Application of Process Capability Indices toMeasure Performance of Manufacturing Process Kwek Yan PingFacultyApplied Science and Technology, University Tun Hussien Onn Malaysia, Malaysia [email protected] process capability indices (PCIs) are used to evaluate and monitorthe process of manufacturing industry. The PCIs that used in the manufacturing industry are , , and . The machine capability is calculated by and which is in short period.
The PCIs areimportant in manufacturing industry.Keywords. Processcapability indices; Machine capability.Introduction A process in the manufacturing industry is very unique because the process needs the linkage of manpower, raw materials, machines, andmethods. The linkage are the variation ofthe process that are hard to control. In order to improve the quality level ofproducts, the capability of the process plays an important role in the manufacturing industry. Rezaie, K. 6.
The process capability indices(PCIs) is also known as process capabilityanalysis which is widely used in manufacturing industry. The PCIs isused to measure the ability of the process to meet the requirements of thecustomers which are also known as specification limits. The analyst used PCIsto evaluate and monitor the performance of the process in the manufacturing industry. Besides, the PCIs can use to improve the performance of the process by reducing the variation of theprocess. JH, S. et al. 3.The process capability analysis can be presented graphically by usinghistogram and probability plot.
Montgomery, D. C. 5. The graphics can determine whether the process iswithin specification limits (SLs) or not. Steiner, S.
et al. 9. Thehistogram is the easiest and clear method to determine the performance of the process.
If the process is incontrol and the data is normally distributed, the potential capability analysiscan be used to improve the quality of the process. Spano, A. 8. By referring to thehistogram, we can determine whether the process is off centering or not. ThePCIs is used to compare the output of in control process with the SLs by 6 sigmas. PCIs are quantitative measures based on the performance and potential of theprocess. By doing so, the process can meet the requirements of the customers.Rezaie, K.
et al. 6. The process of the manufacturingindustry is considered as capable when the process is under control. Spano, A.6.
The automotive industry was applying the PCIs in Japan and United States(US). The process capability with sampling properties and estimation proceduresis measured to determine the normality of the process and the process is instatistical control or not. Kane, V.E.
4. The PCIs had been used inmanufacturing industry in quantitative form to ensure the performance of theprocess. The numerical data will help the person in charge can modify the processby using more efficient methods. The cost spent in the process of rework andscrap will cause the loss of the industry. Hence, the high-quality production should practice in industry and try to meetthe requirements or else the reputation of the manufacturing industry. JH, S.et al. 3.
The PCIs is the best method to maintain the quality of the product.According to the requirements of thecustomers with the costs that needed to spendby the suppliers, the suppliers still need to set the suitable cost for theirproduct and ensure that the products are under tolerance limits. If the process capability of the process is undercontrol by the suppliers, we can try toimprove the quality of products or the process by out of control action plan(OCAP). The OCAP is necessary as the defective or nonconforming items willendanger the safety of the customers. The process capability indices can helpto analyze the capability and try to predict the machining process that canmeet the expectations and tolerance limits of the customers.
The processcapability indices consist of , , and . Abhijit S. et al., Montgomery, D. C. 1, 5.
The valuewill represent the performance of process. Table 1 summaries the six qualitycondition with its value.?hsan K.
2. The different valuewill represent different quality condition.Table 1: Quality conditions and value Quality condition value Super excellent 2.00 ? Excellent 1.
67 ? ? 2.00 Satisfactory 1.33 ? ? 1.67 Capable 1.00 ? ? 1.
33 Inadequate 0.67 ? ? 1.00 Poor < 0.67 In manufacturing process, value is used to identify the variation of thecharacteristics of the product within SLs that set by the engineers or thedesigner. Rupesh C.
et al. 7. Although many manufacturing industry had use value to determine the process capability ofthe process, but value cannot used to determine whether theprocess is off-centering and we do not know the actual performance of theprocess.
Thus, the value is used to indicate that the process isin the SLs. The and always used in manufacturing industry in orderto determine the number of the product that within the specification limits andensure the process is well perform. ?hsan K.2. and have the same statistical properties. Sincethe consists of the and , most of the manufacturingindustry will used instead of and .
The value widely used in manufacturing industry.The value can assume the normality and independencyof the target value, T. If the sampling distribution had skewed slightly or offcenter slightly, the process can still considered as capable. Zhang, N.
F. et al. 10. Since thereare many manufacturing industry had uses the process capability, there are manystudy that try to use the capability indices to determine the performance ofmachining process. More undertakings have been given to utilizations andinvestigations of process capacity indices. Abhijit S. et al., JH, S.
et al., Rezaie, K. et al. 1, 3,6. Actually, the PCIs is not limited in the overall process of the manufacturingfactory but PCIs also can use in the specific item which is machine. In thefuture, the manufacturing industry might implement the capability indices todetermine machining process performance. MethodsProcess Capability Indices Inthe manufacturing industry, the PCIs areused to monitor the capability of the process.
