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Cp & Cpk question...

Cp & Cpk question...

Cp & Cpk question...

(OP)
Where does the value 1.33 come from when calculating Cp and Cpk ?

RE: Cp & Cpk question...

At best, experience with real processes, more likely laziness. It allows for the fact that although a Cpk >= 1 ought to be OK, in practice your estimate of sigma may be wrong, your process may drift over the long term, and so on.

Of course it also assumes that your spec limits are meaningful. I remember that when we specified Cpk=1.67 for a new process the manufacturer immediately asked us to widen the specs. Idiots.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Cp & Cpk question...

(OP)
Thank you for the reply GregLock. I am a new Process Engineer fresh out of college in the automotive industry. I have been doing some Cp & Cpk studies recently. I do know the significance of that value in terms of "is your process capable?" or "how close to center it is?". Although, I was just curious as to why 1.33 ? How is the number 1.33 derived ?

Thanx

RE: Cp & Cpk question...

It means use 4 standard deviations from the nearest spec limit to the process mean. As I said, somewhere between experience and laziness. If someone came back with 1.30 I wouldn't scream as I have no faith that the spec limits are that well defined typically. You might want to instigate a temporary (or permanent) SPC plan on that process tho, to increase your confidence.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Cp & Cpk question...

Greg,

So are you are saying

Cpk= abs value of (process mean-speclimit)/3sigma

If so, aren't there well defined processes where the speclimit can be stated with confidence?

Just asking.

RE: Cp & Cpk question...

(OP)
Zeke,

Upper Limit
Cpk = [U.L - process mean]/3std

Lower Limit
Cpk = [Process mean - L.L]/3std

Cpk = min of Upper or Lower

Maybe that helps.

RE: Cp & Cpk question...

Oh greg, you've got it all backwards. Everyone knows that the process capability must drive the design! smile

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