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Appropriate Sigma Target for Conventional Manufacturing Process?

Six Sigma – iSixSigma Forums Operations Manufacturing Appropriate Sigma Target for Conventional Manufacturing Process?

This topic contains 15 replies, has 7 voices, and was last updated by  Chuck White 5 months, 1 week ago.

Viewing 16 posts - 1 through 16 (of 16 total)
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  • #209335

    tbs11
    Participant

    Hello,

    Can 3sigma as a Target be used for Z-Score for Conventional Manufacturing process? It would be great help if someone can send information about it.

    Thanks,
    Tejas

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    #209355

    Mike Carnell
    Participant

    I have no real clue what you are asking. If you are asking about a Cpk of 1 which is 3 sigma, of course you can target that. You can target whatever you want. Couple questions. First does your customer have an expectation? If so you might want to work to that. Just as an fyi the automotive world has been running to Cpk of 1.33 for about 25 years. You are shooting at a moving target and aiming behind the target.

    Here is the second part most people have forgotten over the years. Lets say you have a 10 step process and you decide that Cpk of 1 is ok so you set that for every step. I assume you understand rolled throughput yield. So now you have 10 steps each with a yield of 99.73%. My rty on that process is 97.3%. basically I now have 2.7% of my product dropping out of the process. So if I get my capability to a Cpk of 1.33 then every step has a yield of 99.99%. The rty becomes 99.9% and I now have 0.1 going to rework. This assumes a centered distribution with both tails going out. You need to plug in your actual numbers to understand the effect. It also assumes you catch all the defects. with a Cpk of 1.0 it becomes more complex catching all the defects because there are more of them so you need to evaluate the effect of escapes. When I was at Motorola we manufactured bomb fuses (read the book Six Sigma by Mario Perez Wilson it closely parallels the bomb fuse project). One bomb going off when it should not (some fuses are powered on the runway before takeoff) kills people and puts you out of the business. If you make those little umbrellas that they put in drinks it might not make the customers angry if they don’t open but the volume might be significant in cost because that is in all probability a very high volume business. The whole yield process can be very important to your business. If you turn this into some math problem and leave out the financial implications you will be as clueless as some of these guys ranting and raving about a 1.5 sigma shift.

    Now you have some insight into why this whole idea of sigma level is important. There is a small minded band of old timers that think they are flexing statistical muscle when they want to discuss the sigma shift. NOBODY except them has cared about the 1.5 sigma shift since the 90’s. This is about capability and the risk/cost of various levels of capability. Let’s say you are about to have a pacemaker installed and the last step of the operation is stopping your heart before they close your chest and letting the pacemaker start it (not sure if that is still the procedure but I was told it was the process). Which one do you want? There is one where every process is 3 sigma and one where every process is 4 sigma. How much cheaper does the 3 sigma pacemaker need to be for you to select it?

    Even more basic. You are in mining and you have a precious metal refinery for your gold. and you process is 3 sigma. You produce 5,000,000 ounces of gold. Lets say it is the 10 step process (which it isn’t) so now you have lost 135,000 ounces of gold with a processing cost of over $800 per ounce so not only have you lost the gold but you lost $108,000,000 in processing cost. todays price is $1243.26 per ounce so your gold cost is $14,580,000,000,000.

    Don’t get wrapped around the axel in some esoteric debate with a dinosaur. Analyze your process. Understand the business and the technology and make an informed decision. Leave the emotion and hype. In they words of Sam Peckinpaw ” they can cut a tin bill and pick sXXt with the chickens.

    Good luck

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    #209473

    Burns
    Participant

    tbs11,

    Mike is ranting about the 1.5 sigma shift that forms the “six sigma” of Six Sigma. It is utter nonsense. Six Sigma is a farce.

    Cp is a poor attempt to replace a control chart with a number.

    Focus on using a control chart to keep your process ‘on target with minimum variance’. This is the definition of World Class Quality.

    A specification at 3 sigma gives very little ‘elbow room’.

