What Is Capability? What Is Cp And Cpk?


Finley, John C.   (1992, ASQC)   GenCorp Automotive, Wabash, IN

Annual Quality Congress, Nashville TN    Vol. 46    No. 0
QICID: 9756    May 1992    pp. 186-192
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Article Abstract

Of fundamental importance in the quality management of manufacturing processes is statistical process control (SPC). Similarly, of crucial significance to SPC is the concept of potential capability (Cp) and actual capability during production (Cpk). Eschewing a purely mathematical explanation of Cp and Cpk, this article employs the production of one dimension of a part (automotive hood-to-cowl seal) to explain Cp and Cpk. It is the job of the manufacturer to produce the part in conformance with a target or nominal value specified by the customer. The customer and manufacturer must then agree on tolerance limits for producing the part. Capability denotes a process which can produce a part consistently within a specification and its tolerance limits. Once manufacturing of the part begins, the total process variation and the total tolerance width is used to calculate the Capability Potential (Cp). Cp reveals whether the process variation for producing a part is able to fit inside the tolerance width specified. Cp does not reveal if the part actually is inside specification. To determine if the part meets specification, and to improve quality, it is necessary to measure Centered Process Capability (Cpk), which compares the actual average of a manufacturing process with the closest specification limit, and balances it against the process width. The closer a process is to specification, the better the quality. Cp and Cpk are valuable tools for evaluating initial and on-going capability of parts and processes.


Capability study,Statistical process control (SPC),Manufacturing

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