A value greater than positive 1 means that the spec limit is far enough away that the process should be capable and be within specs. The value of the Cpk ratio can take on any value ranging from 0 to any positive or negative number. Finally, you report out the minimum of the two values since that will be your “worst” condition and lowest process capability. Then you divide that distance by half the width of the distribution or 3 standard deviations. Essentially what you are doing is measuring the distance from the mean to the different specs. Since the process mean is now off center, you will report out the minimum condition depending on whether the process has shifted closer to the upper spec or closer to the lower spec. That’s when we need to use the Cpk to assess the process capability.Ĭpk is similar to Cp in that it utilizes the customer specs and process variation, as measured by the standard deviation, but it also includes the location of the mean relative to the specs. Interestingly, if the process mean is off-center, the Cp will still be the same since the Cp does not consider the location of the process. More often than not, the process mean ( x-bar ) will not be centered within the specs. However, all of this assumes that the process is centered within the specs. If the value is less than 1, it means that the width of the process distribution is wider than the specs, and items will be outside the specs. A value greater than 1 would mean that the specs are wider than the distribution of the process, so we are comfortably within spec. Therefore, if we look at the formula above, a value of 1 would mean that the width of the distribution is equal to the width of the specifications or specs. The standard deviation that is used is called a “pooled standard deviation” or “within variation.”Ĭp can take on values equal to 1, greater than 1, or less than 1. We are assuming that the sample distribution is normally distributed, and its width is calculated by adding and subtracting 3 standard deviations on either side of the sample data mean. You can say this as Cp is equal to the Customer’s Upper Specification Limit minus the Lower Specification Limit divided by 6 standard deviations. If there is only a one-sided spec, then Cp cannot be calculated. It represents the best the process capability can be for the specs and process distribution. Since Cp and Cpk are related, we will need to spend a little time talking about Cp before discussing Cpk in order to provide context for Cpk.Ĭp, or capability potential, is a measure of the potential capability with the assumption that the distribution of the sample process data is normally distributed and centered within the customer’s upper specification limit and lower specification limit. The goal of process capability is to come up with a quantified measure that informs you as to whether you are meeting your customer expectations or not as well as a relative value of how well you are meeting or how badly you are missing.īesides Cpk or Capability Index, there are a number of other process capability metrics. Process capability is the quantifiable comparison of the Voice of the Customer (specs, requirements, or expectations of your customer) and the Voice of the Process ( control limits ). We will explore its application, benefits and provide some tips and best practices so that you can begin assessing how well your process is doing when measured against the requirements specified by your customer. This article will explain how Cpk is calculated and interpreted. Cpk is a common measure used in process capability analysis that allows you to know how well your process is meeting your customers’ expectations and specifications.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |