Process Capability: An Overview for You
Processes variation is a fact of life in a manufacturing plant and so knowing the capability of your processes is essential information for every factory leader. But what is this process capability thing all about? Well, in its simplest form, process capability is just a mathematical comparison between your product specifications and the real-life variation of your process.
If a process is producing products that are centered in the middle of your specifications and the product-to-product variation is small compared to the width of your specs, then we would say that your process is very capable. But if your process is producing products that are not centered to your specifications and/or the variation is very wide as compared to your specs, then we would say that your process is not very capable.
What is a process capability study? A process capability study provides a statistical method to measure the relationship between your specs and the actual variation of your process. Measuring and understanding your process capability is then the first step in process improvement.
Process Capability is measured using four different process measurements and these are Cp, Cpk, Pp and Ppk. Each of these four measurements are calculated slightly differently and they work together to solve the mysteries of process variation. But before understanding these measurements it is important to understand mean, standard deviation and the normal distribution curve. You can learn about these statistical tools by reading our T4T articles that cover these topics.
Performing process capability studies allows us to determine three things:
First: A process capability study allows us to determine the capability of our process under ideal conditions. Measuring our process capability under ideal conditions helps us to establish our process entitlement or in other words, our capability benchmark. In this first step, we want to know that if all process conditions are perfect, how does our process perform?
Second: A process capability study will tell us how our process is actually performing over the long-term. This study will include variation changes due to all process inputs including raw material lots, room temperatures, machine conditions and operator differences. Really anything that affects process performance over the long-term.
Third: A process capability study can provide us with data that can be used as a launching pad for continuous improvement. You see when we compare our process as measured under ideal conditions to that of normal operating conditions, we can then highlight the difference between our ideal and our actual performance. We can then consider this gap to be our process opportunity.
The concept is that if we can do something once under ideal conditions, then why can’t we do it again and again over the long term? When we identify the differences in our process conditions between the ideal and the actual, we can then identify our opportunities for improvement by asking "why?" or "what changed?".
To learn about the three main measurements used in a process capability study (Cp, Cpk & Ppk), read our T4T article and watch our video about these process measurements.
It takes capable processes to produce quality products!
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