Minumim number of parts to conduct the Process Capability Study

R

remsqa

#11
Sample Size

Dear Mr steve

Thank you for your info to understand better.


I do have a another Question with me.?WE are using the same formula for the Machine capability and the process capability.Is there is any other unity in this other than we are trying to capture the stablity of the machine /process or the varianblity of the machine/process.

Expecting your reply

REGARDS

R.LS.ATTHISH KUMAR
sathish_801980 at yahoo dot com


Steve Prevette said:
The 25 sets of 5 (or 4) is a "correct" answer, but not for the reasons given. It is not related to a histogram, but to a control chart. Dr. Shewhart originally gave the suggestion that do not declare a process stable until you have 25 stable points from subsamples of 4 on a control chart. And process capability is meaningless unless the process is indeed stable.
 
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P

psavijay

#12
Machine Capability

Dear Sathish I hope you know what is Process sigma in this replace X bar with your Target value to be achieved, remaining calculations are same, incase of unilateal tolerance with tolerance band on both side same then machine capability and Process capability both are same

Regards
A.Vijayakumar
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#13
remsqa said:
I do have a another Question with me.?WE are using the same formula for the Machine capability and the process capability.Is there is any other unity in this other than we are trying to capture the stablity of the machine /process or the varianblity of the machine/process.

R.LS.ATTHISH KUMAR
Generally all that the various "process capability" formulae give you is a measure of how likely the variable being monitored will fall within specifications. Stability of the process or machine is judged by the control chart itself, and variability also. Stability and variability are assessed without even needing to know the specifications. Given that the process/machine is stable, I can then take the variability data and use it in the process capability forumulae.
 

Caster

An Early Cover
Trusted Information Resource
#14
(Ir)Rational Subgrouping

Steve Prevette said:
The 25 sets of 5 (or 4) is a "correct" answer....
I once got "heck" from a big 3 supplier quality engineer as follows.

We collected 100 plus parts in series order from our machining line. I used Individual and Moving range charts since to my way of thinking there was no subgroup as such. We had all the parts.

The SQE demanded I used X bar and R with subgroups of 3 "because it is better".

To me there is no justifcation for such a grouping. The subgroup of 3 is arbitrary in this case. If I had collected a sample of 3 parts every hour during a long run, his position would have made complete sense to me. But, I had all the parts due to long cycle times.

Any thoughts?

Of course, we did it his way.

Caster
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#15
Of course, if there is some way to "rationally" do the subgrouping, that is best. But I admit oftentimes it is more arbitrary than not.

Shewhart leaned towards 4 or 5. The issue is you want enough data in the subgroup so that the Range gives a reasonable estimate of the standard deviation for the subgroup. You can go down to a moving range, basically the difference between each successive datum. But the difference between two points has a lot of variability. As subgroup size increases, the range (or even the sigma) for the subgroup becomes more "reasonable". But, countering this, as subgroup size increases, I lose sensitivity. It takes longer to get the subgroup before analyzing the result, and a single point change can be "buried" in the subgroup.

I think that Shewhart leaned towards 5, but acknowledged 4 was easier to deal with in the pre-computer days (square root of 4 is 2).

I would not be terribly offended that the customer specified 3. It would be easy enough to run for other subgroup sizes and see if you get a substantially different interpretation.
 
G

Geese

#16
Bear in mind I'm new to PPAPs, but reading through the manual I found that the standard PPAP Capability Study can be found here. :read: NOTE 4 pg. 6 (100 minimum readings)

The Truck-OEM specific requirements can be found here. :read: II.4.1 Significant Production Run. pg. 47 (30 minimum readings)

Correct me if I'm wrong.
 

Ron Rompen

Trusted Information Resource
#17
Customer requirements will always supercede the PPAP manual or any other 'reference' documentation.

I have convinced our customers over the years that 30 random samples (6 subgroups of 5 pcs) is sufficient to demonstrate initial process capability (or lack thereof) on our parts. There is little part-to-part variation in OUR process....most of it results from outsourced processes (heat treat, plating, e-coating, encapsulation, etc) which is not as controllable as our process (fineblanking).

However, I did have one customer, many years ago, who insisted on the letter of the law; 125 consecutive samples, serialized and tracked through the processes. It was what he wanted to approve my submission, so that's what he got.

Another point to bear in mind (which was already raised) was the number of SC/CC/KC characteristics, and how feasible it is to actually DO capability analysis. As an example, many of our customers now designate material type as an SC/CC. How do you do statistical analysis on a single lot of material?

Oh well....rant off now. (in case you wondered, I'm stuck at home working on 6 PPAP submissions this weekend, and have another 6 to do over the Xmas break...some people get NO luck)
 
A

Arvind

#18
Confidence intervals needed decide the sample size

How many parts you need to use for calculating capability depends on how narrow confidence limits you need for capability.
If you look at Chi square tables and find out confidence interval on capability, here are following C.I.

Sample size---------- Tolerance on Cpk confidence intervals
10 ------------- +/- 45 %
30 ------------- +/- 25 %
100 ------------- +/- 14 %

You can then decide appropriate sample size
 
G

Geese

#19
Just from short experience, I've noticed that if you are the direct supplier to a customer, you can negotiate (develop customer requirements) for your part, but this can be time consuming and you'll need to get documentation that the customer only required 30 pieces for the capability study as to the proposed 100 by the PPAP manual just to cover your bases. Some people I've met in industry tie their shoes twice just so their shoelace loops are symetric. The guy before him might not have, and this new guy might come knocking with dissatisfaction at the 30 readings.

However, for the sake of doing generic processes in quality control when dealing with several PPAPs and distributors to end customers, it is easiest to just do the standard 100 readings, and go beyond that if your customer specifies more. Judge the time required to measure more parts versus negotiating for a smaller sample to study. I could measure 1 attribute on 100 parts in roughly 15 minutes on some devices and 60 minutes on others, and still longer on others.

Plus, when your auditor comes around, he might notice the extra effort and reward you for your diligence...maybe.... Never hurts to be on someones goodside.
 
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