SPC and GD&T - Machined parts - Is Cpk is the correct index?

[Tim]

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.

 

[Don Winton]

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.

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.

http://Elsmar.com/pdf_files/

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

Regards,
Don

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Just the ramblings of an Old Wizard Warrior.

 

[Tim]

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.

 

[Lassitude]

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).]

 

[Don Winton]

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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Just the ramblings of an Old Wizard Warrior.

 

[Tim]

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.

 

[Kevin Mader]

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?

 

[Don Winton]

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

 

[Kevin Mader]

Don,

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

Regards,

Kevin

 

[Tim]

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.