Are Capability Requirement(s) Valid for All Characteristics?

#1
Dear all,

beside of special characteristic (SC, CC, ...) there are always many other characteristics in the drawing (let's say "normal"). The process capability indices are usually evaluated and controlled for special characteristics only (+ some additionally defined char.)

My question:
Is there any minimum requirement for Cpk/Ppk of "normal" characteristics? If so, where is it stated?

Thank you
 
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#3
Hello howste,

I searched an answer in customer requirements of EOMs (VW, BMW, GM..) and so far did not find anything specific. BUT does it mean it's acceptable to have characteristics with Cpk below 1,00? (or 1,33?, ...)
 

howste

Thaumaturge
Super Moderator
#4
There may or may not be requirements from your customers or from a QMS standard that apply. It looks like you're in the automotive industry, so maybe IATF 16949 or VDA apply to your company?

Let's look at the situation from a risk-based perspective. If the Cp and Cpk values are determined correctly (normal distribution and in control) then you would expect roughly 2700 PPM nonconforming with a Cpk of 1.0, and it would be double that if the process is centered and still has Cpk =1.0. If we cared enough about the characteristic to do a study on its capability, then we probably wouldn't want to be shipping that much bad product to the customer. Depending on the criticality of the characteristic for product function or safety (or other risks) we would want to determine if we should work to reduce the variation or center the process better.

If we find that the Cpk=1.33 it's closer to 64 PPM nonconforming. Still not perfect, and you would want to make a decision about the risks to you and the customer and decide if it's an acceptable level or some work needs to be done. Cpk=1.67 is less than 1 PPM, and Cpk=2.0 would be roughly 2 parts per billion NC. Use the data to make decisions on the risks.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
in general (and your customer's requirements may differ), for characteristics that are not identified as critical the supplier must still ship parts that meet the specification. Some Customers have stated acceptable defect rates and others hew to the zero shipped defect standard. So its not about capability indices. These are notoriously unreliable at predicting defect rates beyond + 2 SD even when the distribution is roughly bell shaped. They also give no indication or protection against any defect rate up to and including 100% during an out-of-control period. :mg:

It's really about how you control the actual shipped defect rate
 

Golfman25

Trusted Information Resource
#6
Dear all,

beside of special characteristic (SC, CC, ...) there are always many other characteristics in the drawing (let's say "normal"). The process capability indices are usually evaluated and controlled for special characteristics only (+ some additionally defined char.)

My question:
Is there any minimum requirement for Cpk/Ppk of "normal" characteristics? If so, where is it stated?

Thank you
So here's the thing. Basically a new customer quality guru will state "min. cpk 1.33 required" and that will be applicable to every dimension on the print. Then when he realizes what a nightmare he created, you'll negotiate down to critical and/or significant characteristics. If those aren't identified, I would look at the tightest tolerance items. Good luck.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
Section 2.2.11.1 spells out the requirement: the customer spells out the requirement. The down side? They typically have NO IDEA how capability indices relate to your process. None. They just rubber stamp 1.33 all over their prints - even on unilateral tolerances (concentricity, etc.), which clearly does not apply! Remember this - because I ASSURE you your customer will NOT - Ppk and Cpk are ONLY valid for normally distributed and two-sided specifications with the target in the center. See AIAG PPAP 4th edition section 2.2.11.5 NOTE. Unfortunately, it tells you to go to the SPC book for guidance, and the guidance there is poor at best.
 
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