Machine issue in Cpk/Ppk and Control Plan - PPAP Submissions

J

Jimmy_Nie

#1
Dear pros,

In the PPAP submissions, machine numbers for a specified process are mentioned in both Control Plan and and Capability studies(Cpk or Ppk). My question is,

Is the machine number for a specified process mentioned in Control Plan the same as the one mentioned in the Capability studies for the same process ?

is it Cpk report or Ppk report to be submitted for the Capability studies in the PPAP submissions?

Thanks,

Jimmy
 
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sinned

Registered Visitor
#2
Hi Jimmy,

i do not fully understand your first question. But the fact is i typically do not mark the m/c number on the control plan.

To your 2nd question, i submit PPAP package with Ppk studies where a number of samples for measurement were selected at random from the PPAP batch of parts. This PPAP batch will not be large in most of the time. The key difference between the two is in the way you collect your samples. If you are able to use control chart in your PPAP run, you can compute Cpk as well.

In fact, i found it a dilemma when chosing between Ppk and Cpk for PPAP submission. Because the PPAP run is normally short in time and small in qty, you can't obtain a representative control chart for Cpk calculation. Hence, you go for Ppk. Ppk is supposed to cover all variations in the process. But in the short PPAP run, variation from mat'l etc. may not enter into the picture. Giving Ppk in PPAP only tell the customer your capability at that particular point in time. Ppk says nothing about future performance. So customer gets no information at all on your future performance by looking at your PPAP. On the other hand, Cpk gives you the best possible capability of your process which, in the eyes of the customer, may be more valuable for them to consider approving your PPAP.

Maybe i still need some light here.
 

Howard Atkins

Forum Administrator
Staff member
Admin
#3
Dear pros,

In the PPAP submissions, machine numbers for a specified process are mentioned in both Control Plan and and Capability studies(Cpk or Ppk). My question is,

Is the machine number for a specified process mentioned in Control Plan the same as the one mentioned in the Capability studies for the same process ?

is it Cpk report or Ppk report to be submitted for the Capability studies in the PPAP submissions?

Thanks,

Jimmy
Welcome to the cove.
You are supposed to define the machine that you use for the production as different machines can give different results.
Companies that use different machines for production usually declare this and say they are equivalent.
Change of machine could require a new PPAP.

I always submit Ppk as the PPAP run needs to be only 300 parts.
 
J

Jimmy_Nie

#4
Many thanks to sinned and Howard Atkins.

we may find that we need to identify/describe Machine, Device, Jig, Tools for Mfg. in the Control Plan (please see Control Plan in APQP manual for details). Also on the Ppk study sheet (or X-bar and R control chart), we need to describe machine No. for the certain process which in some case, the the same characteristics as the one in the Control Plan). This is why I ask the previous question.

Personnally I agree with Howard Atkins.

But where can I find proof (somewhere in PPAP or APQP, etc.) so that I can convince others that I am correct?

Thanks again. Happy New Year.

Jimmy
 
D

Dave Dunn

#5
Regarding Cpk/Ppk for PPAP submissions, review your customer's supplier quality manual if they have one. They may require capability be reported using a specific index.
 

Howard Atkins

Forum Administrator
Staff member
Admin
#6
From the 4th edition PPAP Handbook

I
nitial Process Studies.
The purpose of the initial process study is to understand the process variation, not just to achieve a specific index value. When historical data are available or enough initial data exist to plot a control chart (at least 100 individual samples), Cpk can be calculated when the process is stable.
Otherwise, for processes with known and predictable special causes and output meeting specifications, Ppk should be used. When not enough data are available (< 100 samples) or there are unknown sources of variation, contact the authorized customer representative to develop a suitable plan.
 
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