Questions on use of SPC, Ppk, Cpk during PPAP

bobdoering

Stop X-bar/R Madness!!
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#11
For those just coming across this issue in the archives, I am updating the responses to current requirements:

Since, I have tried to ask the following question to our consultant but nothing clear for me.
Go figure...:rolleyes:

My question is quite simple and I would like to ask by using the following model.
Model: Suppose that I have one new machine which operate only one process , let say drilling dia 10 +/- 0.1 mm and roughness is 6.3Rz . Our customer request us to use SPC for both value where the requirements are

1) For PPAP, Ppk must greater than 1.67
2) For ongoing SPC Cpk must greater than 1.33

Question: 1)Why we use Ppk at the PPAP stage? (Ppk is for long term study but doe not require for stability???)[/
It is probably not applicable at all. Drilling is not a normal distributions, and surface finish is not a bilateral specification. See below:

AIAG PPAP 4th Edition
2.2.11.5 Processes with One-Sided Specifications or Non-Normal Distributions

NOTE: The above mentioned acceptance criteria (2.2.11.3) assume normality and a two-sided specification (target in the center).

When this is not true, using this analysis may result in unreliable information.

NOTE (cont.): These alternate acceptance criteria could require a different type of index or some method of transformation of the data. The focus should be on understanding reasons for the non-normality (e.g. is it stable over time?) and managing variation.


2) For ongoing SPC Cpk must greater than 1.33
Question: 2)Do we need to use X bar R chart during our run for PPAP?
For drilling, use a X hi/lo-R chart. Xbar -R chart is the worst chart you can use. See Statistical process control for precision machining . You might be able to use an X-bar-R chart for surface finish if it is bad enough to exhibit a normal distribution.

Question 3)If yes in 2, How can we set a UCL and LCL?
You need to do a capability study to determine the true control limits for the surface finish.

Question 4) For one side spec (roughness in this case),if the requirement is Ppk greater than 1.67. It means that the average value will near 3.2Rz which also means that we have to design our process not for 6.3Rz but for 3.2Rz Otherwise, we will not earn those. For this case, it will cost a lot more, so can we use Pp for this.
See Question 1.

Question 5) When will we apply Cpk instead of Ppk (when it stable??)
See Question 1.

Question 6) UCL, and LCL of X bar and R chart of Cp, Cpk come from???
They originate from the capability study.

Question 7) Since Cpk is a study of short term, so how can we use Cp for ongoing process??
See Question 1.
 
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A

atulsharma98

#12
Dear
As per aiag guidelines during ppap run we have to take 300 consicutive parts
and for consicutive data only ppk is valid
and secondly as per ppap we have to go for initial process study where all causes can be captured

2) and secondly for ppap you no need to go for xbar r chart
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#13
Yes, per AIAG the minimum run size requirement is 300 consecutive parts. The common problem is people stop reading when they get the simple answer. One of the Big 3s customer specific requirements does not agree that the 300 pcs are consecutive, but rather the PPAP run is long enough to capture all variability (shift, tool, machine, etc.) and that if it exceeds 300 the capability samples are to be spread out evenly over the entire run.

Also, in AIAG PPAP 4th edition, 2.2.11.4 requires the process has to be stable. Guess how that is established? Yep, a control chart. The correct control chart - may not necessarily be X-bar/R. Also, it states all special causes are to be removed. How are they identified? Yes, control charts.

Sections 2.2.11.1 specify the evidence is needed for special characteristics. Remember, the customer print may not be the only source of special characteristics. APQP 2nd edition section 2.11 states they are a concurrence of the customer and the supplier and often discussed in the Customer Specific Requirements. CSRs may also expand the need for evidence beyond just the special characteristics. The most correct answer to any question: it depends.

So, I agree the x bar/R chart itself is not required (and shouldn't be - it is not always the right one), but the correct control chart is. It may not be mandated to be included in the PPAP package (and not everything is) but you would need it to support your claim the process is stable.

Clarity and conciseness (and, in cases accuracy) is not the AIAG's strong suit. The rule on verboseness, though!
 
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