1. What is Process Capability (Cp)?

The Process Capability Index (Cp) measures how well a process can meet specification limits, comparing the width of the specification limits to the width of the process variation. It indicates the potential capability of a process assuming it is centered between the specification limits.

2. How Does the Calculator Work?

The calculator uses the Cp formula:

Where:

• \( USL \) — Upper Specification Limit
• \( LSL \) — Lower Specification Limit
• \( \sigma \) — Standard deviation of the process

Interpretation:

• Cp < 1.0: Process not capable (variation exceeds specifications)
• 1.0 ≤ Cp < 1.33: Marginally capable
• Cp ≥ 1.33: Capable process
• Cp ≥ 2.0: Six Sigma quality

3. Importance of Cp Calculation

Details: Cp is crucial for quality control, helping manufacturers determine if their processes can consistently produce within specification limits. It's used across industries to measure and improve process performance.

4. Using the Calculator

Tips: Enter the upper and lower specification limits and the process standard deviation. All values must be valid (standard deviation > 0, USL > LSL).

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between Cp and Cpk?
A: Cp measures potential capability assuming the process is centered, while Cpk considers both variation and centering of the process.

Q2: How is standard deviation (σ) determined?
A: σ is typically calculated from process data using statistical methods. For ongoing processes, it's often estimated from control charts.

Q3: What if my process isn't normally distributed?
A: Cp assumes normal distribution. For non-normal data, consider transformations or non-parametric capability indices.

Q4: Can Cp be greater than 1 if my process has defects?
A: Yes, if the process isn't centered between the limits. That's why Cpk is often used alongside Cp.

Q5: What's a good target for Cp?
A: Many industries aim for Cp ≥ 1.33. Automotive often requires Cp ≥ 1.67 for critical characteristics.