Ppk and Pp on unilateral or unequal tolerance - DCx says Pp can not be calculated

[qualitytrec]

As I get to know the PSO requirements I have again been reminded that there is variation in approach to Ppk and Pp. Some places I have worked require us to find the center of a tolerance zone and work Pp and Ppk from it. I did not like this but our SQE insisted that this is how it was to be done (customer may not be right but act like it anyway).
The PSO for DCx says that Pp can not be calculated for unilateral tolerance. This makes sense to me but leaves me with the question "How can we calculate Ppk and be accurate if there is no Pp value?" Take for example an unilateral profile call out of 1.0mm where the process is in spec at .25mm mean and has s of .1. The formula prescribed would look like Ppk=(1.0-0.25)/(3*0.1)=2.5. This seems strange to me since you would be running so close to the lower specification.
I do understand how it would apply to flatness or max radius type call outs but it seems wrong for others such as the one I gave.
What have you done with this stuff and why?
Mark

[Marc]

Any takers on this one?

[Mike S.]

Do a search for several threads on the subject of cpk and ppk -- I know there are a few. You can calculate Cpk or PPK on a unilateral tolerance.

[qualitytrec]

Originally Posted by Mike S.

Do a search for several threads on the subject of cpk and ppk -- I know there are a few. You can calculate Cpk or PPK on a unilateral tolerance.

Thats not the question.

[Rob Nix]

1. Center of tolerance zone: That makes no sense since neither the Pp nor the Ppk is calculated using the "target" value. Also, if the tolerance is unilateral, there IS NO center of the tolerance zone. How does the SQE propose to 'work the Pp and Ppk from the tolerance zone'?

2. You CAN calculate Ppk without a Pp - you just showed it! Profile is unilateral simply because you cannot go less than zero (or perfect fit) - REMEMBER: zero is NOT a "lower specification". Your concern here is that you don't EXCEED 1.0mm. Your process appears acceptable since at 3 std devs greater than the average your highest measurement would be .55mm, a long shot from 1.0mm.

The only problem with these types of measurements is that the distribution is likely not normal, but skewed.

[qualitytrec]

Originally Posted by Rob Nix

1. Center of tolerance zone: That makes no sense since neither the Pp nor the Ppk is calculated using the "target" value. Also, if the tolerance is unilateral, there IS NO center of the tolerance zone. How does the SQE propose to 'work the Pp and Ppk from the tolerance zone'?

2. You CAN calculate Ppk without a Pp - you just showed it! Profile is unilateral simply because you cannot go less than zero (or perfect fit) - REMEMBER: zero is NOT a "lower specification". Your concern here is that you don't EXCEED 1.0mm. Your process appears acceptable since at 3 std devs greater than the average your highest measurement would be .55mm, a long shot from 1.0mm.

The only problem with these types of measurements is that the distribution is likely not normal, but skewed.

Rob,
With a profile we are dealing with a 3D part which means the surface can in fact be shy or full to where it is supposed to be (nominal). How can I say that the part is in control when we are riding near nominal which in this case means we could fall out of spec by being shy material?
The only options I see is that the print is wrong in how it is defining the surface, the meathod is wrong for determining control, or I have no idea what I am talking about though I think I do.
Mark

[Rob Nix]

It sounds like you are talking about an "edge", like the sheared edge of a formed sheet of metal. In that case, it seems to me that the tolerance is indeed bilateral (I might agree that the print is drawn wrong) - i.e. you can be shy up to so much material and you can be full, or in excess, so much material.

How far "shy" can you go before it is rejectable? Whatever that amount is, that is your "other" tolerance boundary. If it is zero, or none, then the Ppk in your example is really 0.833, which is out of tolerance.

Am I missing anything?

[qualitytrec]

Nope that is exactly my point. Things like profile really do have an upper and lower spec limit. Call outs like flatness, concentricity, perpendicularity, parallelism etc.. are really unilateral and as such are the only GD&T that the rule would apply to for not having a Pp. Am I right? If so then back to my original question if I have a Profile call out that is Said on the print to be unilateral meaning the trim edge can be long but not short etc... What do you do and why?
Mark