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Author Topic:   SPC and GD&T
Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 28 July 1999 02:37 PM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
I am a mechanical engineer recently thrown into the SPC arena. I know little or nothing about the subject, but have been reading voraciously. I have started to gain a basic understanding of the topics, however, I can find no literature that connects GD&T to SPC. I am primarily interested in Cpk. My problem is that I don't know if Cpk is the correct index. Most of the parts are machined and we have developed key characteristics. The problem comes in trying to figure out the math. Any help will be greatly appreciated. Thanks.

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
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posted 29 July 1999 07:49 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
quote:
I can find no literature that connects GD&T to SPC.

Yea, that would be sorta tuff. Most traditional SPC text rarely takes GD&T into account because SPC is supposed to focus on the process aspect of the operation. However, there are some statistical aspects to GD&T. Chapter 11 of Grant and Leavenworth's Statistical Quality Control and Chapter 23 of Juran's Quality Control Handbook cover the topic rather well. If you do not have these books, visit a library. A local college or university should have them.

quote:
My problem is that I don't know if Cpk is the correct index.

Visit here and download the file CPK.PDF. Read thru that and if you have any further questions, ask.

/pdf_files/

There are other good files there as well that may be of some help.

Regards,
Don

------------------
Just the ramblings of an Old Wizard Warrior.

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Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 29 July 1999 08:35 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
Thanks for the info.
When I finish reading all of it, I'm sure that I'll have more questions.
After all, the hardest part of a long journey is the first step.

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Lassitude
Forum Contributor

Posts: 89
From:
Registered: Jun 99

posted 29 July 1999 09:28 AM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
Just stop back by and ask your questions and we (particularly Don - our local statistical wizard) will try to help out.

[This message has been edited by Lassitude (edited 29 July 1999).]

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Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 03 August 1999 12:35 PM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
Ok, I've done some reading and feel like I'm starting to get a handle on some of this stuff. I am having some trouble finding a way to determine my USL and LSL values when applying the max material condition modifier to hole locations. As you all know, the MMC modifier allows bonus tolerance to hole position based on the hole size. How can you come up with a mean and standard deviation when the allowable values float directly proportional to feature size? My brain is almost fried on this one. This statistical stuff is more than a gear head can comprehend.

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
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posted 03 August 1999 01:08 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Sometime back, I received a paper on that very subject. I will try to find and forward. Stay tuned.

Regards,
Don

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Check Out dWizard's Lair:
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Kevin Mader
Forum Wizard

Posts: 575
From:Seymour, CT USA
Registered: Nov 98

posted 04 August 1999 07:18 AM     Click Here to See the Profile for Kevin Mader   Click Here to Email Kevin Mader     Edit/Delete Message   Reply w/Quote
Interesting question. My brain keeps racing over Virtual Condition (worst case condition). I am thinking that this would be your tolerance range when figuring your Cpk. What do you think Don?

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Don Winton
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Posts: 498
From:Tullahoma, TN
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posted 04 August 1999 07:42 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
I have found the paper that demonstrates the Cpk calculation for MMC. I admit I find it interesting. The paper is in PowerPoint 4.0 and I can forward to anyone who is interested.

Regards,
Don

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Lassitude
Forum Contributor

Posts: 89
From:
Registered: Jun 99

posted 04 August 1999 08:53 AM     Click Here to See the Profile for Lassitude   Click Here to Email Lassitude     Edit/Delete Message   Reply w/Quote
I'd like a copy.

Marc

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Kevin Mader
Forum Wizard

Posts: 575
From:Seymour, CT USA
Registered: Nov 98

posted 04 August 1999 10:47 AM     Click Here to See the Profile for Kevin Mader   Click Here to Email Kevin Mader     Edit/Delete Message   Reply w/Quote
Don,

Please send it my way. I would like to give it a read.

Regards,

Kevin

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Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 04 August 1999 11:42 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
I have given the paper a once over. At first glance it appears to have solved my problem. I really appreciate everyone's input. As I get deeper into the application I'll let everyone know how it's going. A particular thanks to Don. The paper was a real life saver.

