SPC: What is process control?

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By Ken Roney | Jul 25, 2019
Process control in Statistical process control

This blog is the second in our series on statistical process control (SPC). In the previous blog (read here), we learned about the fundamentals of statistical tools and techniques. In this blog, we will be focusing on one of the most commonly employed statistical tools in the manufacturing industry – process capability indexes (Cpk).

Process capability index is an indicator of how the production process is performing with regards to key process control parameters and customer specifications. Besides the obvious reason of satisfying customer requirements, there is another reason to develop and use capability indexes. They can be critical predictors of future performance and a robust method of measuring the ability of production processes to produce high-quality parts consistently.

The letter “C” in SPC stands for control. But, what are we controlling? Statistical analysis of a production process is all about controlling and reducing variation in the process. It aims at identifying and separating the natural causes of variation, that are inherent to the process, from the special or assignable causes that can be controlled, adjusted, and/or eliminated. Process capability is the ability of a process to produce a product that is both accurate and precise. The accuracy and precision of a set of measurements can be illustrated as follows:

Accuracy versus precision

 

Cpk takes into account both the accuracy and the precision of the measurement around the average. Statistical software takes measurement data and shows the process capability as a single number that represents the process’ ability to provide both – an accurate and precise product. At GMN Plastics, we aim for a Cpk of 1.33 or greater for most of our customers. The core benefits of creating a process that delivers high-level capability include optimizing manufacturing time and materials, reducing scraps and re-work, and equipping the production system to create more products in the least amount of time possible.

Ultimately, process capability is all about creating products that meet customer specifications with a minimum variation over time. Stay tuned to learn more about control charts in our next blog. Until then, check out our other blog on Qualification process: Four parts to ensure part consistency.