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Pre-control charting

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  • #37092

    BillyBobDubya
    Participant

    Hi everyone,
    I am wondering if anyone has had any experience with introducing Pre-Control charting on the shop floor. Do you guys haveΒ  any recommendations for this? Is there an international or regulatory standard for Pre-Control charting?Β  I’d appreciate the help.
    Thanks

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    #108542

    Dog Sxxt
    Participant

    If people are keen on international standard, they shall always stick to ISO standards.

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    #108554

    RubberDude
    Member

    Pre-control charts have long been misused as a way to introduce “control” of the process without any analysis of the process.Β  It is believed that establishment of “pre” control limits at points two-thirds the distance either side of the center point of specification limits will somehow keep the process in control.
    While this CAN be the case, it is not a replacement for proper statistical analysis using actual data from the process not under the guidelines of pre-control.Β  Also, my experience has been that use of pre-control charts has allowed operators to “pick and choose” the data points they put on the chart.
    PROPER use of “pre-control” limits is when the control limits are calculated from actual data, and set in order to show in real-time when the process is running with only common cause variation and will show special cause variation in real time as well.
    There is no “international or regulatory” standard for the use of pre-control limits.
    My advice – stick with the “real thing” ofΒ  established control limits based on actual statistical evidence and analysis.Β  Pre-control is one of those systems that belongs in the ASQ museum.

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    #108584

    Mike Carnell
    Participant

    RubberDude,
    Just an additional comment (or two).
    I am not sure where Ford is at this time but they used to get this huge emotional response to a PreControl Chart. If you follow the set up of splitting the distance between the LSL and USL into 4 equal areas and make the two in the center green and the two outside yellow and everything outside the spec limits red you can be setting the process up to run at a Cpk of 1.0 (letting it wander rail to rail). They didn’t respond well to that at all.
    I have used it in a facility where literacy was a serious issue and it worked well.
    Where it seems to very handy is on machine setup. If you follow the 2 green = go, etc. it will keep your setup people from getting a reading just inside the spec limits and turning the machine on based on that (it will force them to center the setup – assuming they follow it).
    Just a thought.
    Regards

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    #108594

    Sammy
    Member

    Might do some good to read and “reflect” on the concepts given by T.PyzdekΒ  and F. Breyfogle in their well known books “Six Sigma Handbook” (pp.661-664)Β  and “Implementing Six Sigma” (pp. 703-707).Β Β  Course they donΒ΄t talk about high nor profound stat nor math, but they surely leave us a good PRACTICALΒ message that could be taken advantage of by whoever wanted to capitalize it.
    Regards.Β Β  Sammy

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    #108595

    RubberDude
    Member

    Mike,
    I also used “green, yellow, red” charts in the early 80s in a “less than average literacy” environment.Β Β To your point of being able to “control” or stabilize the process to some degree by controlling the operator (a part of the process) by giving him guidelines for “turning knobs”, I found it to be worth little value if the control limits were set to the “four equal areas” standard.Β  They DID have some value when the control limits were calculated from historical data and reset when process improvements were put in place.Β  Once this system was set up and fully implemented, my Ford “SQA”, GM rep, and Chrysler rep were all “happy” with what we were doing…… As long as I backed it up with capability studies proving a 1.0 or greater Cpk (remember…. this was the early 80s and Cpk min of 1.0 was “acceptable.”
    Of course, the processes with standard Xbar & R charts were more easlily accepted by them (assuming the Cpk value was OK.)
    I’m sure, even though they are now retired, if those same SQA guys were around and up to date, they would have a good laugh at a pre-control chart in one of their supplier plants…. right before they went into orbit……

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    #108597

    Mike Carnell
    Participant

    RubberDude,
    I just stuck to the standard control charts rather than have to listen to the dogma. We basically ran about 80 charts to keep them happy – we actually used about 5 to run the process. I had to carry 3 headcount to maintain and train all these charts and still cough up 5% price reduction per year – very enlightened. We won’t even talk about the trips to the ballet.
    We used the PreControl a lot when we were building bowling balls (Motorola West). Worked out ok. Got more mileage from the focused cells.
    Regards

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    #108598

    RubberDude
    Member

    “We won’t even talk about the trips to the ballet.”
    IsΒ “The Ballet”Β the name of the local “gentleman’s club?”Β  I found a bottle of Jack and some serious study of the “bimodal curves” around town always helped my Q-101 score a few points……
    (In case any Ford guys are listening….. I’m just kidding….. )

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    #108599

    Mike Carnell
    Participant

    RubberDude,
    One in the same.

