Process Capability Index: Questions And Answers

Chapter 1

Process Capability Index

Process capability index is a continuous variable with a known probability distribution that is used for analyzing the production process thus it can be used for designing sampling plans. It have been used in various industrial environments as capability measures that are obtained based on process departure from a target, process yield, process consistency and process loss. Process capability compares the output of an in-control process to the specification limits and the process output with the customer’s specification. Two parts of process capability are: i) Measure the variability of the output of a process, and ii) Compare that variability with requirement specification or product tolerance. The comparison

These deviations may be categorized as being due to two causes. They are Assignable Causes of variation and Chance Causes of variation. A chance cause of variation is the natural variability that present in every production processes. The variation exists in production process due to randomness as purely random variability from one product to another. The variation attributed to any production process due to non-random is called Assignable Causes of variation. These factors cause heterogeneity in the process and as a result they affect it, leading to low quality product.

Statistical In- Control and out-of control

The process that operates with only Chance cause variability is said to be state in-control. The process that operates in the presence of assignable causes of variation is said to be state out-of control. The assignable causes of variability can be detected and eliminated that would reduce the overall variability. As a result of eliminating this variation leads to an improvement of the quality of the product. SPC methods are helpful in deciding whether the production processes are stable or in a state of statistical control. Process control is achieved by employing control chart as a statistical tool. J

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Control Limits, Specification Limits, and Natural Tolerance

The OC Curve defines the ability of the certain SPC methods to notice shifts in the process mean. A plot of â versus the shifting process parameter gives the operating characteristic (OC) curve of a control chart. The OC curve is a measure of the ability of a control chart to detect the change in process parameters. For small changes in the process parameter, the probability of non-detection (â) by the control chart is high. For large changes in the process parameter â should be small so that it is detected and corrected by the control