The , , and will relate with the process parameter thatset by the industry. The PCIs always used in assessing and predicting theprocess performance. The PCIs provide the measurements in numerical value whichcan help to improve the quality of products and process easily.1. The estimated standard deviationThetrue standard deviation in a production process is always unknown.
The value is always replaced by the value from the sample measurement. The value is calculated by using the equationbelow: = Average of range = Factors of center line Thefunction of is depend on the sample size of the processand the value of can determine by referring the Appendix VI. Montgomery, D. C.
5. The value is used when calculating the and value. 2. The value Themain reason for using value to assess the process ability to produceproducts with small amount of variation. The 6? is the natural tolerance of theprocess. The value can determine whether the manufacturingindustry produce the defect items or not.
The quality of the product can bemaintained by industry as long as the process is statistically in control. Ifthe process is out of control, the OCAP can be taken by manufacturing industry.The higher the value of , the higher the quality conditionof the process.
The estimate of the value is calculated by using the equationbelow: 3. The valueThemain reason for using value to assess the process ability to produceproducts that within the SLs. The is calculated by setting the 3? between theaverage of the process and the closest SLs. The is determined by the minimum value of and .
The value can determine the whether the industryproduce the products within SLs or not. The value should closer with value because the process should not far fromthe target value. The value is calculated by using the equationbelow: One-sidedprocess-capability ratios for upper specification limit One-sidedprocess-capability ratios for lower specification limit µ =Mean Results and Discussion Bad Good Bad Good Process not capable change tool or adjust for good accuracy. The process is capable but average need to be adjusted. Not possible. The process capable and well adjusted.
Figure 1: Therelation between and There are many manufacturing industries inMalaysia. Hence, the manufacturing industries are keep trying to improve thequality level of their products. The industries try to produce the productsthat can surpass the expectation of the customers. Hence, the PCIs are veryimportant in order to improve the performannce of the process and products. Figure 1 is about the relation between and . JH, S. et al., Rezaie, K.
et al. 3, 6. The processin manufacturing industry is considered capable if the value is equal to 1. The value should as high as possible. This isbecause the higher value means the variation of the process islower. In manufacturing process, the variation of the process must be reducedin order to produce the products with constant quality level.
If the value for a process is greater than 2, we canconsidered the process that undergo by the manufacturing industry is qualifiedas six sigma process. The value can only determine the variation of the process but value cannot ensurethe production of the process is within the SLs. If the process is notcentering, the process is considered faultily. In order to ensure the process of themanufacturing industry is in good condition, the value is calculated. The process is consideras capable as the process is within the SLs and value must larger than 1 in manufacturingsector.
If the value is equal to 1, the manufacturingindustry will accept 99.73% of the product. The higher value, the process distribution becomenarrower. In the manufacturing industry, the quality of the products need moreuniform and within the SLs. Hence, the value is considered a good measurement inmanufacturing industry because it can ensure the quality of products are alwayswithin the expectation of industry. If the value is greater than 3, most of themanufacturing industry will do nothing because the process is very capable. If the value is equal to value, the process will consideredcentralized.
In most of the manufacturing industry, the value will not exactly equal to value. Some of the manufacturing industry willset minimum value for and as 1. Actually, and value should be more than 1.33 which means theprocess of the manufacturing industry is satisfied. The value will never higher than value. If the value is slightly lower than value, the process is still acceptable formanufacturing industry. The and value is important to determine the processcapability in the manufacturing industry. MachineCapability Process Variation Machine Method Manpower Measurement Environment Material Figure 2: Ishikawadiagram The environment of manufacturing sector will affect the capabilities of the process in a longperiod.
The machine capability will not consider whether the machine is incontrol or not as the problems of the machine can determine by the controlchart. However, the machine capability cannot use to predict the status of theprocess in long-term by short-term sampling plan. Thus, the capability of the machineis not suitable calculated in a long period. Generally, the machinecapability is used plus and minus three standard deviations (± 3 ). This is the baseline for the manufacturing process. Byusing the machine capability,the manufacturing industry can determine the condition of every single machine.
The machine capability is similar with PCIs. The machine capaility incides arevariation of the machine itself. The calculation for is represent while represent . Hence,the value is used to determine the capability ofthe process while the value is used to determine the capability ofthe machine.
There are some elements whichwill cause the variation of the process which is shown in Figure 2. All of thevariation that mentioned in Figure 2 might influence the outcome. The machinecapability we only determine the dependency and the centering of the machineparameters.
JH,S. et al., Rezaie, K. et al. 3, 6.
ConclusionThe processcapability indices can use in the manufacturing industry to ensure the processcapability. The process capability indices can ensure the products is withinthe specification limits. If the products donot meet the specification limits, the corrective actions can be taken. Hence,the quality of the products can be maintainedin a uniform manner.
Besides, the manufacturingindustry can use the process capability indices to monitor the process. Thequality of the products can be improved. The machine capability needed tocalculate in short term. The variation of the process should be eliminated.
Theoverall process of the manufacturing industries can be maintained or evenimproved.AcknowledgementsThe author very appreciates the referees because the informationand knowledge they gave are very useful in orderto accomplish this paper. References1.
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