25 Dec 2013

Process Capability


Process capability compares the output of an in-control process to the specification limits by using capability indices. The comparison is made by forming the ratio of the spread between the process specifications (the specification “width”) to the spread of the process values, as measured by 6 process standard deviation units (the process “width”).



Why Should I know the Capability of  Processes?

Process capability measurements allow us to summarize process capability in terms of meaningful percentages and metrics.
1. To predict the extent to which the process will be able to hold tolerance or customer requirements. Based on the law of probability, you can compute how often the process will meet the specification or the expectation of your customer.
2. You may learn that bringing your process under statistical control requires fundamental changes – even redesigning and implementing a new process that eliminates the sources of variability now at work.
3. It helps you choose from among competing processes, the most appropriate one for meeting customers’ expectation.
4. Knowing the capability of your processes, you can specify better the quality performance requirements for new machines, parts and processes.
 


Why Should I know the Capability of Supplier’s Processes?

1. To set realistic cost effective part specifications based upon the customer's needs and the costs associated by the supplier at meeting those needs.

2. To understand hidden supplier costs. Suppliers may not know or hide their natural capability limits in an effort to keep business. This could mean that unnecessary costs could occur such as sorting to actually meet customer needs.

3. To be pro-active. For example, a Cpk estimation made using injection molding pressure measurements during a molding cycle may help reveal a faulty piston pressure valve ready to malfunction before the actual molded part measurements go out of specifications. thus saving time and money.

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