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
Registered:

posted 04 August 1999 08:30 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
quote:
particularly Don - our local statistical wizard

Thanks for the kind word words. I appreciate it.

quote:
This statistical stuff is more than a gear head can comprehend.

Years ago, I was a fixture, tool and jig designer for the company I worked for at the time. If I had thought at the time I would be in this field, I would have said no way. Time does change things, donât it. Stick to it.

quote:
A particular thanks to Don.

I do not know if I would go that far. I did not author the paper and there should be thanks to the author. I just forward the information I have and hope there are those that can use it. I had never heard of MMC before this paper.

Regards,
Don

BTW, whaddya think of the new signature?

Just the Ramblings of and Old Wizard Warrior

Visit dWizard's Lair:*** Dead Link Removed ***

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Kevin Mader
Forum Wizard

Posts: 575
From:Seymour, CT USA
Registered: Nov 98

posted 05 August 1999 07:02 AM     Click Here to See the Profile for Kevin Mader   Click Here to Email Kevin Mader     Edit/Delete Message   Reply w/Quote
Don,

Do you know what you want to be when you grow up yet (LOL)? Funny thing, life. The direction I picked out in my teens is much different from the course I am on today. I believe this is true for most folks. Chances are, I will shift again at some point.

Statistics is an interesting subject, which, in my teens, I hadn't an interest. I realize today that life is made up of a multitude of processes. Business, no different. As in life and business, the choices we make are still ours. Better to make choices based on accurate, predictable processes. I believe the use of statistics (basic or advanced) helps to make folks make better decisions.

Through heavy mathematics in college, I still did not dabble with statistics, and their powers were still beyond me. Only after being introduced to SQC in a follow-up college course did I begin to understand their powers. Subsequent studying for the CQE, I built on the foundation. After reading several Deming books, I am convinced in their powers. My understanding of Statistics: I would consider myself a novice, desperately not trying to become a hack. But I agree with you Don, we are never too old (not smart enough) to enjoy a change.

Regards,

Kevin

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
Registered:

posted 05 August 1999 09:38 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote

Do you know what you want to be when you grow up yet...

Not really. Just an Old Wizard trying to find his way.

BTW, I have forwarded about ten or twelve copies of this thing. I ask only that if those I have sent it to forward it to someone else, please credit the author. As I stated above, it was not mine and will not nor want to attempt to take credit for it.

Thanks all.

Regards,
Don

Just the Ramblings of and Old Wizard Warrior

Visit dWizard's Lair:*** Dead Link Removed ***

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Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 06 August 1999 06:38 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
This is really a long story so I'll try and give the condensed version while attempting to provide enough detail to still make sense. I work as a consultant to the government for engineering issues. The contractor that provides our system has started to implement SPC. We put this effort on contract with advice from another consultant. He specified Cpk as the capability index. The contractor has been negotiating with a separate consultant who has told them that Cpk will not work in conjunction with GD&T. This is what prompted my first post. I have reviewed the paper and feel that it will adequately describe the process capability. The problem is that I shared teh paper with our contractor who in turn shared it with their consultant. His response was, "Interesting concept but their is no way this can provide a normal distribution." As far as I can tell from all of my research, the distribution must be normal for Cpk to mean anything. I have also gathered that there is a test for normality. My concern is if this paper is adequate it will save the government and the contractor a lot of money. On the other hand, is the consultant unscrupulus enough to discount the paper for the intended purpose of getting business. If this method is valid his services will not be needed. I hope this makes sense. My gut feeling is make some measurements, get some numbers and do a normality check. Is this possible? Any advice on how to do a normality check?

