In this article we will discuss the most common used indices cp, cpk, pp and ppk. Process capability cp, cpk and process performance pp. You have worked hard at getting rid of the special causes of variation. Process capability cp, cpk and process performance pp, ppk. A free powerpoint ppt presentation displayed as a flash slide show on id. Capability analysis of normally distributed data chapter 5. There is quite some confusion about the use of these indices. 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 capabilitya managers tool for 6 sigma quality.
In process improvement efforts, the process capability index or process capability ratio is a. An important part of any spc implementation is the use of process capability indices. Process capability can be expressed as percent nonconforming or in terms of the natural spread related to the specification spread. The process capability or parameters pertaining to process capability that include machine accuracy of a machine or manufacturing the calculations of the process capability ratios cp, cpk, process is the minimum tolerance to which machine and ckm. Connection between the process capability and six sigma. In a later modules we will discuss capability analysis for nonnormal data. Pdf process capability can be evaluated through the computations of various process capability ratios and indices. A process capability study is a systematic procedure for determining the capability of a process. Process capability answers the question of how well our process meets our customers. A process capability analysis compares the distribution of output from an incontrol process to its speci. The distribution of the process considered is normal. Comparison between process capability index and dynamic process capability. A capability index relates the engineering specification determined by the customer to the observed behaviour of the process.
T ake representative sample of process output statistical analysis of samples ma thematical tools, scattered plot, pareto chart. S chart for x group group summary statistics 1 4 7 10 16 19 22 25 28 31 34 37 40 0. Pdf process capability and capability index cp and cpk. First, managers construct a plan to decrease the difference between customer needs voice of. Free online process capability evaluator by symphonytech. Pdf what is process capability, how to calculate and how to use during development stage and at commercial stage find, read and cite all. A proposal for a managementoriented process capability index. Process capability cp is a statistical measurement of a process s ability to produce parts within specified limits on a consistent basis. Experiments and capability analysis in process industry diva. Pdf measurement of manufacturing process capability. Iap 21 process capability in blackboard page 1 individual activity packet. Process capability analysis is a statistical technique that is used to identify and reduce the variability of a manufacturing process in order to produce items that meet certain specifications. Process capability study cpk study with excel template. Process capability analysis and process analytical technology.
The lower and upper engineering specification limits are 1. Usl lsl cp process is not a capable process since cp process capability analysis 68 process capability analysis l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l process spread lsl usl specifications the process is out of control but it is capable. Figure 3 shows these 3 cases for cp process capability measurement can be done in the short term as well as in the long term. Ppt process capability powerpoint presentation free to. Autocorrelation when testing for process capability. It is the ratio of the specification width to 6 x process standard deviation. Standard practice for process capability and performance. A process capability study uses data from an initial run of. A process capability study is the end result of the analysis done on process output data. The cp and cpk indices are used to evaluate existing, established processes in statistical control. Calculating process capability reports for normal and nonnormal data using. Process capability cp usllsl 6sp pp usllsl 6st sp pooled standard deviation st total standard deviation usl lsl cp, pp compares specification range toleranceto process width regardless of where the process is centred. Pdf the process capability analysis a tool for process.
Process capability free download as powerpoint presentation. For more information on these and on pp and ppk, please see our threepart series on process capability in our spc knowledge base. When the study is completed, you will identify the natural variability of the. Process capability index example a manufacturing process produces a certain part with a mean diameter of 2 inches and a standard deviation of 0. Capability analysis based on rate of nonconformities chapter 4. It will also attempt to explain the differences between process capability and process performance. Otherwise, one may think data are unstable when in reality may be the result of. In the absence of any special or assignable causes of variation, a process. Your process is in statistical control with respect to a key output variable. In this case, chance of producing nonconforming parts is very low. The performances of process capability indices in the six. For the capability analysis to be performed the process needs to be under statistical control.
It actually measures the capability of the process to perform the defectfree. The difference between specifications is called the tolerance when the tolerance is established by the design engineer without regard to the spread of the process, undesirable situations can result. 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. Abed schokry islamic university, gaza palestine process capability refers to the ability of a process to produce products or provide services capable of meeting the specifications set by the customer or designer. The basic three capability indices commonly used in manufacturing industries are cp. Process improvement studies follow the deming cycle of plan, do, study, act. But the statistical measurements of cp, cpk, pp, and ppk may provide more insight into the. The output of this measurement is usually illustrated by a chronological control sample, histogram and calculations that predict how many parts will be produced out of specification oos. Process capability analysis for six sigma 6 6 length frequency 5. During the past three decades, pcis process capability indices have inspired hundreds of pages of scientific research. Iso, aiag, asq, astm published their guideline or manual on process capability index calculation. Process capability indices for non normal distributions. If these assumptions are not met, the resulting statistics may be highly unreliable.
Many different process capability indices have been developed to measure the capability of a manufacturing process. Usl lsl cp process is not a capable process since cp process capability indices. Process capability assesses the relationship between natural variation of a process and design specifications an indication of process performance with respect to upper and lower design specifications application of process capability design products that can be manufactured with existing resources identify process weaknesses. Now only common causes are present most of the time. Measuring your process capability symphony technologies.
Process capability can be evaluated through the computations of various process capability ratios and indices. Six sigma process performance is reported in terms of sigma. Capability analysis capability analysis, or process capability analysis, is the comparison of the distribution of sample values to the specification limits, and possibly also the s pecification target. Iap 21 process capability in sigmaxl conducting process capability in sigmaxl is from the iassc lss gb bok section 2. Process capability and six sigma methodology including fuzzy and. There are several capability indices cp, cpk, ppk, cpm, ncpk. Process capability study is a scientific and a systematic procedure that uses control charts to detect and eliminate the unnatural causes of variation until a state of statistical control is reached. It could be purity of a chemical, hardness of a steel tube, or a key part dimension. The process capability is a measurable property of a process to the specification, expressed as a process capability index e. One basic measure of the capability of the process is the proportion of values falling inside or outside the specification limits. Following the numbering system under the title steps for a capability analysis, the correct order for performing a process capability analysis should be. Process capability analysis for normal distributions cp and ppm interpretation the figure below shows control charts for a process. This is determined by plotting data on a control chart.
189 1002 238 83 3 1153 665 780 1490 1301 1091 47 944 1310 719 503 1532 802 1215 736 1402 1060 1418 1481 769 1605 1315 892