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]

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]

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

 

[Rob Nix]

PLEASE DISREGARD MY PREVIOUS POST (#7)! I crossed over the line from thinking "total deviation" to specific measurements. That's why I hate it when customers want capability studies done on GD&T callouts!

Your TOTAL profile deviation is what you are concerned with - and that has nothing to do with location. The drawing probably shows a profile of a line callout in the feature control frame with a phantom line entirely inside the material zone.

My first comments were correct, in that, if the deviation (or worst/furthest points along the profile line) stay far away from 1.0mm, then you are holding the profile OK. The assumption of the measure is that the location of that most deviant point is still in spec. That is why YOU CANNOT DO CAPABILITY STUDIES ON GD&T FEATURES! {In other threads I've argued against TP}

There is really only two accurate things you can do. 1) Pick a point (or two or three) along the profile and measure its actual location and collect the data for SPC and Ppk studies. Or, 2) Use a GO/NO GO gage and do attribute studies on them (better yet, design an automatic in-process gage station for 100% checking).

I feel your pain. Sorry about the additional confusion I've caused. NOTE: I worked in a metal stamping plant for 10 years, but that was 10 years ago. :mg:

 

[Bill Ryan - 2007]

Alright, now I'm confused enough to join in. I'll start with Profile of a Surface (I believe measuring either one is the same except for the 2D vs. 3D thingy). This callout has linear dimensions as basics.

I understand what both Mark and Rob are saying - in a sense. However, when "reporting" a profile you don't report a negative value. Therefore, I'm with Rob and his statement of hating capability studies on GD&T callouts. If my unilateral tolerance zone is 1.0 and I get a linear readings between 0 and 1.0, the largest reading would be my profile. If I get readings of 1.1, 1.2, 1.3 (for discussion's sake), my profile is 1.1, 1.2, 1.3 (resp.). If my readings are less than "zero" (-0.1, -0.2, -0.3) my profile is 1.1, 1.2, 1.3 (resp.). In other words, it would make more sense to me to "study" the actual linear distance callout than it would the profile callout. Am I completely off my rocker??

However, that doesn't address Mark's original question. When I have to submit a capability study on callouts with natural limits I submit the PpU or PpL. I actually used to submit Cpk and Ppk values with zero as the lower spec. limit. I would get calls asking why I submitted with a Ppk of .05 (for the PpL) and would have to explain that I couldn't produce a part with a True Position (or Profile) of less than zero so why were they even looking at that index. I now only supply the PpU (or PpL).