Cpk vs. "Bad" Parts - Unreasonable Diameter Tolerance and a 1.67 Cpk

bobdoering

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#11
I am not convinced that the part is actually out of spec. Measurement on these types of parts is always very tricky. So I want to confirm on our equipment. However, if they are out of spec., then I am afraid that there is no way we can guarantee "good" parts. The variation would be intermittent and most likely caused by material springback. Good luck with that one. Thanks for your help.
An issue I have come across in such a situation was that the customer was using an air gage, and their probe had worn more than our probe, giving deviating data. That wear would make a good part look bad. Hopefully they master and calibrate at a reasonable schedule, but some people think air gages do not wear and extend their check to unreasonable lengths - creating such a problem.

I am not saying that is the case, just something to look into. Have you verified the specific parts in question measure the same? Is there any effect of shipping? Are they packed in bulk? What is the roundness of the "bad parts"?

If it is true your process is not capable, sorting may be the endpoint. That being the case, I would make sure the distribution is applicable to Cpk (and with springback, it would be), and ensure the measurement and/or gage error is not masking the true dimension and characteristic.
 
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Jim Wynne

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#12
The diameter is on the drawn feature. We measure it using an average -- high, low divide by 2. Even have a specific spot to check it. Use a special air gage. Roundness is not an issue (been there, done that -- thus the average). I am going to guess it is round within .002 to .004. Our run data was all good -- some within plus .001, some within minus .001. Thus, the good cpk. They found a few pcs. out of tens of thousands.

I am not convinced that the part is actually out of spec. Measurement on these types of parts is always very tricky. So I want to confirm on our equipment. However, if they are out of spec., then I am afraid that there is no way we can guarantee "good" parts. The variation would be intermitent and most likely caused by material springback. Good luck with that one. Thanks for your help.
How did they find the "bad" parts?
 

Golfman25

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#13
Ok, here is the update.

I received a few samples. Using the air gage, only one is out by .0001. On the cmm all parts are within spec.

Bob, in speaking with the customer, their gage probe (the part which holds the part for measurment) is .001 smaller diameter than ours. Could that cause the discrepency?? Some of the parts are tight, so we know we are influencing the part when it is put on the gage. Thus, we like the cmm since there is no part influence.

Back to my original question though. We have taken all the cpk data which shows capability -- not perfection. You're still subject to a "bad" part from time to time. In this case, I see only 1 pc. out by a tenth. So what is the point behind the cpk if you're going to make me sort cause 1 pcs. was slightly out of spec.? Not to mention that the cpk is done using a "bogus" tolerance. Thanks.
 

Jim Wynne

Staff member
Admin
#14
Ok, here is the update.

I received a few samples. Using the air gage, only one is out by .0001. On the cmm all parts are within spec.

Bob, in speaking with the customer, their gage probe (the part which holds the part for measurment) is .001 smaller diameter than ours. Could that cause the discrepency?? Some of the parts are tight, so we know we are influencing the part when it is put on the gage. Thus, we like the cmm since there is no part influence.

Back to my original question though. We have taken all the cpk data which shows capability -- not perfection. You're still subject to a "bad" part from time to time. In this case, I see only 1 pc. out by a tenth. So what is the point behind the cpk if you're going to make me sort cause 1 pcs. was slightly out of spec.? Not to mention that the cpk is done using a "bogus" tolerance. Thanks.
How did the customer find the "bad" parts? This is important, because if the parts were in the bowels of an assembly and the customer had to go through a big production to get them out, it's a different case than if they found a bad one on the line and could just toss it aside and pick up another.

As to the liability question, you should read the contract (which could include a supplier manual by reference). If the customer says that if they find out-of-tolerance parts then you have to sort, then you'll probably have to do it. Be careful what you ask for.

On the other hand, it's true that when a customer specifies a Cpk level, they are at least tacitly acknowledging that there'll be some nonconforming parts. I say "tacitly" because there's a good chance that whomever made the rule doesn't understand what he was specifying. Again, but on the other side, it's be careful what you ask for.

If there is no agreement in writing on sorting, they are within their rights to return "bad" parts--subject to inspection by you--and get credit for them, but that's all they're entitled to. My experience has been that customers like this may not be worth the trouble.
 
R

rexch

#15
Ok, I need help with this situation. We have a customer's part which called for an unreasonable diameter tolerance and a 1.67 cpk based on that tolerance. Eventually (although hesitantly), we agreed that the part diameter would be measured with an average and that the cpk would be based on a "larger" tolerance. I thought it was absurd, but we do what we need to do to get the business.

Fast forward and we are in production. After 40,000 plus pcs. customer has found 3-5 pcs. "out of specification" by a few tenths. Yet our cpk numbers are good. They want to reject and return and 100% inspect.

Anyone with advice on how to hanlde would be appreciated. Thanks in advance.

CPK value means the process capability. If the process capability is not enough, you set procedure to conduct 100% inspection to avoid defective product be packed. I don't know if this method matches your product.
If you don't conduct 100% percent inspection, you need to make sure that the CPK is good and is stable.
 

bobdoering

Stop X-bar/R Madness!!
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#16
Ok, here is the update.

I received a few samples. Using the air gage, only one is out by .0001. On the CMM all parts are within spec.

