FAQ: What is the difference between Cp & Cpk?Īns.: Cp & CpK are termed as process capability and process capability index. downloading links are provided at top of this Article. Note: Download the Cp & Cpk excel template or format and deploy it in manufacturing process. Standard Deviation = Average of Range / d2 = 4.23Ĭp = (USL-LSL)/6*Standard Deviation = 1.2ĬPU = ((USL-Average of Mean)/3 x Standard Deviation) = 0.872ĬPL = ((Average of Mean-LSL)/3 x Standard Deviation) = 1.489ĬpK = 0.872(minimum of CPU or CPL). Laddle Temperature Specification= 600 ± 15☌ The process engineer has collected the 100 nos laddle temperature reading and the same is mentioned in the below table. Process Capability Analysis with Manufacturing Example (20 times X 5-readings at a time).Īfter doing the Process Capability Analysis on Scratch hardness readings, we got the below result value: Characteristics: Scratch Hardness In the above two tables (Table-1 &2), we have taken the 100 reading i.e. Scratch hardness readings Table: Table-1 Sl.No. Further Ppk can be calculated when there is any combination of spec limits: lower only, upper only and both upper and lower spec limits.Tolerance is 20. Note: Ppk depends on both the average and standard deviation. The acceptance criteria for a variable sampling plan can be stated in terms of the capability indexes Pp and Ppk. Ppk estimates actual performance, while Cpk estimates the capability the process could achieve if made stable over time. Cpk requires that the units be collected in subgroups over time and uses the within subgroup standard deviation, ignoring the effects of shifts between subgroups. However, they use different methods for estimating the standard deviation. They are both calculated using the formula above. Ppk is similar to the capability index Cpk. The larger Ppk is, the better the capability. A Ppk value of 2 means the distance between the average and the nearest spec limit is 6 standard deviations and thus the process fills only half this interval. A Ppk value of 1 means the distance between the average and the nearest spec limit is 3 standard deviations and thus the process fills this interval and touches the spec limit. For the normal distribution 99.7% of values fall within +/-3 standard deviations of the average or into an interval 6 standard deviations wide. The denominator is 3 standard deviations, which can be thought of as the half the width of the process. For a one-sided spec limit (lower spec only or upper spec only) only use the portion of the formula for that spec limit. The numerator is the distance to the nearest spec limit. Capability index that measure the relative distance to the nearest spec limit:
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