Cp, Cpk, Dpm, or other? Diameter of a Hole - Non-Normal Distribution

Bev D

Heretical Statistician
Staff member
Super Moderator
#11
Forget all this nonsense about Normal distributions and capability indices. how much variation does the process have vs the spec limits? Plot the histogram. Plot the time series / control chart is it stable?
If the hole is at it's minimum specified size, and the thing that goes into it is at it's maximum will fit it correctly or are there stackup concerns?

Now how much variation can you allow? that is your goal.
You should spend MORE TIME working on determining what the product truly requires to meet customer performance needs and determining the critical input factors for your process and how to control them...once youknow hat to do and how to accomplish it consistantly the capability index is irrelevent. the plot of the data vs teh spec limits will tell the story. capability indexes only hide the true nature of your variation...
 
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J

jospe.at.liu

#12
You should spend MORE TIME working on determining what the product truly requires to meet customer performance needs and determining the critical input factors for your process and how to control them...
Very true words, my friend! Quality control is more important than quality inspection!
 
G

Geoff Withnell

#13
Forget all this nonsense about Normal distributions and capability indices. how much variation does the process have vs the spec limits? Plot the histogram. Plot the time series / control chart is it stable?
If the hole is at it's minimum specified size, and the thing that goes into it is at it's maximum will fit it correctly or are there stackup concerns?

Now how much variation can you allow? that is your goal.
You should spend MORE TIME working on determining what the product truly requires to meet customer performance needs and determining the critical input factors for your process and how to control them...once youknow hat to do and how to accomplish it consistantly the capability index is irrelevent. the plot of the data vs teh spec limits will tell the story. capability indexes only hide the true nature of your variation...
Bev,

The entire POINT of a Cp, or Cpk calculation is to compare the spec limits with the process variability is some sort of meaningful fashion, beyond "Does the histogram fit inside the spec limits? YES! Whoopee, let's run!"

You assume that the hole has a mating part, we don't know that, might be a flow orifice, or something else. And finally, the specs should be what the customer truly needs, or they should be changed. It is not at all unusual, however, for the product spec to be the only input available to the manufacturer as to the customer need. And while data plots and so forth do tell the story, if one has numerous characteristics on multiple parts on which to convey capability information, they are a bit clumsy. I'd much rather review (as a customer judging a vendor's performance) a list of 100 capability indices than try to make sense of a hundred run plots. Cpks are not "nonsense".

Geoff Withnell
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Bev,

The entire POINT of a Cp, or Cpk calculation is to compare the spec limits with the process variability is some sort of meaningful fashion, beyond "Does the histogram fit inside the spec limits? YES! Whoopee, let's run!"

You assume that the hole has a mating part, we don't know that, might be a flow orifice, or something else. And finally, the specs should be what the customer truly needs, or they should be changed. It is not at all unusual, however, for the product spec to be the only input available to the manufacturer as to the customer need. And while data plots and so forth do tell the story, if one has numerous characteristics on multiple parts on which to convey capability information, they are a bit clumsy. I'd much rather review (as a customer judging a vendor's performance) a list of 100 capability indices than try to make sense of a hundred run plots. Cpks are not "nonsense".

Geoff Withnell
Geoff - You are correct about the hole being an orifice - I stepped over that. And I understand that sometimes the customer won't tell you anyhting about the mating parts - this is a shame since knowledge is always more powerful than ignorance and too often custoemrs treat their suppliers like ignorant indentured servants instead of partners.

Specs are often anatomically derived or copied from previous similar designs. not all specs are engineered...Additionally not all specs are from customers but are our own designs, we mustn't forget that many in the cove make their own designs...most of the companies I have worked for were their own design house...

On the use of capability indices, you and I will simjply have to agree to disagree. My supplier engineers get control charts or multi-vari charts; we inspect the charts, not the parts. *I* do not find capability indices helpful; as I've posted before: using a single number to describe variation is silly to me. and there are so many ways to 'cheat' the system - without the charts you can't detect it. you can actually see if a process is unstable or non normal. many of our posters come here looking for help with a customer that won't listen to reason and just wants the Cpk to be less than 1.33 so they fudge the numbers...I will always ask to see the data, and I wil always require those who report to me to look at the plotted data. For me statistical summaries are no excuse for the doing the real hard work of variation analysis. but that's me. you choose a different path and that's your choice...

For those who are truly curious about my position - I would refer you first to a series of articles written by Bert Gunter, “The Use and Abuse of Cpk, Parts 1, 2,3 and 4”, Quality
Progress, January 1989, March, 1989, May 1989, July 1989
It's well worth your time to track this series down...
 