Bob, in speaking with the customer, their gage probe (the part which holds the part for measurement) is .001 smaller diameter than ours. Could that cause the discrepancy?? Some of the parts are tight, so we know we are influencing the part when it is put on the gage. Thus, we like the CMM since there is no part influence.
If the parts are tight, that is a problem. It should be free floating on an air gage.

As far as the probe diameter, how close does the master ring read? How about their gage - how close does it read to their master ring value? That is key.

You are right, the CMM would be a good referee method.

Back to my original question though. We have taken all the Cpk data which shows capability -- not perfection. You're still subject to a "bad" part from time to time. In this case, I see only 1 pc. out by a tenth. So what is the point behind the cpk if you're going to make me sort cause 1 pcs. was slightly out of spec.? Not to mention that the cpk is done using a "bogus" tolerance.
If you are required to show a Cpk of 1.67, it pretty much reduces your usable portion tolerance by 25% - more if you are not centered. That accommodates the portion of your probability that lies out in the tails of the distribution, as well as sampling error (because you sample and do not measure 100%), gage error, etc. (see total variance equation)

So, do you sort because 1 pc. was slightly out of spec? No, you sort because there is a probability that there are more pieces out of spec, and by doing that sort you may find some even more out of spec than the relatively small sample you have now. Bad parts are like roaches - if you find one, you can bet there are more.

If you ran to 75% of the spec (or could), there would be no argument now. With the contributing variations, you never really "own" the whole tolerance on the shop floor. Only dock inspection and customer incoming does - and even then only theoretically because they still have gage error.

Is the tolerance bogus? If they really need it that tight, no. Are you capable? Hard to say. You have gaging issues, but even so, you do not have the kind of process you can "dial in" like machining. Are you running high, low, or are you using the whole spec? It would still be good to see a capability study.

The real problem is, if your company signed up for a job they are not statistically capable on, this will be an ongoing problem....
 

Golfman25

Trusted Information Resource
#17
A few points of clarification from my first post.

We are holding the required cpk. However, there are two "tolerances." The part tolerance is p/m .001. The cpk tolerance is p/m .0015.

Also, we tried hard not to take this part. We told them they where being unrealistic. We told them what would be required. We asked numerous times for better tolerances. We priced it appropriately. We knew it would be a nightmare. Unfortunately, those who where involved are no longer with the company. Sometimes you do what you have to in order to earn a living. I just don't like headaches at this stage of my life.

Bob, the gage readout is adjustable. You set the probe in the master rings -- one high, one low -- and "turn the dial" until the readout is correct. I call the thing a "magic wand" cause I have no idea how it works.

Jim, apparently they "found" the one part durring their assembly. It was "tight" so they checked it. When that was "bad" they checked more and started sorting. Next thing you know all heck breaks loose. Problem is, the samples they submitted as "bad" are actually good on our gage and even better on the cmm.

I guess my issue is that we spend all this time maintaining a cpk value. Yet, even though it is good, we are still told to do 100% inspection. So what is the point?
 

bobdoering

Stop X-bar/R Madness!!
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#18
I guess my issue is that we spend all this time maintaining a cpk value. Yet, even though it is good, we are still told to do 100% inspection. So what is the point?
If your Cpk is always good (as determined with adequate measurement system with statistically insignificant measurement and gage error), yet you still have "bad" parts, then you have a few possible issues:

1. The sample size you use to determine the Cpk is too small

2. The problem is truly intermittent and is either a special cause that needs eliminated or your process is not completely stable and the only way to remove those parts from shipment is sorting.

3. The parts are being damaged after the process measurement (e.g. shipping) - maybe not destroyed, but perhaps deformed enough to affect the characteristic.
 

Jim Wynne

Staff member
Admin
#19
If your Cpk is always good (as determined with adequate measurement system with statistically insignificant measurement and gage error), yet you still have "bad" parts, then you have a few possible issues:

1. The sample size you use to determine the Cpk is too small

2. The problem is truly intermittent and is either a special cause that needs eliminated or your process is not completely stable and the only way to remove those parts from shipment is sorting.

3. The parts are being damaged after the process measurement (e.g. shipping) - maybe not destroyed, but perhaps deformed enough to affect the characteristic.
A "good" Cpk result doesn't mean there won't be nonconforming parts. It's supposed to help predict the number of nonconforming parts produced by a stable, normally-distributed process. As we know, there are serious problems with using Cpk for predictions (or using it at all). If the number of nonconforming parts is greater than expected, your list of possibilities probably holds the answer. We don't know at this point if what the customer found is what the customer asked for in terms of Cpk.
 
T

True Position

#20
Ok, here is the update.

I received a few samples. Using the air gage, only one is out by .0001. On the cmm all parts are within spec.

Bob, in speaking with the customer, their gage probe (the part which holds the part for measurment) is .001 smaller diameter than ours. Could that cause the discrepency?? Some of the parts are tight, so we know we are influencing the part when it is put on the gage. Thus, we like the cmm since there is no part influence.

Back to my original question though. We have taken all the cpk data which shows capability -- not perfection. You're still subject to a "bad" part from time to time. In this case, I see only 1 pc. out by a tenth. So what is the point behind the cpk if you're going to make me sort cause 1 pcs. was slightly out of spec.? Not to mention that the cpk is done using a "bogus" tolerance. Thanks.
How are you evaluating the part on the CMM? How much form error is there in the feature?
 
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