Tips on Performing a Process Capability Study

A process capability study is performed for two primary reasons:

1. to predict future capability of the process to produce product within specification.
2. to identify process improvement opportunities.

Process capability is a statistical analysis of variation of the process output compared to the allowable specification limits. Cpk is most often used in a manufacturing process to express process capability.

There are common problems that occur in process capability studies that can be overcome with proper planning and implementation before the study begins.

Process Potential Study vs. Process Capability Study
Should process potential (Ppk) be calculated or process capability (Cpk)?

A Ppk calculation is used to define, in statistical terms, what has already happened in a single lot of product. It is often used in the early phases of a project to obtain an estimate of process variation when only a small number of parts have been produced. The data points represent only a “snapshot” of the process and only a small window of its potential; hence the term “process potential study”.

A process capability study (Cpk), on the other hand, results from sampling of a continuous process over planned intervals. It is a prediction of what is likely to occur in the future based on sample subgroups taken over a length of time. It is a method of predicting future capability of the process; hence the term “process capability study”.

Sample Frequency and Handling during the Capability Study
For statistical accuracy, the capability study samples should be taken in subgroups at planned intervals throughout the production run. One way to assist in the accuracy of sampling is to establish a container or storage location for each sample set to be taken. Number the container to identify the sample sets in sequence or prepare tags for the samples in advance so that your capability study is not adversely affected by miss-handling of the samples. Identifying the sequence of your samples may prove to be valuable in the problem-solving portion of your capability study.

Process Interruptions During the Capability Study
More often than not, the production process is interrupted for an un-planned reason during your capability study. Before the study begins, establish a minimum number of sample sets that you will perform statistics on, and a “goal” number of sample sets you would like to collect. If the process was interrupted after the minimum samples were collected, end the study at that point. Remember that your Cpk numbers are based on the assumption that your data is gather from a “normal” process, represented by the bell curve labeled “Process Capability”, above. If an interruption occurs, there is a greater risk that you’ve introduced a “special cause” into your capability study. If your minimum sample number has not been met, it’s best that you start over with the study. Determine ahead of time what is an acceptable process interruption, and which interruptions will not be tolerated during the study.

Measurement System Capability
Before beginning your process capability study, determine the bias, stability, linearity, repeatability, and reproducibility of your measurement system. At a minimum, know your gage R&R as a percentage of the part tolerance. If an in-depth study of the measurement methods has not been done, get assistance to perform a capability study on the measurement system. It’s important to understand the total error that will be introduced into your study by the inspection equipment and people performing the work. A certified inspection lab can assist with this task as needed.

Process Changes and Improvements
Process changes (for the right reasons and at the right time) can have a positive outcome on the product. If they are done during the capability study, a process change only serves to introduce error and uncertainty. Involve the operators of the process in your plan so they will understand the importance of holding the process stable during the study period. Once the study is complete and special causes have been identified, introduce changes that will improve the process. Then perform another process capability study to confirm the improvements.

Planning for Success
If this is your first time developing a capability study, look for assistance to gain insight from others’ past experiences. Pitfalls range from cultural resistance to statistical errors, to process instability. A seasoned expert that has applied real statistics “in the trenches” can be a great savings to you in time and in the company’s resources. In any case, success lies with the implementation of what was learned from the study. Proper implementation will make your capability study move from an organizational cost to a value-added activity.
 

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