Let's fix Six Sigma!

A

artichoke

I believe it boils down to you should not use specification as a "basis", nor should you only use "target" as a basis. You should consider both.
The target should be the basis for process management but naturally the specification must be attained. There is no point in having a tightly controlled process where the specification limits are inside control limits. If the process is in-control and in-spec, we can be sure that in-specification product will be made both now and in the immediate future. That is, the process is predictable.

If we base process management on setting very wide specification limits (ie Six Sigma) to allow for "drifts" or "shifts", the latter will result in an out-of-control process. We will have no guarantee that specification will be met, no matter where specification limits have been set.

 
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bobdoering

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The target should be the basis for process management but naturally the specification must be attained. There is no point in having a tightly controlled process where the specification limits are inside control limits. If the process is in-control and in-spec, we can be sure that in-specification product will be made both now and in the immediate future. That is, the process is predictable.
I'll buy that.

If we base process management on setting very wide specification limits (ie Six Sigma) to allow for "drifts" or "shifts", the latter will result in an out-of-control process. We will have no guarantee that specification will be met, no matter where specification limits have been set.
If we ignore allowing for natural "drifts" or "shifts" in process management, the controls (e.g. process specifications, control limits, etc.) will also be false. The biggest fallacy of the common capability study is that it generally only envelopes one setup, one lot of material, one or two operators, etc. What would one think is going to be the ongoing effect of the natural variation of these parameters? Material variation will likely provide drifts, operator changes can provide shifts. Now, hopefully the process will permit adjustment to accommodate these shifts - and hold them between economically and statistically valid control limits. If not, then you truly will have no guarantee that the design specification will be met, no matter where the process specification limits have been set and with any attempt of process control. You would have to walk away from that product. The design specifications can not be met - and sometime that is the bottom line, no matter how hard the process is tweaked. Or, they may need to be reconsidered - such as opened up -if the design criteria was too strict.
 
A

artichoke

.... Material variation will likely provide drifts, operator changes can provide shifts. ... etc
Yes, processes and attitudes to process management, have gone back to the bad old days with 6 Sigma, where the only solution was/is to broaden specifications and/or inspect finished product for defects.

Deming recognised these issues and provided a path out of such problems ... such as page 35 "Out of the Crisis" discussing the benefits of single suppliers ... and many others ... his books are worth re-reading.
 

bobdoering

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Yes, processes and attitudes to process management, have gone back to the bad old days with 6 Sigma, where the only solution was/is to broaden specifications and/or inspect finished product for defects.
If you interpret 6 Sigma as where the only solution is to broaden specifications and inspect finished product for defects, then I recommend you find another set of tools to meet your needs.

Deming recognised these issues and provided a path out of such problems ... such as page 35 "Out of the Crisis" discussing the benefits of single suppliers ... and many others ... his books are worth re-reading.
Yes, as blue sky some of those ideas were handy for the time. Single suppliers these days borders on extreme wishful thinking: either wishful that they never go out of business, wishful that they never raise their prices, wishful that they are not bought out and obsolete your raw material, wishful that your customer will immediately accept a new supplier when the previous one closes up in bankruptcy or your customer will willingly accept a line shut down while a new supplier is being verified.

Deming was a deep thinker, and we can learn from his writings. But, we must always look at any work with a critical eye for historical changes affecting its application or new knowledge countering the content. Deming, Six Sigma - all the authors, all the concepts - all should be viewed critically. Otherwise, if it was that easy, one book written in the 1800's would suffice for today.
 

Miner

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It always gives me a chuckle when people claim that Six Sigma is not about six sigma. :D
Who said that? You are making too big of a deal about the 3.4 PPM. If you don't like it, forget it. Consider Six Sigma as having a Cp index of 2.0.

There are two ways to obtain a Cp of 2.0. If the specifications accurately reflect the true customer requirements, you must reduce the process variation. However, there are many situations where engineers picked a standard tolerance that in no way reflected the customer's requirements, but were much tighter than necessary. In these situations , it is 100% legitimate to open the tolerance to achieve capability. There is nothing wrong with this approach. In fact it is foolish to ignore it.
 

Stijloor

Staff member
Super Moderator
<snip>However, there are many situations where engineers picked a standard tolerance that in no way reflected the customer's requirements, but were much tighter than necessary. In these situations, it is 100% legitimate to open the tolerance to achieve capability. There is nothing wrong with this approach. In fact it is foolish to ignore it.
Regretfully, this usually happens after excessive tooling/equipment costs already have been incurred. A result of poor design/process evaluations or rather the lack of it.

Stijloor.
 
A

artichoke

Who said that? You are making too big of a deal about the 3.4 PPM. If you don't like it, forget it.
Who said that ... there's still thousands of web sites such as isixsigma.com who claim:

"The objective of Six Sigma Quality is to reduce process output variation so that on a long term basis, which is the customer's aggregate experience with our process over time, this will result in no more than 3.4 defect Parts Per Million (PPM) opportunities."

Perhaps some of the true believers here would like to present their evidence for their belief in 3.4 dpmo ? Or does everyone here now accept that 3.4 dpmo is historical rubbish ... and we can move on ?
 

bobdoering

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Or does everyone here now accept that 3.4 dpmo is historical rubbish ... and we can move on ?
I will buy that a process of no more than 3.4 defect Parts Per Million (PPM) opportunities is a poor long term target. I like 0 PPM. I have some customers that dig that, too. Handling process capability in the short term utilizing the concept may be acceptable for a normal process, or distributions that are similarly acceptable.

Of course, with the continuous uniform distribution found in properly controlled precision machining operations, 0 ppm from common causes is rather easy to achieve - short term or long term. I can live with that, too. And, true, one does not achieve that with +/-3 standard deviations from the mean. The mean is not even considered in controlling that process. So, it clearly does not apply to all processes, if that makes you feel better.

I will also buy that Taguchi's loss function is useless in the short term, because it needs historical data to determine both the process distribution and the determination of the applicable variation to apply. It can not be derived from thin air. It may better direct long term variation control - IF one bothers to properly develop the total variation (that is, the sum of the individual cost variation contributors) that contributes to the loss function. If not, it is just exploitation of academic jargon.
 
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