What if my process has only one specification limit?
A: For one-sided specifications, use Cpk with the available limit. Cp cannot be calculated with only one specification limit.

Q5: Are there limitations to these indices?
A: They assume normal distribution and stable process. For non-normal distributions, other capability indices should be used.