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Roger Eastin
Forum Wizard

Posts: 345
From:Greenville, SC
Registered:

posted 06 August 1999 07:14 AM     Click Here to See the Profile for Roger Eastin   Click Here to Email Roger Eastin     Edit/Delete Message   Reply w/Quote
There's actually been a LOT of material written in the Cove about the topic of the application of Cpk's to processes - normal and non-normal. I believe, in a nutshell, there are two broad areas to investigate: the use of PPM(non-parametric approach) or using an approximation to Cpk based on how "non-normal" your distribution is. Regarding the second approach, your Cpk can be fairly accurate even if the distribution is non-normal. The robustness of your Cpk depends on how "robust" the standard deviation is and that depends, in part, on the type of distribution your process follows. A good SPC book (Grant and Leavenworth, Wheeler, etc) would help. Generally, though, the more "tailed" your distribution is, the more problems are created for Cpk calculations. You should search the old forum for Cpk discussions because I'm sure that this has been covered before. Also, a while back, Bert Gunter put together an excellent series of articles in Quality Progress (when he was still writing for the "Statistics Corner") on Cpk applications. Try calling the ASQ and seeing if they can give you a transcript or, if I still have them, I can fax them to you.

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
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posted 06 August 1999 09:00 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Good answer, Roger, thanks.

Tim,

Try these for now and I will have more over the weekend.

Regards,
Don

https://elsmar.com/ubb/Forum10/HTML/000025.html
https://elsmar.com/ubb/Forum10/HTML/000024.html
https://elsmar.com/ubb/Forum10/HTML/000015.html

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
Registered:

posted 06 August 1999 09:23 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
quote:
"Interesting concept but there is no way this can provide a normal distribution."

What I find interesting is that someone would say this without either data nor prior experience to support this statement. I usually view this as a Îwarningâ sign that either the personâs mind is already made up or they are not trained in statistical techniques.

Either way, I would view this comment as suspect. As Roger well stated, it is not necessarily normality that matters, but how strictly you wish to follow the Îrules.â Those that have read my posts know I have bent and broken more that I have followed, with somewhat of success (although I view myself as rather less that Îexpertâ).

BTW, the test for normality is a Skewness test. If the data are not normal, there are ways to Îconvertâ it to normal. Too detailed to go into here, but any good statistical test will give the procedure.

The contractor has been negotiating with a separate consultant who has told them that Cpk will not work in conjunction with GD&T.

Wrong! Wrong! Find another consultant (would I do )

As far as I can tell from all of my research, the distribution must be normal for Cpk to mean anything.

No, it must not. It should be, but as I stated above, the normality depends upon how far you are willing to stretch the rules. And, if you are getting Cpk from a control chart (which you could do in the case of MMC), the law of subgroups would apply. This states that regardless of the population distribution, the subgroup sample data would reasonably simulate a normal (see the links above).

The one fixed rule for Cpk is that the process must be in a state of statistical control.

My advice: ditch this consultant and find one trained in advanced statistical techniques, or one that has at least read Grant and Leavenworth.

Ask your questions here. I will help as best I can. If your data are available, e-mail them and I will also help as best I can.

Good luck.

Regards,
Don

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Batman
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Posts: 111
From:Kane, PA 16735
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posted 08 August 1999 10:38 AM     Click Here to See the Profile for Batman   Click Here to Email Batman     Edit/Delete Message   Reply w/Quote
May I heartily agree with Don, throw out that consultant.

As I recall, the MMC Cpk paper is how to calculate Cpk based on the allowable bonuses. How can that restrict a process from being normally distributed or not?

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
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posted 08 August 1999 08:34 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote

How can that restrict a process from being normally distributed or not?

I thank you Batman for your support. I had thought my ramblings had been of a negative nature, which had been the intent. I was absolutely upset that someone would presume this much.

Any process, regardless of the origin and/or source, will either become normal over a continuous period of time or the source of non-normality will sought out and corrected. For anyone to presume that this is not so is either non-enlightened or cares not to be.

BTW, check out the Wizards Way here:*** Dead Link Removed ***

Just some stuff I put together that has nothing to do with this forum.

