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Ppk/Cpk for geometrical true position of hole with MMC

sinned

Registered Visitor
#1
Can anyone light me up on how to calculate Ppk/Cpk for true position of, say a hole, with respect to other datum hole at MMC position when bonus tol. come into play ?

- dennis
 
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Jim Wynne

Staff member
Admin
#2
B

Bill Ryan - 2007

#3
sinned said:
Can anyone light me up on how to calculate Ppk/Cpk for true position of, say a hole, with respect to other datum hole at MMC position when bonus tol. come into play ?

- dennis
I don't remember if I spouted off in another thread or not - so just to make sure......

In my (always?) humble opinion - Performing a statistical study on a geometrically toleranced feature with a "bonus tolerance" callout is a complete waste of time and energy. GD&T came about (in part anyway) to better identify how parts actually work together (ie: reduce scrap). With the MMC designation, it allows an "easier, quicker" method to discern part fit and functionality (a fixture gage of some type). I realize it is probably a customer requirement that has brought you to raise the question. In my mind, it makes more sense to "study" the Basic dimensions as they relate to the feature location RFS.

Sorry, but I sure feel better (and I cut myself short of a mini-novel). This always raises hackles on my neck when I get into it with a customer. It has always been just a "window dressing" type of study to get a part approved.
 

Jim Wynne

Staff member
Admin
#5
Bill Ryan said:
I don't remember if I spouted off in another thread or not - so just to make sure......

In my (always?) humble opinion - Performing a statistical study on a geometrically toleranced feature with a "bonus tolerance" callout is a complete waste of time and energy. GD&T came about (in part anyway) to better identify how parts actually work together (ie: reduce scrap). With the MMC designation, it allows an "easier, quicker" method to discern part fit and functionality (a fixture gage of some type). I realize it is probably a customer requirement that has brought you to raise the question. In my mind, it makes more sense to "study" the Basic dimensions as they relate to the feature location RFS.

Sorry, but I sure feel better (and I cut myself short of a mini-novel). This always raises hackles on my neck when I get into it with a customer. It has always been just a "window dressing" type of study to get a part approved.
I agree wholeheartedly, and think it's particularly ridiculous for cast and stamped holes that ain't never going to move. There might be some application in formed or machined parts, but it almost always makes better sense to analyze where the features are wrt a nominal dimension.
 

sinned

Registered Visitor
#6
Thanks for your opinion, Bill.
It is true that my question came from a customer requirement on a machining part print. Actually, i've measured the true position of the hole with CMM and used the data (distance error from the theoretical center) to calculate Ppk. Unsupprisingly, the results came up with a Ppk well below 1.0 and the customer replied that the way i computed the value was not correct.

Now if i forget about the MMC designation and go check with RFS, should i break down the distance error into x- and y- portion and compute the Ppk separately, or is there other method to come down to one single Ppk by, say, treating the data as bivariate variable ?
 

apestate

Quite Involved in Discussions
#7
Wow.

I just read the referenced thread where Paul F. Jackson talks about the technique to calculate Ppk or Cpk of a position tolerance. Good grief.

Tell your customer you're working on it, but it took a very bright person a whole summer and a flash of genius to pull it off and the technique is still floating around the upper echelons of the elite and maybe you need a little more time to work out that particular problem.

I wanted to reply to mention a technique I use for reporting a geometric position tolerance, but don't know if it is correct or common.

The way I do it is to report an error diameter. Let's say a hole is on 1 x 1 position and is .500 in size. The tolerances are .500 +/- .015 and the position tolerance is diameter .005 MMC. The hole's perfect, let's say at .999, .999 position, and at .515 diameter.

I would compute the position error in one direction, .0014. x2, .0028. The hole position error is a diameter of .0028. However, the bonus tolerance applies and I would report -0.0122 for the position tolerance.

How wrong is that?

Might be better to do 2 capabilities, one for hole size and one for position regardless of feature size. Report the two?
 
B

Bill Ryan - 2007

#8
sinned said:
Thanks for your opinion, Bill.
It is true that my question came from a customer requirement on a machining part print. Actually, i've measured the true position of the hole with CMM and used the data (distance error from the theoretical center) to calculate Ppk. Unsupprisingly, the results came up with a Ppk well below 1.0 and the customer replied that the way i computed the value was not correct.

Now if i forget about the MMC designation and go check with RFS, should i break down the distance error into x- and y- portion and compute the Ppk separately, or is there other method to come down to one single Ppk by, say, treating the data as bivariate variable ?
To "satisy" your customer, I would go through the excersize of actually figuring out the bonus tolerance for each part measured. If you still aren't capable, you have some issues to deal with. To deal with those, you need to work on the basic dimensions RFS. If you can show capability RFS, MMC is a given. (Sorry but I'm not sure what you mean by "bivariate variable")
 
B

Bill Ryan - 2007

#9
atetsade said:
However, the bonus tolerance applies and I would report -0.0122 for the position tolerance.

How wrong is that?
VVVeeerrryyy wrong!!!!! It's impossible to report True position as a negative value.
 

sinned

Registered Visitor
#10
Bill Ryan said:
To "satisy" your customer, I would go through the excersize of actually figuring out the bonus tolerance for each part measured. If you still aren't capable, you have some issues to deal with. To deal with those, you need to work on the basic dimensions RFS. If you can show capability RFS, MMC is a given. (Sorry but I'm not sure what you mean by "bivariate variable")
If i can dig out the bonus tol for each part measured, how do they fit in to the equation of Ppk ?
 
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