Process Capability Metrics
Introduction :
Quality control is the cornerstone of any successful business, ensuring that products or services meet customer expectations. Within the realm of quality control, Process Capability Metrics are invaluable tools for assessing and quantifying the performance of a process. In this blog post, we’ll dive into the Process Capability Metrics
What Are Process Capability Metrics?
Process Capability Metrics are statistical tools that provide insights into a process’s ability to consistently produce items within specified limits or tolerances. They gauge how well a process can meet quality requirements. Key metrics include Capability Indices (Cp and Cpk) ,Capability Ratio (Ca) and Defective Parts per Million (DPPM)
The Key Metrics
- Capability Indices (Cp and Cpk): These metrics measure the overall capability of a process to produce items within specified limits. Cp assesses potential capability, while Cpk considers both potential and centering within limits. Cp greater than 1 indicates that the process is capable of producing within specification limits.
How to calculate CPK and CP
CPK=
CPU: (average of Upper Limit - average of VALUE)÷(3σ of VALUE)
CPL: (average of VALUE - average of Lower Limit )÷(3σ of VALUE)
CPK: MIN(CPU, CPL)
CP= (average of Upper Limit - average of Lower Limit)÷(3σ of VALUE)- Defective Parts per Million (DPPM): DPPM quantifies process performance and quality in terms of defects or errors per million opportunities. The lower the DPPM value, the higher the quality of the process.
PPM= (NORMSDIST(-CPK*3)+(1-NORMSDIST(CP*6-CPK*3)))*1000000- Capability Ratio (Ca): The Capability Ratio (Ca) is another key metric that assesses the process capability by comparing the process’s inherent variability to the specification limits. A Ca value greater than 1 indicates that the process can meet the specifications.
Ca= 2*(average of VALUE-(average of Upper Limit + average of Lower Limit)/2))÷
(average of Lower Limit - average of Upper Limit) Real Time Example of CPK metrics
The Parking a Vehicle in the Garage :
Think of the walls of your garage — where you have to fit your Vehicle — they become the customer specification limits. If you go past those limits, you will crash, and the customer will not be happy!
When your process has a lot of variation, the process average is all over the place. Not good for parking a Vehicle or any other process. To give your parking process the best chance of success, you should reduce variation and centering.
If the Vehicle is too wide for the garage, nothing you do to center the process will help. You have to change the dispersion of the process (make the Vehicle smaller.)
If the Vehicle is a lot smaller than the garage, it doesn’t matter if you park it exactly in the middle; it will fit, and you have plenty of room on either side.
If you have a process that is in control and with little variation, you should be able to park the Vehicle easily within the garage and thus meet customer requirements. Cpk tells you the relationship between the Vehicle’s size, the garage’s size, and how far away from the middle of the garage you parked the Vehicle.
Benefits of Using Process Capability Metrics
Implementing Process Capability Metrics offers several advantages:
- Improved Product Quality: By understanding and monitoring process capability, organizations can consistently deliver high-quality products or services.
- Cost Reduction: Identifying and addressing process variability can lead to cost savings, as it reduces waste and rework.
- Risk Mitigation: Using these metrics can help organizations reduce the risk of defects, non-compliance, and customer dissatisfaction.
Conclusion
Process capability metrics are powerful tools for understanding and improving manufacturing processes. By measuring the capability and performance of a process, businesses can ensure that their processes are capable of consistently producing good parts, leading to improved quality, efficiency, and customer satisfaction. Remember, the goal is to have a manufacturing process width that is narrow and well centered relative to the specification limits.