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Reducing a Cpk which is higher than 1.33

O

oldman

#1
Hello all,

I have a customer asking what we are going to do to get the Cpk results that are higher than 1.33, down to 1.33.

Is this something that anyone has come across before?
 
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Jim Wynne

Staff member
Admin
#2
Hello all,

I have a customer asking what we are going to do to get the Cpk results that are higher than 1.33, down to 1.33.

Is this something that anyone has come across before?

Welcome to the Cove. :bigwave:

Your customer is obviously confused. You should just reply and explain that for CpK results, the higher the better.

In a similar vein, a while back I submitted a PPAP package (to an automotive customer) that included an attributes gage study. The study indicated that there was 100% agreement between operators, which is the goal. The submission was rejected because, and I quote, "MSA results greater than 30% are not exceptable [sic]. See AIAG MSA 4th addition [sic]."
 
O

oldman

#3
Thanks....that was my reaction, I was just caught a little off guard by the question and wanted to see if anyome else had run into it.
 
D

DrM2u

#5
:lol: I like the responses from the other Covers, you have to take the situation with a grain of salt!

Unfortunately, I had a similar situation but there was some validity to it after further investigation. The problem was, the supplier had a process with very high Cp/Cpk, but targeted above the mean. The customer also had very high Cp/Cpk values, but targeted below the mean. Guess what: some of the parts would fail at assembly, even though they were well within the tolerances, so the customer asked for lower Cpk so more of the parts would fit. The supplier was a little smarter and re-targeted the process to match the customer's process and eliminate the problem.

What it comes down to is to know and understand your customer's (unspecified) requirements and how the parts you produced are used further.
 

Jim Wynne

Staff member
Admin
#6
The problem was, the supplier had a process with very high Cp/Cpk, but targeted above the mean. The customer also had very high Cp/Cpk values, but targeted below the mean.
How can a process be "targeted above the mean"? Do you mean "nominal"?

Guess what: some of the parts would fail at assembly, even though they were well within the tolerances, so the customer asked for lower Cpk so more of the parts would fit. The supplier was a little smarter and re-targeted the process to match the customer's process and eliminate the problem.
What it comes down to is to know and understand your customer's (unspecified) requirements and how the parts you produced are used further.[/QUOTE]
It sounds like this was a request on the part of the customer for the supplier to give up half (or thereabouts) of the tolerance previously agreed to in order to avoid doing the obvious and changing the specifications.
 
O

oldman

#7
Thank you all for your input.

The customer realized the error of his question when I stated that we would have to add more variation to the process to decrease the Cpk of 2.5 and higher.

It was a duh momment!
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#8
Thank you all for your input.

The customer realized the error of his question when I stated that we would have to add more variation to the process to decrease the Cpk of 2.5 and higher.

It was a duh momment!
A good story. This fits right in with the customer may not always be right, but the customer is always the customer. Sometimes the supplier needs to educate the customer. Glad to hear there was a happy ending.:agree1:
 

Statistical Steven

Statistician
Staff member
Super Moderator
#10
Hello all,

I have a customer asking what we are going to do to get the Cpk results that are higher than 1.33, down to 1.33.

Is this something that anyone has come across before?
It is not an unreasonable request actually....what they meant to say was, can you tigthen your specifications so the Cpk with the given variation would be 1.33....that you can do!
 
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