Jim Wynne

Staff member
Admin
#15
On the use of capability indices, you and I will simjply have to agree to disagree. My supplier engineers get control charts or multi-vari charts; we inspect the charts, not the parts. *I* do not find capability indices helpful; as I've posted before: using a single number to describe variation is silly to me. and there are so many ways to 'cheat' the system - without the charts you can't detect it. you can actually see if a process is unstable or non normal. many of our posters come here looking for help with a customer that won't listen to reason and just wants the Cpk to be less than 1.33 so they fudge the numbers...I will always ask to see the data, and I wil always require those who report to me to look at the plotted data. For me statistical summaries are no excuse for the doing the real hard work of variation analysis. but that's me. you choose a different path and that's your choice...
I'm with you, Bev--show me the charts and I won't need a Cpk calculation. Nonetheless, there is widespread demand for it, so it has to be done in many cases.


BTW, I think you meant "...just wants the Cpk to be greater than 1.33..." :D
 

Jim Wynne

Staff member
Admin
#16
Bev,

The entire POINT of a Cp, or Cpk calculation is to compare the spec limits with the process variability is some sort of meaningful fashion, beyond "Does the histogram fit inside the spec limits? YES! Whoopee, let's run!"

You assume that the hole has a mating part, we don't know that, might be a flow orifice, or something else. And finally, the specs should be what the customer truly needs, or they should be changed. It is not at all unusual, however, for the product spec to be the only input available to the manufacturer as to the customer need. And while data plots and so forth do tell the story, if one has numerous characteristics on multiple parts on which to convey capability information, they are a bit clumsy. I'd much rather review (as a customer judging a vendor's performance) a list of 100 capability indices than try to make sense of a hundred run plots. Cpks are not "nonsense".

Geoff Withnell
Before you can put any stock in a Cpk number from a supplier, you need to have some confidence in their understanding of the requirements. I can't tell you how many times, when a supplier has reported a Cpk number >1.67, I asked for the charts and found obvious evidence of instability, rendering the Cpk results useless. Trust but verify.
 
G

Geoff Withnell

#17
Bev D. and Jim,
Of Course more information is better. I'm glad your supplier quality Engineers have time to look at the charts. What would you do if you were dealing with hundreds of suppliers, with mulitple thousands of parts, and many dimensions per part? This is the environment that generated the use of Cpk. It is essentially impossible in some industries, the automotive in particular, to review that many charts. Can the system be gamed? You bet. Does the use of charts prevent gaming the system? Not a chance. It just needs to be a bit more creative. The Cpk requirement of 1.33 is just a quick and easy method for the customer to say "We want you to have a std dev. of cushion minimum on your process capability". And providing the number to the customer is a quick and easy way for the supplier to say "We checked it, and we have it". Cpk is based on normal distribution, which is many processes, does not occur. A good supplier quality engineer will in this case just want to see how the supplier is assuring an equivalent capability.
Someone who reports a Cpk > 1.67 on an instable process either does not understand the concept of Cpk, since it can only be calculated against a normal distribution, which an unstable process won't have, or is a barefaced liar. In either case, a supplier one would need to "counsel".
In any case, you don't like em and don't need em, fine, don't use em. But they are not "nonsense" when appropriately used.

Geoff Withnell
 

Jim Wynne

Staff member
Admin
#18
Bev D. and Jim,
Of Course more information is better. I'm glad your supplier quality Engineers have time to look at the charts. What would you do if you were dealing with hundreds of suppliers, with mulitple thousands of parts, and many dimensions per part?
I would periodically audit the charts. Just because you can't do all of them doesn't mean you can't do any.
 
G

Geoff Withnell

#19
Very good idea. Periodic process auditing is one of the best ways to avoid the "gaming" of the system we were discussing.

Geoff Withnell
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#20
Bev D. and Jim,
Of Course more information is better. I'm glad your supplier quality Engineers have time to look at the charts. What would you do if you were dealing with hundreds of suppliers, with mulitple thousands of parts, and many dimensions per part? .

Geoff Withnell
To answer your questions:
It takes surprisingly little time to look at the charts - and we gain tons of information that is unavailable in a single index. Liek anything, catching somethign early is far cheaper and faster than catching it later downstream. So the small amount of extra time is very value add.

lots of suppliers and many parts: got it now. had it before. I hire enough supplier engineers; I only require high scrutiny on the critical dimensions, Really good suppliers reach the point of not having to submit charts as they have proven themselves, Corrective actions result in individual attention by the supplier engineers to drive capability.
And I've worked in the automotive industry twice. I worked at Honda and for a tier one supplier...so I understand the concerns. You and I simply disagree on the usefullness of the capability indices. 'nuff said.
 
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