Now viewing: capability indexes

Ken Roney
By Ken Roney
Tuesday, August 20, 2013 - 00:00

An increasing number of our customers at Elite Plastics require that we develop, record, and make available upon request, process capability data for key characteristics found on the print. Process capability indexes (most commonly Cpk) are an indicator of how a production process is performed with regards to key process control dimensions and their relation to customer specifications. Besides the obvious reason of satisfying customer requirements there is another reason to develop, and use, capability indexes. They can be used as predictors of future performance and are a good way of measuring the ability of our production process to produce good parts consistently.

Accuracy and Precision

The letter “C” in SPC stands for “control,” but what are we controlling? Statistical analysis of a production process is ultimately all about reducing variation in that process and to identify, and separate, the natural causes of variation that are inherent in all processes, from the special, or assignable, causes that can be controlled, adjusted and/or eliminated. 

The accuracy and precision of a set of measurements can be illustrated as follows:

The goal is to confirm the consistency of the process. With that in mind, it is extremely important that the company has a good handle on the inputs and variables of the given process and have the ability to make sound adjustments at the qualification stage to the inputs that are within their control. Process capability is also about both accuracy and precision. 

A Cpk index takes into account both the accuracy of the measurement and the precision of the measurements 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 accurate and precise product. For most of our customers we aim for a process capability index (Cpk) of 1.33 or greater.

Process Capability is the ability of the process to produce product that is both accurate and precise, that it consistently produces product that meets customer requirements with a minimum of variation over time. The benefits of creating robust processes that deliver a high level capability are obvious – it means a maximization of production time and materials, reduced scrap and/or rework, and the ability of the production system to deliver more products in the minimum amount of time possible.

Stay tuned for an upcoming blog discussing control charts and their use in our production processes.