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    #209480

    Mike Carnell
    Participant

    Burns you don’t seem to understand what a complete joke you are to the world. You have been locked onto this 1.5 sigma shift thing for years. You go off like a deranged person and you think it is a argument that changes anyone’s mind? You are about the only one that even cares about the shift. The pathetic part is hatred seems to be what keeps you alive. I started posting here in 2001 and have continuously told people to avoid getting into some esoteric discussion with an old dinosaur like yourself who has no other reason to live than to use the word “stupid.”

    Lets look at how much of a liar you are. You said I ranted and raved about a 1.5 sigma shift. What I posted is still there. You posted previously that you had spoken to me about something. That was also a lie. I would not waste my time speaking to you.

    You are as welcome as anyone, my opinion, to post here. It would be interesting to see if you have ever had an original thought. You don’t seem to be able to make a statement without citing Dr. Wheeler. If I want to know what Dr. Wheeler thinks I can read his books. Nobody needs you to tell us what you read. You can even continue to lie if you choose. You do need to leave me out of your lies particularly when it is such a blatant lie someone can scroll up one post and see you are lying.

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    #209498

    Strayer
    Participant

    Crosby noted that the only appropriate goal is zero defects. Say you want 3 sigma (93%) that’s the same as saying your goal is 7% defects, which is crazy. Even if your goal is 99.99% that’s still saying you want to create some defects. That said, you can set lower interim progress goals as long as you understand that continuous improvement doesn’t stop there. We may never get to zero defects but we should never be satisfied with any, even six sigma.

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    #209505

    Mike Carnell
    Participant

    Strayer Part of the reason people backed off of the Zero Defect idea was because for most manufactures they are already over the target for the year by January 2 (every year). There was always the idea that there was this “hidden factory” and nobody understood the effect and cost. Rolled Throughput Yield was really a first shot at quantifying that hidden factory i.e. if I start this material in the line what is the probability of it coming out the other end without going through any rework cycles.

    Zero Defects was basically a good concept that didn’t work. We have had everyone go to C=0 sampling because the concept sounds right. It sounds right but if you look at the OC curves it is actually a pretty ineffective sampling concept so what we did was choose style over substance.

    The concept I spoke about in my first post on this thread is what drove the Motorola program. It is basically built around a rolled throughput yield and the cost of your quality level because we always had those people (in the 80’s) that believed there was this point where it was no longer profitable to improve quality.

    Just my opinion

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    #209511

    Chris Seider
    Participant

    How about identifying where the defects are in your process and using business critical needs, financial analysis, and basic pareto tools to decide where to improve first. Get improvements going, gain traction, and more projects will bloom to further increase your sigma level of your processes and products.

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    #209798

    Strayer
    Participant

    The reason zero defects fell out of favor is, of course, that it cannot be achieved. So we chose lesser goals that we could pat ourselves on the back for achieving. There’s a sound argument for “good enough”. That is, what’s good enough for customers to accept. Only our customers can tell us what’s good enough. If we set our own goals based on process capability, cost, or whatever we’re still missing the boat.

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    #209812

    Mike Carnell
    Participant

    Strayer Deming’s 10th point: Eliminate slogans, exhortations and targets for the work force asking for zero defects and new levels of productivity. Such exhortations only create adversarial relationships, as the bulk of the causes of low quality and low productivity belong to the system and thus lie beyond the power of the work force. That probably did as much to kill Zero Defects programs as much as anything.

    It fell out of favor for a lot of reasons. Crosby started it with Quality is Free but he eventually backed away (my opinion) from driving the use of statistics even after being a Chair at ASQ in 1979. Referring to yourself as the Funny Uncle of Quality doesn’t help either. Regardless of that he had an overall positive impact on Quality.

    Nobody chose Six Sigma as a lesser goal so we could pat ourselves on the back. It was a target with a process attached to it. When I was setting targets for my quality improvement if I had a 1 sigma process I could target a big improvement. If I had a 3.5 sigma process I would target a lower level of improvement but with a lot more work. Eventually the metric inside Motorola was your rate of improvement as opposed to your actual sigma/ppm level. It was a pragmatic approach and I didn’t have to buy floor mats and posters that said “do it right the first time.”

