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......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
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.
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")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 ?
VVVeeerrryyy wrong!!!!! It's impossible to report True position as a negative value.atetsade said:However, the bonus tolerance applies and I would report -0.0122 for the position tolerance.
How wrong is that?
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")