9.3 Process Capability Analysis

Process capability analysis is a crucial tool for assessing how well a process meets customer specifications. It quantifies consistency and predicts future performance, helping businesses identify areas for improvement and ensure product quality.

Calculating capability indices like Cp and Cpk provides insights into potential and actual process performance. These metrics guide improvement strategies, from reducing variation to centering processes, ultimately leading to better customer satisfaction and more efficient operations.

Process Capability Analysis Fundamentals

Definition of process capability

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• Process capability quantifies process's consistency in producing output within specified limits comparing process variation to customer specifications
• Importance in assessing process performance
  • Predicts future process performance guiding decision-making for improvements
  • Identifies potential for defects or non-conforming products helping determine if process meets customer requirements
• Key components
  • Process centering locates process mean
  • Process spread measures variation around the mean

Calculation of capability indices

• Process capability index (Cp) measures potential capability
  • Formula: $Cp = \frac{USL - LSL}{6\sigma}$ (USL: Upper Specification Limit, LSL: Lower Specification Limit, σ: Process standard deviation)
  • Interpretation
    • Cp < 1: Not capable
    • Cp = 1: Marginally capable
    • Cp > 1: Capable (higher values indicate better capability)
• Process capability index (Cpk) measures actual capability accounting for process centering
  • Formula: $Cpk = min(\frac{USL - \mu}{3\sigma}, \frac{\mu - LSL}{3\sigma})$ (μ: Process mean)
  • Interpretation similar to Cp
  • Cpk always less than or equal to Cp (equal when process perfectly centered)

Process Capability and Improvement Strategies

Capability vs customer specifications

• Customer specifications define acceptable range of product characteristics (target value with upper and lower limits)
• Relationship to process capability
  • Capable process produces output within specifications
  • Capability indices compare process performance to specifications
• Impact on customer satisfaction
  • Higher capability indicates lower defect probability leading to satisfied customers
• Specification limits vs process limits
  • Specification limits set by customers or design requirements
  • Process limits reflect natural variation (±3σ from mean)

Strategies for capability improvement

1. Reduce process variation
   • Implement statistical process control (SPC)
   • Eliminate special causes of variation
   • Standardize procedures and work methods
2. Center the process
   • Adjust process mean to target value
   • Use process control techniques (feedback loops)
3. Improve measurement systems
   • Enhance accuracy and precision
   • Implement gauge R&R studies
4. Redesign the process
   • Eliminate bottlenecks
   • Simplify process steps
   • Implement new technologies (automation)
5. Modify specifications (if feasible)
   • Collaborate with customers to adjust specifications
   • Align specifications with process capabilities and customer needs
6. Implement continuous improvement initiatives
   • Six Sigma methodologies
   • Lean manufacturing principles
   • Kaizen events for ongoing process refinement