Understanding Process Capability & Six Sigma Standards
Process capability evaluates whether an in-control manufacturing or service process can consistently produce parts that fall within customer engineering tolerances. Industry benchmark guidelines:
- Cpk < 1.00: Incapable process; significant defects produced.
- 1.00 ≤ Cpk < 1.33: Barely capable; requires active statistical monitoring.
- 1.33 ≤ Cpk < 1.67: Capable process; automotive and aerospace benchmark for existing parts.
- Cpk ≥ 1.67: Six Sigma quality level; benchmark for new critical parameters.
Formulas
Potential Capability: Cp = (USL - LSL) / (6σ)
Upper Capability: Cpu = (USL - μ) / (3σ)
Lower Capability: Cpl = (μ - LSL) / (3σ)
Actual Capability: Cpk = Min(Cpu, Cpl)
Taguchi Capability: Cpm = (USL - LSL) / (6 × √(σ² + (μ - Target)²))