1. What is CPK?

The Process Capability Index (CPK) measures how well a process can produce output within specification limits. It compares the spread of the process variation to the specification limits while considering how centered the process is.

2. How Does the Calculator Work?

The calculator uses the CPK formula:

Where:

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

Explanation: The formula calculates two values - how close the mean is to the USL and how close it is to the LSL, then takes the minimum of these two values.

3. Importance of CPK Calculation

Details: CPK is crucial for quality control in manufacturing and process improvement. Higher CPK values indicate more capable processes (CPK ≥ 1.33 is generally considered acceptable).

4. Using the Calculator

Tips: Enter all four required values. Standard deviation must be greater than zero. The calculator will compute how well your process fits within the specification limits.

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 measures actual capability accounting for process centering.

Q2: What is a good CPK value?
A: CPK ≥ 1.33 is generally acceptable, ≥ 1.67 is good, and ≥ 2.00 is excellent. Below 1.00 indicates the process needs improvement.

Q3: How does CPK relate to sigma levels?
A: CPK of 1.0 equals 3σ (99.73% within specs), 1.33 equals 4σ (99.9937%), and 2.0 equals 6σ (99.9999998%).

Q4: Can CPK be negative?
A: Yes, negative CPK indicates the process mean is outside the specification limits.

Q5: What if my CPK is too low?
A: Consider reducing process variation (σ) or centering the process mean between the specification limits.