Cpk 1.99 But Out of Specification Measurement Detected in Control Chart

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

The control charts, UCL, LCL, CP and CPK are automatically calculated by the system. They are 60 subgroups, and 8 of them are way off UCL measuring 0.099. CP is 3.08. My thought for this is, when a small portion of a good data is out of control, it will still give you good cpk. A very important rule often overlooked by the operator is that they must first look at the control charts before looking at the cpk.

An even better rule for the operator is to stop charting when out-of-spec conditions are encountered. Operators also need to watch for trends (excursions away from the mean in particular). The whole point is to use statistical data as a guide in centering the process variation as far away from the spec limits as is reasonably possible. Having crazy, unintelligible charts after the fact isn't helpful.

 

[David DeLong]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

See Steve Prevette's comment in this thread; the most likely prospect is that the mean and control limits need to be recalculated. The first question to ask is why out-of-spec conditions were charted in the first place. If the process is producing material that doesn't meet specifications, we know that the process is grossly incapable, regardless of statistical capability. At the point where the out-of-spec measurements took place, someone should have stopped and done something about it.

I think that we should reflect all the sample information even if the reading is out of specification but the reaction to this out of control situation is a bit different. We certainly should correct the process and also place the parts that were made from the last good check in quarantine. The suspect parts should then be 100% sorted.

Cp & Cpk should only be calculated if the process is in control and I cannot think of how a process could be in statistical control achieving a Cpk of 1.99.

 

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

I think that we should reflect all the sample information even if the reading is out of specification but the reaction to this out of control situation is a bit different.

While I agree that in a continuous situation the charts should show everything that happens, I suspect that given a 300-piece population perhaps a PPAP submission is involved. I could be wrong about this, but if I'm right, the PPAP SPC report should show a stable, capable process, and not a lot of wacky stuff.

Cp & Cpk should only be calculated if the process is in control and I cannot think of how a process could be in statistical control achieving a Cpk of 1.99.

I still don't understand your reasoning here. Why shouldn't a process that's in control yield a Cpk value of 1.99? (Note that my question has nothing to do with this particular case.)

 

[David DeLong]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

I still don't understand your reasoning here. Why shouldn't a process that's in control yield a Cpk value of 1.99? (Note that my question has nothing to do with this particular case.)

I am referring to a situation where one has a Cpk of 1.99 and parts out of specification in my previous answer.

Of course a process that is in statistical control can have a Cpk of 1.99.

 

[fEArmE]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

An even better rule for the operator is to stop charting when out-of-spec conditions are encountered. Operators also need to watch for trends (excursions away from the mean in particular). The whole point is to use statistical data as a guide in centering the process variation as far away from the spec limits as is reasonably possible. Having crazy, unintelligible charts after the fact isn't helpful.

Chart is plotted automatically. Operators shall be trained adequately to analyze control charts

I've attached the control chart to clarify some confusions. Actual CPK is 1.87, not 1.99 because I received the chart from our supplier only after I posted this message, but they are close.

 

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

I am referring to a situation where one has a Cpk of 1.99 and parts out of specification in my previous answer.

Of course a process that is in statistical control can have a Cpk of 1.99.

Thanks for the clarification, but you made no mention of out-of-spec parts being a condition of your statements. Here is what you said:

If one has 60 sub-groups of 5 samples on an X bar and R chart and somehow arrives at a Cpk of 1.99, something is wrong.

Cp & Cpk should only be calculated if the process is in control and I cannot think of how a process could be in statistical control achieving a Cpk of 1.99.

It's helpful if you quote at least part of the post you're responding to so that people don't read comments like those above and not understand the underlying context.

 

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Chart is plotted automatically. Operators shall be trained adequately to analyze control charts

I've attached the control chart to clarify some confusions. Actual CPK is 1.87, not 1.99 because I received the chart from our supplier only after I posted this message, but they are close.

It appears that whomever sent you the chart just plugged a bunch of numbers into the software, hit "Print" and didn't bother to analyze the results. It's not really even worth reviewing in detail. I've seen a lot of this--people who think that (A) no one is going to parse the data, and (B) all you're looking for is a Cpk number above some defined limit. I also note that the spec limits aren't mentioned or charted. If you have already explicitly defined what you expect to see wrt SPC reports, and this report meets those requirements, you need to revisit them. On the other hand, if you haven't provided an explicit definition, you need to reject this report and tell the sender what you expect to see.

 

[David DeLong]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

Chart is plotted automatically. Operators shall be trained adequately to analyze control charts

I've attached the control chart to clarify some confusions. Actual CPK is 1.87, not 1.99 because I received the chart from our supplier only after I posted this message, but they are close.

I reviewed the chart and it is sooooooo out of control in the averages that the Cpk is not valid.

Here are some of the conditions that could give you such a chart:

1
Your gauge uses too much of the tolerance. One must perform an R & R study and then achieve no more than 10% of the total variation (or state of the art).

2
Your sample size is not homogeneous. If one has a 12 station machine (each station affects the dimension) and you are using a 5 piece sub group, the averages would be reflect as shown in your sample. A 6 station machine should have a sample size of 6 while the 12 station machine uses 12.

3

 

[David DeLong]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

3 The samples were taken in different locations.

Sorry but I hit the wrong button previously.

By the way, one does not have a LCL on the ranges chart unless your sample size is very large, say 15.

Hope these thoughts help out.

 

[Jim Wynne]

Re: CPK 1.99 But Out of Specs Detected in Control Chart

By the way, one does not have a LCL on the ranges chart unless your sample size is very large, say 15.

The chart shows zero as the lower bound, which is correct. While you're correct in stating that for this case there is no LCL, the software apparently adds the label anyway, which makes no difference one way or the other because the range can't be a negative number.