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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