Regards,
Don

Just the Ramblings of and Old Wizard Warrior

Visit dWizard's Lair:*** Dead Link Removed ***

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Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 09 August 1999 07:16 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
Don and Batman,

Thanks for the vote of confidence. I have given this a lot of thought over the weekend and come to the conclusion that this consultant has a method to sell. The paper I received and forwarded basically cuts him out of a contract. I believe his business half of the brain overrode his judgement. I'll get some data and do some tests.

Thanks Again,
Tim

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
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posted 03 January 2001 03:46 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
Does anyone remember anything about the paper? Did anyone ever get a copy? I've also been unsuccessful in attempting to contact Don. The pictures of the wizards he placed in many of his posts were links to his web site - and they're broken so Don might have gone the way of the winds.

*************************************

From: "Jackson, Paul (P.F.)"
To: "'timothy.floyd@robins.af.mil'" ,
"'lt@Elsmar.com'" ,
"'kevin.mader@thuleus.com'"
Subject: FW:
Date: Wed, 3 Jan 2001 14:36:43 -0500

Tim, Lassitude, or Kevin,

Each of you requested the paper mentioned below from Don Winton. If you still have it could you share it with me?
Thanks,

Paul F. Jackson
Product Engineer, GDT Strategy (734) 45-80414
ATEO CAD Cube-E234A maimailto:Pjackso4@ford.com

> -----Original Message-----
> From: Jackson, Paul (P.F.)
> Sent: Wednesday, January 03, 2001 2:18 PM
> To: 'dwizard@spacemail.com'
> Subject:
>
> Don,
>
> I e-mailed you a month ago or so about the note that you posted on the
> Cove Forum about a paper that you have.
>
> I have found the paper that demonstrates the Cpk calculation for MMC. I
> admit I find it interesting. The paper is in PowerPoint 4.0 and I can
> forward to anyone who is interested.
>
> Regards,
> Don
>
> I have discovered a solution to the problem as well. It will be published
> in Quality magazine in February 2001. The only references that I have
> discovered in my searches of related works are the one that you have
> indicated and another that mentioned an article by "Glen Gruner."
>
> Naturally I am curious to see if the solutions are similar. Could you
> forward me a copy of the article or, if it is unavailable, describe the
> solution.
>
>
> Paul F. Jackson
> Product Engineer, GDT Strategy (734) 45-80414
> ATEO CAD Cube-E234A maimailto:Pjackso4@ford.com

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Rick Goodson
Forum Wizard

Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 03 January 2001 05:13 PM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
Marc,

His website is still up although I do not believe it has been updated recently.
*** Dead Link Removed ***

Have you tried the E-mail address at the site?

*** Dead Link Removed ***


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Sam
Forum Contributor

Posts: 244
From:
Registered: Sep 1999

posted 03 January 2001 05:25 PM     Click Here to See the Profile for Sam   Click Here to Email Sam     Edit/Delete Message   Reply w/Quote

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 03 January 2001 10:37 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
I checked the site and its counter is 'expired' over a year, I think. And I did send mail to that address. We'll see. People do disappear.

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Paul Jackson
unregistered
posted 04 January 2001 01:41 PM           Edit/Delete Message   Reply w/Quote
Thanks for looking for Don Winton for me guys. Kevin Mader and Timothy Floyd both sent me the Powerpoint Presentation.

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 04 January 2001 02:11 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
What's the name of the file? I'm wondering if I ever got it.

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Paul Jackson
unregistered
posted 05 January 2001 08:09 AM           Edit/Delete Message   Reply w/Quote
The file is named "Mmc_cpk2.ppt"

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Sam
Forum Contributor

Posts: 244
From:
Registered: Sep 1999

posted 05 January 2001 08:49 AM     Click Here to See the Profile for Sam   Click Here to Email Sam     Edit/Delete Message   Reply w/Quote
Since there are so many request for this paper I think it should be posted on the FTP site.

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 05 January 2001 10:07 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
I don't have a copy to post - if someone will please e-mail me a copy.....

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