    I do agree we need to understand customer expectations and we need to meet them. Somewhere we seem to have lost the concept of the Voice of the Business, VOB. You remember that time when everyone spoke about maximizing Shareholder Value and Michael Douglas told us “greed, for lack of a better word, is good.” Your customers first interest generally is not the financial success of your company. It had better be in your top 3 because that is what pays your bills. The whole Taguchi Loss Function doesn’t have anything to do with your customer. It is about minimizing your companies loss and it is good for everybody’s business.

    Meeting customer spec is like a membership card to the Mediocre Success Club of Manufacturing. Profitability is what allows a company to provide good health insurance, pay raises, day care and all the other amenities they say people today want. https://business.financialpost.com/business-insider/17-companies-with-awesome-perks-that-will-make-you-jealous If you want to compete for talent with these companies you better be making more than mediocre profit.

    Just my opinion

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    #209850

    Strayer
    Participant

    Your criticism of my zero defects comment is well taken, Mike. The appropriate improvement goal depends on what all the “voices” say, considering cost/benefit, despite what Crosby said in Quality is Free. But have we answered the original question? I took it to mean that he was considering 3 sigma as a “conventional” target but maybe he was asking more about the relation between sigma and z scores, which you addressed in your first response and maybe the rest of us should have let it go at that.

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    #209880

    Mike Carnell
    Participant

    Strayer I apologize I didn’t mean for that to appear as criticism. When I was first starting out “Quality is Free” was the first true quality book I ever read. I went through the frustration of blowing my target typically within the first hour of starting up after New Years shut down. Then you think you will reset to zero again the next day and before long it is resetting every day until you are sitting in a corner goint wtf!

    We made the most progress when we stopped even trying to hit 6 sigma and retargeted to an improvement rate of 68% reduction of defects per year. That was based of the idea that they wanted a 10X improvement in the first 2 years and 100X in 4 years. For some reason when we targeted rate the monthly incremental improvements were more palatable.

    When I answered the first time it was exactly what you said. I told him I had no clue what he was asking for. At this point we are really just winking at people in a dark room. We know what we are doing but nobody else does.

    Regards,
    Mike

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    #210006

    MBBinWI
    Participant

    Good points all. I agree that RTY is probably the best way to evaluate your overall quality. You apply DMAIC continuous improvement methods to improve process capabilities until the costs of better improvement outweigh the benefits. Then (or before, depending on the focus of the organization) you should apply DfSS to change the underlying design of the system to one less sensitive to the variations that exist.
    To go back to the original question – it really doesn’t matter the target level you choose, rather that you are measuring it. If you are measuring honestly, you can then have discussions with your customer and management on whether you are currently good enough (you aren’t). But more importantly, by measuring, you will see where you can improve and begin to take steps to get better in that area. You should never be satisfied with a level – there will always be defects that can be eliminated. You should be striving for a method to identify and help prioritize where to go after improvement as that will bring you the biggest bang for your investment. Just my humble opinion.

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    #210873

    Chris Seider
    Participant

    Don’t for a Z-score of 3. No one can accept those results–whether they are “shifted” or not.

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    #210990

    Mike Carnell
    Participant

    @cseider What?

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    #211357

    Chris Seider
    Participant

    @mike-carnell I had interpreted them asking if a goal of 3 for Z was a good target and I said no.

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    #211361

    Chuck White
    Participant

    Sorry to jump in late on the middle portion of this thread. In my thinking, the zero defects controversy boils down to the distinction between a target and a goal. Zero defects should always be the target (whether explicit or not), but it isn’t a reasonable goal. To use a design characteristic on a part as an analogy, the target is the characteristic nominal, and the goal is the manufacturing tolerance. While the manufacturing target is to produce a part exactly to nominal, realistically that never happens. With fine enough resolution, you will always find some deviation. But the goal is to manufacture the characteristic within the tolerance limits, and that should be reasonable if it was engineered correctly.

    So I don’t mind stating that zero defects is the target (even though that should be obvious), as long there is also a separate achievable goal, and the distinction is clear. For some, explicitly stating the zero defect target serves as a reminder that the goal shouldn’t be static, just as focusing on a characteristic nominal can help drive efforts on variation reduction.

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