Oct 11, 2015 process capability index it is a dimensionless number that is used to represent the ability to meet customer specification limits. From the cpk value, one can derive that machine 1 is better than 2. Spc practitioners applying control charts to processes with high cpk 2. Process capability cp, cpk and process performance pp, ppk. Process capability analysis for nonnormal processes with. Columns four and five include the lower and upper confidence limits for cpk. One process capability is cpu, which is the process capability based on the upper specification limit. Lean six sigma programs 10, process capability indices. Machine performance studies for measured data on discrete parts 8. Capability indices there are several statistics that can be used to measure the capability of a process. Process capability cpk and realtime spc software infinityqs.
Acceptance limits, process capability, gauge reproducibility and repeatability. The process is running too close to the lower specification limit. The value of cpk is the minimum of two process capability indices. Process capability cpk is the ability for a process to meet specification limits. Since cpk uses specification limits and parts variation sigma, we can also arrive at. If cp and cpk differ, then the process is not centered. Process capability by sanjaya kumar saxena the capability of a process is a statistical indicator of how well it is functioning, or, in other words, how successful it is at running within its specified limits. From a manufacturing perspective, a capable process is one that can tolerate a lot.
The capability of a process is defined as the ratio of the distance from the process center to the nearest specification limit divided by a measure of the process variability. Process capability analysis using minitab ii by keith m. Process capability of pipe diameter production run 2 case 3. Process capability analysis in this chapter we give the mathematical background of process capability analysis, in particular the capability indices cp and cpk and related topics like tolerance intervals and density estimation. The process capability analysis a tool for process performance measures and metrics a case study article pdf available in international journal for quality research 80620 october 2014. Process capability of pipe diameter production run 3 case 4. Should you calculate cpk when your process is not in. This implies that it has to be based on some sort of collection of data from the process.
Calculate capability indices and determine the level of quality 4. These indices are shown to form a complementary system of measures of process performance, and can. Process capability analysis with focus on indices for one. Abstract this paper builds on the content covered in the previous edition of extraordinary sense. In a later modules we will discuss capability analysis for nonnormal data. Cpk represents the difference between the actual process average and the closest specification limit over the standard deviation, times three.
Process capability analysis for nonnormal processes with lower specification limits masters thesis master of science in quality and operations management duygu korkusuz advisor. 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. Use cpk to evaluate the potential capability of your process based on both the process location and the process spread. An important part of any spc implementation is the use of process capability indices. Capability suite of six charts use the capability suite or the x chart templates to evaluate. Hendry raharjo division of quality sciences department of technology management and economics chalmers university of technology gothenburg, sweden 2011. Cp and cpk are considered shortterm potential capability measures for a process.
In the absence of any special or assignable causes of variation, a process will still have some inherent variability. Verify that the distribution of the data is normal 3. Based on the process capability analysis one can determine how the process performs relative to its product requirements or specifications. In simple words, it measures producers capability to produce a product within customers tolerance range. There is quite some confusion about the use of these indices. These indices are shown to form a complementary system of. Consider how difficult it is to physically picture the capability of a process that, for example, has reported process capability index cp 1. Process capability indices pcis, cp, ca, cpk, cpm, and cpmk have been developed in certain manufacturing.
By convention, when the cpk is less than one, the process is referred to as incapable. In both cases we want to try to verify if the process can meet to meet customer ctqs requirements. Automotive industrie requires a ppk better than 1,67 and cpk better than 1,33 or ppk better than 2,0 and cpk better than 1,67 f. Iso 225142, statistical methods in process management. Process capability indices cp, cpk, provide a common. Cpu usl xbar 3s process capability relative to upper specification limit. Generally you use this when a process is under statistical control. Basic wrap up of lecture on process capability index covering a few items we missed in class today. The process capability index, or cpk, measures a processs ability to create product within specification limits. Chapter 1 process capability analysis in this chapter we give the mathematical background of process capability analysis, in particular the capability indices cp and cpk and related topics like tolerance intervals and density estimation. A process capability study is a systematic procedure for determining the capability of a process. An interactive look at process capability spc for excel software. The other is cpl, which is the process capability based on the lower specification limit. Other approaches to describe how a process is performing includes a bar chart, pie chart, redyellowgreen scorecards or stoplight scorecards, and a table of numbers.
Chapter 255 capability analysis introduction this procedure summarizes the performance of a process based on userspecified specification limits. Process capability index or process performance relative to customer desires can be reported using process capability indices e. Potential within capability for normal capability analysis. This index compares the variability of characteristics to the specification limits. Importance of process capability and process performance. Process capability can be expressed as percent nonconforming or in terms of the natural spread related to the specification spread. The capability indices cp, cpu, cpl, k and cpk are presented and related to process parameters. The conclusion of combining knowledge within these four areas with the purpose of. In this article we will discuss the most common used indices cp, cpk, pp and ppk. The cpk associated with a process or a group of items is either the value for cpu or cpl, whichever calculation results in the smallest number. Process capability and performance of timedependent process models 7 iso 225143, statistical methods in process management.
