Process Capability (CP) and Performance (CPK) Chart in Real-Time
To succeed in today’s global competitive manufacturing sector, businesses must thoroughly understand and manage their processes. Using Statistical Process Control (SPC) methodologies, organizations can use process data to get an accurate picture of how a process functions and what actions can be taken to improve it. Process improvements, the goal of SPC efforts, support essential business goals such as cost reduction and customer satisfaction.
There are several SPC tools that help you better understand how your process is functioning. You can use run or pre-control charts to check quality data against customer specifications. Additionally, you can monitor process variation using specified control limits on a control chart. Control charting with control limits gives you a visual showing your process stability. You can investigate data outside the control limits to find and solve problems that will improve stability as you reduce waste. As your process becomes more stable, indices for capability and performance are tools for understanding how well your process meets specification requirements over time. These analyses help you to monitor your process performance with regard to mean and standard deviation, determine how centered your process is around the nominal and see opportunities for reduction of scrap, rework and waste.
Capability indices are used to predict process behavior. They represent the fit between the expected future production and the specifications. To understand what a process is doing and what it can do, we need both the Cp and Cpk indices. Cp is the process capability. It refers to how well the process can satisfy the specifications. It is equal to the range of the specifications divided by the six sigma spread (+/- 3 sigma). This index compares the spread of the specifications to that of the natural tolerance. The larger the Cp value, the better the capability. However, Cp doesn’t tell us how centered the process is. It remains the same, regardless of the center line’s location or how much product is out-of-specification. It only responds to changes in the process variability.
Process centering is important, since a poorly centered process makes more out-of-specification parts than a well centered one. Manufacturers use the Cpk index to measure process centering. It is equal to the range of the process average, to the nearest specification limit, divided by half of the natural tolerance (3 sigma). Because Cpk is the limit closest to the average, it gives the worst end of the process. The Cpk value represents how centered a process is.
To understand what the process is doing and what it can do, we need both the Cp and Cpk indices. The Cp tells us if the process is capable of producing within the tolerances, and the Cpk tells us if it does. If both indices are the same, the process is completely centered. Cp and Cpk values are only valid for a stable process. Don’t be in a hurry to obtain a capability index. Many people err in basing a capability index from control chart information before they find out if the process is stable. Capability represents the fit between the expected future production and the specifications. If we can’t predict the process behavior, the capability means nothing.
Performance indices, Pp and Ppk, describe the data distribution of product that has already been manufactured and shipped to customers. Pp is the process performance variation due to common causes. Calculated similar to Cp, except, it includes all measured data. Ppk is the process variation and centering due to common and special causes. Ppk is calculated similar to Cpk, except, it also includes all data.
A process does not have to be stable before calculating Pp and Ppk values. Therefore, there is no assurance that future production will follow the same distribution. In order to prioritize process improvements, performance indices should only be used to check against capability indices (Cp and Cpk).
SynergySPC Capability Charts Predict Process Performance and Show Areas of Improvement in Real-Time
All capability charts are not created equal and many SPC software programs do not include this SPC tool. Be cautious if others claim to have this tool. We can assure you that SynergySPC capability charts are so uniquely designed that they will help you make lasting positive process improvements that will make significant improvements to the company’s bottom line – like no other.
SynergySPC suite of products has a variety of high-performance capability charts including:
- Calculated Capability (N) – calculates Sigma to determine process capability (Pp/Ppk)
- Cal. Capability (n-1) – calculates Sigma to determine process capability (Pp/Ppk)
- Estimated Capability – estimates Sigma to determine process capability (Pc/Cpk)
- Estimated Capability (calculated range) – uses the same formula above, but the value is based on all available samples shown on the chart, including samples that were not covered by the latest control limit calculation (Cp/Cpk)
These real-time charts communicate complex process information with clarity in just one-click. You are able to quickly access your current or historical process performance data against the mean and standard deviation in an easy-to-understand visual. You also see the exact calculated values for all critical capability metrics right on the chart. SynergySPC software does the time-consuming analytic work, in real-time, letting you focus on solving problems and implementing solutions.
Designed so you can quickly visualize your process capability and critical capability metrics, SynergySPC capability charts clearly identify where process improvements can take your quality to the next level.
At-a-glance and with one-click, you gain instant visibility into your process with Zontec capability charts.These information-rich capability charts give you actionable and meaningful insights into your process and help you identify areas where significant quality improvements can be made.
Over the years, our customers have saved millions of dollars improving their processes using SynergySPC capability charts. Zontec customers benefit from savings as they monitor their process for improved quality. Making process adjustments and sometimes modifications to equipment, highly talented engineers use the rich SynergySPC insights to increase yield while reducing scrap, waste and rework.
SynergySPC real-time software features a Cpk dashboard
Zontec has made it even easier for quality managers and engineers to have confidence in their process capability. Using the real-time Cpk dashboard, you immediately know the status of Cpk levels for any number of parts. Red indicators flag unexpected Cpk levels to quickly direct your attention to problem areas. Right from the dashboard you can easily drill-down to capability charts featuring specific quality metrics. To further free-up your quality team, SynergySPC can monitor Cpk levels and alert you with a text or email when a problem occurs. SynergySPC puts your critical quality information at your fingertips. Request a Zontec SPC software demo.
Free eBook about Statistical Process Control
More information about capability and performance indices is available by referencing The Book of Statistical Process Control by Zontec. This is written for the non-statistician, as a guide for statistical process control. Request a free copy of the Zontec eBook for more about statistical process control techniques and tools.