Process Capability: Examples
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A company producing kitchen worktops, specified the length of one range at 120 cm ± 0.25 cm. As a normal part of production monitoring, Control Charts were kept for such specified measures and all output was inspected against specification limits. A request from the sales team for tighter limits prompted the question of whether the limits could be reduced to ± 0.1cm, as this could result in significant orders from a new customer who preferred not to shave worktops to fit.
The production supervisor used the Control Chart data to draw a Histogram and check for Normal distribution, then calculate Cpk, across samples from a week's work, as in
the figure below . This showed that the process was currently so capable that inspection could be dropped and the new limits still met. This turned out to be as a result of old specification limits coupled with several recent process improvements.
As a result, the new orders were achieved and savings were also made on inspection costs.
Fig. 1. Process Capability example
- A sales completion team, aiming to reduce the shipment time of urgent orders, studies the process, including plotting control charts and using them identifying variables and eliminating common causes of variation. They then measure the process capability and use the results to set delivery targets for each of a set of geographical areas. Knowing their capability, they then guarantee delivery times to customers.
- A lathe operator doing an ongoing process improvement of his work learns the variables involved and how to measure them. He then uses a Control Chart to help stabilize the process. Work pieces are still sometimes failing specification limits, so he uses a Process Capability index measure to identify how capable the overall process is of meeting limits. He finds that the process is centered, but only just capable. Further process improvements help to improve this situation.
- An auctioneer studies the pricing variation on different categories of cars, and uses a Process Capability study to help determine pricing limits. She then uses this data to start bidding prices and advise potential sellers.
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