Maintaining manufactoring process capability



#1 Draft ISOTS 16949:2002 says "The organization shall maintain manufactoring process capability or performance as specified by the customer part aproval process requiremts":
Well, imagine that i have a Cpk of 5.25 mentioned in the PPAP; So, i understand that i can't go below 5.25! What if i mention in PPAP (control plan) that (although CPK of 5.25 was achieved) cpk must be superior to 1.67?:biglaugh:
Best to all
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There was an IASG intrepretation on this point which I am sure will apply to 16949 as well. You will find it listed as C7. It states "The intent of this requirement is based on the maintenance of the process and not the level of the indices' value. To maintain (or exceed) requires two components:
1) Monitoring of the process over time to verify capability and stability; and
2) If the process is capable and stable, then to verify that the process meets the requirements as described in PPAP"

The fact that you had at one point a 5.25 Cpk is immaterial. You are required to provide product meeting the requirements.

Hope this helps.


Laura M

Technically, yes, you need to maintain that Cpk. Which begs the question, why do so good in the first place, right?

But the theory behind it is that your PPAP parts are matched up with other suppliers PPAP parts, and a Cpk of the assembly is established. Variation outside the 5.25 Cpk parts you submit is not considered. So if you add variation, the impact is not known. It's that variation that "gets them into trouble" so to speak.

I know its theoretical, and difficult to accept if the costs of maintaining 5.25 are high, but that's the rules right now.

Al Dyer

Jeez, did the customer let you define the spec limits!

The initial study performed during PPAP is to get an idea about variation. Long term process performance is what is important and that should not deteriorate (sp) although it will vary.

Also, I see no difference in the thought process between holding a process at 1.67 or 5.25 if those are the accurate numbers. A process is a process and performs as designed when special causes are defined and removed.

I don't see the control plan as an appropriate place to put such a disclaimer, >1.67 is the industry "norm". Let sleeping dogs lay.

Plus, don't sweat it too much until the approved TS is released.:bigwave:


Ok, so what it seems is that i need a customer specification regrading cpk; we have some customers (although not in the auto business), that specifie cpk of 1.2 and 1.67.
The question is that we have a ppk in the PPAP process that's not a true cpk (at least i think so).
Just for the record, we make injected plastic parts! so, maintaining a cpk of +5 is unreall for most of the parts, as there is much variation that we CAN?T control (material and the sorts).

Howard Atkins

Forum Administrator
Staff member
Originally posted by Al Dyer
Jeez, did the customer let you define the spec limits!
Believe it or believe it not this has happened a number of times.
If the distribution is good then we have had automotive customers, French and German asking us to give them tolerances that will give a ppk of 1.67 or in one case 2.0.
They have then changed the drawing for us.
I have also seen cases where the same part is produced by diferent suppliers each with their own drawing from the customer with their name on it and diferences in tolerances.


I know someone will be upset.

I have also seen where a part was given to two suppliers and after both have submitted samples they were given different prints. New dimensions and or tolerances so the could both provide better CPK numbers. Is it me or does this sound like the customer is over critical on the specs for both vendors. Both parts work! Why not one print that you can actually manufacture to.
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