Transfering Cpk specification to 100% inspection specifications

R

royua

#1
A key caracteristics requiring a Ppk of 1,33, centered process and full SPC to detect process deviation, goes bad and requires 100% inspections to contain the damage until the situation is corrected.

My question: What tolerances should be used for the 100% inpection?

In theory, the tolerances for 100% inspection should be tighter to keep the part close to nominal and not wind up with parts all grouped close to the original tolerance limits. The initial tolerances set up using a design for Six Sigma approach allows very occasional parts to wind up at either end of the tolerance range but not for too many since this would create assembly issues.

Any guidelines would be appreciated.
 
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P

prototyper

#2
Re: Tranfering Cpk specification to 100% inspection specifications

Your contract is to supply parts which meet the specification. If you are artificially tightening the tolerances, you are rejecting good parts.
100% inspect to the full tolerance and instigate corrective action on the process to restore/improve the capability.
 

bobdoering

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#3
Re: Tranfering Cpk specification to 100% inspection specifications

My question: What tolerances should be used for the 100% inspection?

In theory, the tolerances for 100% inspection should be tighter to keep the part close to nominal and not wind up with parts all grouped close to the original tolerance limits. The initial tolerances set up using a design for Six Sigma approach allows very occasional parts to wind up at either end of the tolerance range but not for too many since this would create assembly issues.
Close. Theoretically 100% sorting should allow the use of the full tolerance. BUT...there is the problem of gage and measurement error. For example, if you are going to sort a round dimension, either you use a pin or ring (MMC), or you have to measure all around the part to ensure that no diameter is out of specification. Otherwise, you will fall into the Rule of Life: If you measure a diameter, you will measure a good one, band your customer will measure a bad one. To accommodate these issues on a gross overall manner, you may want to sort to 75% of the tolerance.

The initial tolerances set up using a design for Six Sigma approach allows very occasional parts to wind up at either end of the tolerance range but not for too many since this would create assembly issues.
This may be true for processes with a normal distribution, in that the tails theoretically can go beyond the specifications, but it should not be true for properly control precision machining processes. That may not apply to this situation, but it is important not to assume this is always true. Only special causes should have the possibility of making product out of specification at all.
 
P

prototyper

#4
Re: Tranfering Cpk specification to 100% inspection specifications

75% of the tolerance band could reject a lot of good parts unnecessarily.
Whilst I agree that the method of gauging and it's inherent inaccuracies should be considered, the tolerance required should be much closer to 100%. It is also worth considering the distribution of the suspect batch. If the spread has increased then you may just be rejecting an odd "flyer". If there has been a mean shift, at 75% of the tolerance, you could be rejecting 100% of the batch. That is commercial suicide!
 

bobdoering

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#5
Re: Tranfering Cpk specification to 100% inspection specifications

75% of the tolerance band could reject a lot of good parts unnecessarily.
Whilst I agree that the method of gauging and it's inherent inaccuracies should be considered, the tolerance required should be much closer to 100%. It is also worth considering the distribution of the suspect batch. If the spread has increased then you may just be rejecting an odd "flyer". If there has been a mean shift, at 75% of the tolerance, you could be rejecting 100% of the batch. That is commercial suicide!
Well, commercial suicide may have already been achieved - which is the basis of the sorting. Maybe 100% of the batch IS bad!

No problem using 100% of the tolerance, as long as you have NO gage or measurement error. Otherwise, you are certain to be accepting bad parts - which is far worse than rejecting good parts, because you accepting the risk of having additional overhead of re-sorting and preparing a corrective action for not sorting correctly. Using 75% is if you are not willing to take the time to determine the gage and measurement error. If you qualify most of your parts as good with that criteria, you are lucky and you will sleep well. If you have high fallout with the 75% criteria, then stop, evaluate your potential sources of error and accurately determine how close you can get without accepting bad product.
 

AlanC

Involved In Discussions
#6
I would consider what out of tolerance did the customer reject at, if they are gross failures then checking at 100% of tolerance maybe ok, if its 2 micron then all the factors mentioned need to be considered and maybe you need to band your stock as step 1, then check again those at 75-100% of tolerance either by different method/different operator, or if its a major problem take some down to the customer to cross check for you. Its not possible to shorten the tolerance otherwise your good parts will be scrap and the cost huge
alan
 
R

royua

#7
Thanks for all the answers. Food for thought.

Now from what I've learned in the past, tolerances set using statistical tolerancing analysis should be typicaly wider than tolerances set using worst case analysis. In worst case analysis, assembly interference is imposssible and in theory all part could be at a tolerance extreme and the parts would still fit. In statistical tolerancing, extreme values could potentially cause interference but as long as the process is maintained centered by adequate SPC, these events are extremely rare and are an acceptable trade off to permit wider tolerances and often lower cost process.

As long as the process is OK everything is fine. When it goes bad, yes we would look into corrective action, but in the mean time, we must often resort to 100% sorting. This might lead to many case of assembly interference instead of the very rare occasion.

In theory we could do a worst case analysis and se what tolerances pop-up but that would take too long and I was hoping for a quick and dirty method.

I get the impression statistical tolerancing is not that often used in this way, but we have had some good succes stories using this approach.
 

bobdoering

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#8
In worst case analysis, assembly interference is impossible and in theory all part could be at a tolerance extreme and the parts would still fit.
This is really the only way to develop tolerances for precision machining.

In theory we could do a worst case analysis and see what tolerances pop-up but that would take too long and I was hoping for a quick and dirty method.
If you are using precision machining, and you are controlling your process correctly, you really can not use statistical tolerancing. You really need to know what the design tolerance is - if it is fit, then what fits? Then, you run within 75% of the tolerance. You can run to as tight of a tolerance as is needed, until your roundness or parallelism becomes too much of your tolerance (in which case you need to find another process).

Now most of the conversation was based on sorting. Again, if you are using precision machining, and you are controlling your process correctly, the only time you will be sorting is if you have a special cause problem - tool broke, bearing blew, gage broke, etc. Then you have to ponder how close you can measure to the given tolerance and still have good product.
 
R

royua

#9
Maybe I should clarify a point. I am with supplier QA, so I am the client. When one of our suppliers screws-up we wind up in a situation where we either have to sort the bad lots on hand or shut down our assembly line. It would not be unheard of for us to sort to a higher standard only to resort to a lower standard later when we find out our supplier cannot meet demand.

Definitively there is a special cause to explain this process shift and our supplier SHOULD have caught it and isolated us from bad product but all too often they don't.

I'm sure I'm not the only one on this forum to find that SPC reaction plans are often lacking.
 

bobdoering

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#10
I'm sure I'm not the only one on this forum to find that SPC reaction plans are often lacking.
...or SPC implemented incorrectly creating more problems than it solves.

The biggest problem I have seen - even in correctly controlled processes - is passing bad product out of machining operations of all types caused by a broken tool and not segregating and/or sorting (effectively) the resulting bad product.
 
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