Process Capability Cpk & Ppk Guide
Process capability indices quantify how well a process produces parts within specification limits.
Key Indices
| Index | What It Measures | Formula |
|---|
| Cp | Potential (if centered) | (USL-LSL)/(6σ) |
| Cpk | Actual + centering | min(USL-μ, μ-LSL)/(3σ) |
| Pp | Long-term capability | (USL-LSL)/(6σlong) |
| Ppk | Long-term + centering | min(USL-μ, μ-LSL)/(3σlong) |
Standards
- Cpk ≥ 1.33: General (~63 ppm)
- Cpk ≥ 1.67: Automotive safety
- Cpk ≥ 2.0: Six Sigma (3.4 ppm)
- Cpk ≥ 2.5: Medical/aerospace
- Cpk < 1.0: Incapable
How to Calculate
- Collect 30-50 samples (short-term) or 100+ across batches (long-term)
- Calculate mean (μ) and std dev (σ)
- Get USL/LSL from drawing
- Cpk = min[(USL-μ)/(3σ), (μ-LSL)/(3σ)]
Common Mistakes
- No gage R&R verification first
- Single setup sampling
- Ignoring non-normal distributions
- Process not in statistical control
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