1. What is Cpk?

The Process Capability Index (Cpk) measures how well a process meets specification limits, considering the process variation and centering. It indicates the capability of a process to produce output within specification limits.

2. How Does the Calculator Work?

The calculator uses the Cpk formula:

Where:

• \( min\ tol \) — Minimum tolerance (difference from target to nearest spec limit)
• \( \sigma \) — Standard deviation of the process

Explanation: The formula calculates how many standard deviations fit between the process mean and the nearest specification limit.

3. Importance of Cpk Calculation

Details: Cpk is crucial for quality control, helping determine if a process is capable of meeting specifications and identifying improvement opportunities.

4. Using the Calculator

Tips: Enter the minimum tolerance (distance from process mean to nearest spec limit) and standard deviation of your process. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a good Cpk value?
A: Generally, Cpk ≥ 1.33 is acceptable, ≥ 1.67 is good, and ≥ 2.00 is excellent. Below 1.0 indicates the process may not meet specifications.

Q2: How is Cpk different from Cp?
A: Cp measures potential capability assuming the process is centered, while Cpk considers both variation and centering.

Q3: When should Cpk be calculated?
A: Calculate Cpk after the process is stable (in statistical control) with sufficient sample size (typically n ≥ 30).

Q4: What are limitations of Cpk?
A: Cpk assumes normal distribution and may be misleading for non-normal processes. It also requires stable process conditions.

Q5: How to improve Cpk?
A: Reduce process variation (σ) or center the process better to increase the minimum tolerance.