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Our office has argued back and forth about this and i don't think we have ever come up with a solid answer. If you have a process running very well and very stable. Our Cpk's are outrageously high like 4.0 for about a month. And then a punch tip breaks without stopping the machine.
So when the operator does his/her inspection he/she finds a dimension that is way out of the spec limits. The operator throws out all of the parts produced between the last good inspection and the bad one and then replaces the punch. Then the operator starts running again and everything is once again very good.
Except the Cpk and Ppk on SPC is now showing around .2 . We are not worried that we have supplied our customer with bad product but we don't know if the outlier should be used to evaluate the process. Punch tips breaking is a part of the actual process. It will happen on occasion and product will run out of spec when it does.
I just finished reading the long and old sticky post about Cpk vs. Ppk and it almost sounds like the answer might be yes and no. Am I right in saying that for Cpk we should throw out this outlier but for Ppk we should not? If Cpk is the process capability when instabilities are removed and Ppk is the actual process as it has performed with subsample variation then.
Either way this fairly uncommon outlier is going to make the population non-normal so using either Cpk or Ppk with the outlier has major issues. I'm just wondering what you guys think should happen in this situation. Do you throw the data out for a Cpk analysis? Do you throw it out for a Ppk analysis?
We don't currently have acceptance based on Cpk in our plant but will be going to it in the next few years. These are the questions that boggle the forward looking people around.
So when the operator does his/her inspection he/she finds a dimension that is way out of the spec limits. The operator throws out all of the parts produced between the last good inspection and the bad one and then replaces the punch. Then the operator starts running again and everything is once again very good.
Except the Cpk and Ppk on SPC is now showing around .2 . We are not worried that we have supplied our customer with bad product but we don't know if the outlier should be used to evaluate the process. Punch tips breaking is a part of the actual process. It will happen on occasion and product will run out of spec when it does.
I just finished reading the long and old sticky post about Cpk vs. Ppk and it almost sounds like the answer might be yes and no. Am I right in saying that for Cpk we should throw out this outlier but for Ppk we should not? If Cpk is the process capability when instabilities are removed and Ppk is the actual process as it has performed with subsample variation then.
Either way this fairly uncommon outlier is going to make the population non-normal so using either Cpk or Ppk with the outlier has major issues. I'm just wondering what you guys think should happen in this situation. Do you throw the data out for a Cpk analysis? Do you throw it out for a Ppk analysis?
We don't currently have acceptance based on Cpk in our plant but will be going to it in the next few years. These are the questions that boggle the forward looking people around.
sorry i cannot share with you the spc form since i'm not with company anymore.
) and post it in this thread 