Interpret the key results for normal capability sixpack. In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability. The idea is illustrated graphically in figure 1 that shows a histogram of the process output along with the specification limits. The output of a process can be product characteristic or process output parameter. Recommend process improvements that increase process capability. As with cp, this measure is appropriate only when the process is stable incontrol. Process capability indices measure how much natural variation. If these assumptions are not met, the resulting statistics may be highly unreliable. Pdf process capability can be evaluated through the computations of various process capability ratios and indices.
Frequently, quality practitioners find that the assumption of modeling a process using the normal distribution is not valid. There are several capability indices cp, cpk, ppk, cpm, ncpk. The concept of process capability only holds meaning for processes that are in a state of statistical control. Process capability analysis and process analytical technology. Process capability cp, cpk and process performance pp. The basic three capability indices commonly used in manufacturing industries are cp, cpk,cpm and cpmk. The outcome of a process capability study is a single metric, which provides an indication of the ability of a process to consistently provide output which is within required specifications. Process capability analysis for six sigma 6 6 length frequency 5. Pdf process capability and capability index cp and cpk. The observed performance as well as the performance relative to the normal distribution are output. The first process, on the other hand, is in control and thus its cpk is a good predictor of process capability. Note these slides are provided by the text book pub. Cpk takes into account where the process is centered. Process capability index cpk is a statistical tool, to measure the ability of a process to produce output within customers specification limits.
Cp and cpk are based on short term variability pp and. The capability analysis in figure 6 shows the corresponding analysis assuming a weibull distribution to the original set of data. Resolving process capability index issues for cp, cpk, pp. Frequently, statisticians make use of the weibull distribution for modeling purposes. Ppk is process potential capability cpk is long term analysis and is continous process capability. A probability plot overcomes this confusion by describing the process in terms that are easy to understand e.
Process capability analysis of pizza delivery case 5. Ppt 03 process capability and cpk index free download as powerpoint presentation. If you do not have the control chart to evaluate for process control, you might be tempted to select the second process as being better on the basis of the higher cpk. Process capability ratios such as c p and c pk are produced. Cpcpk capability 17 february 2017 ammaranass in process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability. In general, the notation for a discrete stochastic process is x i, where x i is the outcome of element no. Process stability using control charts process capability using histogram with cp cpk, capability plot, values plot data normality using probability plot histograms with cp, cpk, pp, ppk 1. Bringing a process into statistical control is putting the process where it should be. Here we discuss the following topics relating to process capability and six sigma. Cpk is a standard index to state the capability of one process, the higher the cpk value the better the process is. Process improvement is not bringing a process into statistical control. Process capability index it is a dimensionless number that is used to represent the ability to meet customer specification limits.
Variability can be stated as either shortterm or longterm. Process capability analysis march 20, 2012 andrea spano andrea. You may download a pdf copy of this publication at this link. The process is not centered, so cpk does not equal cp. The main difference between pp and cp on one side and ppk and cpk on the other side is whether we use complete set of data for calculation pp and ppk where we calculate real performance of the system, or we use sample preproduction, batch, logical subgroups where we calculate capability of the process. The distribution of the process considered is normal. For a discrete process, the index i is normally the number of the item in the generated sequence of items. Cpk is used to summarize how a process is running relative to its spec limits. An important part within process capability analysis is the use of process capability indices.
Process capability compares the output of an incontrol process to the specification limits by using capability indices. Process capability statistics cp and cpk for the six different organizations of the 100 data. In case the characteristic is a property x i, it is a value given to each of the events in the set of events used for characterizing the process. Pdf the process capability analysis a tool for process. Cmk, and in a similar way with a p for process capability, e. Process capability evaluation looking beyond the obvious. In six sigma we want to describe processes quality in terms of sigma.
This promotes senselessly tight control limits, especially in cases where slight shifts in the process mean occur when an input parameter has been changed such as raw materials, setups, lots and so forth. Process capability studies are used for monitoring the capability of a process. An overview of theory and practice on process capability. This thesis focuses on process capability indices in the situation when the specification limits are onesided. Potential capability indicates the capability that could be achieved if process shifts and drifts were eliminated. For a detailed mathematical account of capability indices, we refer to. Combining quality costing and risk based approach en. Cpl xbar lsl 3s process capability relative to lower specification limit.