Process Capability (Cp) is a statistical measurement of a process’s ability to produce parts within specified limits on a consistent basis Its gives us the idea about the width of the Bell curve. This section contains detailed explanations of various capability indices and their interpretation. If, however, a system is stable, capability analysis shows not only the ability of the system in the past, but also, if the system remains stable, predicts the future performance of the system. endobj A free online reference for statistical process control, process capability analysis, measurement systems analysis, control chart interpretation, and other quality metrics. An important technique used to determine how well a process meets a set of specification limits is called a process capability analysis. Capability indices help to change the focus from only meeting requirements to continuous improvement of the process. << /S /GoTo /D (Outline0.3.2.18) >> << /S /GoTo /D (Outline0.3) >> endobj Process Capability (Cp) and Performance (Cpk) Chart Capability (Cp) and performance (Cpk) charts illustrate a process’s ability to meet specifications. The SixSigma R package allows getting significant information about a process capability with just a … << /S /GoTo /D (Outline0.4.3.55) >> Assessing process capability is not easy. (Summary) Process capability analysis measures how well a process is performing compared to given specification limits. 44 0 obj The input of a process usually has at least one or more measurable characteristics that are used to specify outputs. Specifications are numerical requirements, goals, aims, or standards. /Filter /FlateDecode Is it possible to have a Ppk value of 10 and a Pp number of 5? 8 0 obj support@pqsystems.com, 210 B East Spring Valley Rd. To complete the calculations, a set of data is required, usually generated by a control chart; however, data can be collected specifically for this purpose. << /pgfprgb [/Pattern /DeviceRGB] >> 59 0 obj endobj Specifications are the numerical requirements of the system. Capability analysis, or process capability analysis, is the comparison of the distribution of sample values to the specification limits, and possibly also the specification target. (Process Capability Analysis) For example, suppose you have a rotary tablet press that produces 30 tablets, one from each of 30 pockets … endobj (Box-Cox Transformation) >> (References) (CP and PPM interpretation) Can the process performance index Ppk be applied on the ongoing process? endobj An X- R control chart was also completed, showing no special causes of variation. endobj endobj However, nonnormal distributions are available only in the Process Capability platform. << /S /GoTo /D (Outline0.3.5.30) >> Occasionally there is only one limit, a maximum or minimum. The indices are easy to interpret; for example, a Cpk of more than one indicates that the system is producing within the specifications or requirements. What is process capability? << /S /GoTo /D (Outline0.1.1.4) >> >> How can Cpk be good with data outside the specification? (Process Capability Analysis for Non-Normal Distributions) << /S /GoTo /D (Outline0.3.3.19) >> 75 0 obj Process capability analysis represents a great tool for industrial and process engineers for identifying variation within a process with the aim of improving its yield and make it more efficient. Traditionally, the focus has been to reduce the proportion of product or service that does not meet specifications, using measures such as percentage of nonconforming product. Process capability = Voice of the customer / Voice of the process Before capability analysis is completed, a histogram and control chart need to be completed. In other words, the ability of a process to meet the given specifications (e.g. endobj Capability indices help to reduce the variation relative to the specifications or requirements, achieving increasingly higher Cp and Cpk values. Copyright © 2020 Productivity-Quality Systems, Inc. Dayton, OH 45458, English Español Deutsch Português 中文 Français. >> How do we determine process capability if the process isn't normal? What is the difference between Cp and Pp? Use Capability Analysis (Binomial Distribution) to assess the capability of an in-control process when the data are from a binomial distribution . Process Capability Analysis: Estimating Quality presents a systematic exploration of process capability analysis and how it may be used to estimate quality. The measurement of process capability is more complicated than that. Why would I have Cp and Cpk indices well over 1 when some readings are outside the specification limits? Without referencing any of the mathematics, process capability analysis is simply comparing the specification range for a characteristic to its process range. Why do capability indices formulas divide by 3? The Process Capability for a stable process, typically defined as ((USL-LSL)/ (6 x Standard Deviation)). Can I compare two processes based on only the Cpk values of each of them? >> Calculating capability indices with one specification, 800-777-3020
Adjustments must be made when we have a one-sided specification and/or a boundary limit in place… Generally, the final solution of the process capability is specified either in the form of calculations or histograms. endobj customer requirements, engineering tolerances or other specifications). One basic measure of the capability of the process is the proportion of values falling inside (or outside) the specification limits. A process that is capable indicates the car (process) is skinnier than the garage you’re trying to park it in (the specification width). Information for the example is shown below: USL = upper specification limit = 14 minutes, LSL = lower specification limit = 0 minutes (on time). It is essentially a ratio of the customer requirement (specification) and the expected process variation. endobj << /S /GoTo /D (Outline0.3.1.17) >> Capability analysis is summarized in indices; these indices show a system’s ability to meet its numerical requirements. endobj 11 0 obj • In a later modules we will discuss capability analysis … endobj All methods of capability analysis require that the data is statistically stable, with no special causes of variation present. endobj << /S /GoTo /D (Outline0.4) >> ���v���`;��q�Y�+�'xK=i�0�a�+�&ا��jDL�Nj�9�a� ��~|p�=�Џ�K|�� �N�b��E�r����`��y�jp�q�|}��. 56 0 obj Some textbooks teach users to wait until the process reaches equilibrium, take roughly 30 samples and calculate their standard deviation; however, it is difficult to know when the process reaches a state of equilibrium and if the recommended samples are representative of the process. endobj This means that such obvious sources of potential variation as operators and vendors will be controlled while the study is conducted. endobj It is important to remember that specifications are not the same as control limits. 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