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    #108610

    Chris Gatieza
    Participant

    Pre-control is a technique for measuring product during production and continuously gauging to determine if the process has drifted or its variation has increased.Β  It is not a control chart and it does not require charting the measurements.Β  It is a mistake to chart its data.Β  For that go ahead an implement a Shewhart control chart and compare the pattern against the control limits.Β  Pre-control makes is used to make decisions against the Specification Limits and Pre-control limits.
    Read the book of Machine/Process Capability Studies by Mario Perez-Wilson, there is a short chapter about it.
    Good luck
    Β 
    Β 

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    #138299

    Greg Howard
    Participant

    I would recommend you stay clear of pre-control. People who use it do not have a proper understanding of why the customer is requesting SPC. The point of SPC is to optimize a process by reducing the variation to that which is inherent in the process. Too many people have a “make it to print” mentality and don’t appreciate the customer’s need to limit variation.
    If the customer merely wants product to print,Β they are not likely to require SPC (maybe during the 1980’s but not today).Β GM and Delphi state that QCI characteristsΒ (those that require capability) are selected because the product performance deteriorates in proportion to devation from target. Control charting is used to minimize variation and center the process around the customer’s target. Pre-control does not address this issue.
    Another comment is that control charts are based upon averages. Averages will follow a normal distribution making it possible for a control chart to show the stability of a process. Pre-control is based upon individual readings. It is not possible to determine stability. Since there is no range, it is also impossible to know the standard deviation.
    Finally, I find it sad that so many see SPC as little more than creating paperwork to satisfy the customer. SPC works, especially when used for your own benefit. During the 1980’s, I was QC manager for a Tier One Q1 supplier of exhaust manifolds to Ford. We had about 160 control charts on the shop floor and all of them had Cpk’s of 2.0 or higher. Our charting became so sensitive that we could distinguish between tool wear and clamping or location issues. We also installed acoustic sensors to stop machines before cutting tools broke and we used vibration analysis to identify machine components wearing out and schedule downtime for maintenance. Our emphasis was on process control, not detection of defects.
    By doing things right, we had tremndous cost savings, high productivity and a customer ppm < 10 during 1990. Ford's satisfaction was simply a by-product of doing things right. If we had maintained a build it to print attitude, we probably would have lost the business.
    Β 

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    #144504

    Nadeau
    Participant

    I am currently looking at implementing SPC throught our maching areas in our plant. I am dealing with operators (button pushers) who do not want any part of this. How do I implement this as painlessly as possible. I am currently looking at Infinity QS to make things a bit easier. Does anyone have any suggestions? Should I stick to paper charting?

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    #144513

    Chad
    Participant

    I agree that it can be easier.Β  I used it to introduce the concept of control in a setup process as well where calculation of limits by floor people needed some ramping up to.Β  Walk before you run sometimes.
    I believe that Shanin still advocates the use of Precontrol with other tools developed by Dorian (since dead, I think).Β  GM used it at one time and Shanin tools were part of our BB training at GE.Β  1996!Β  Whew! Long time ago.

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    #144518

    Cherukara
    Participant

    Nadeau: SPC is good for you, the operators and the management. I do not think we dispute that.
    In my opinion – What we needΒ in your case is a SPC masterΒ  planner – a system which has the Process Tree, together with the sub processes, to which you will assign the relevant chart. You may call this a schedule. This way you would have an idea what are the processes at the floor which needs SPC andΒ the corresponding charts. No confussion as to what chart to use.
    At the lower level you conduct the SPC.
    So what it effectively means is : You have a master planner on a system and you can them zoom down to any of the cells in the planner and you get to do the right chart. This allows a good management of SPC.
    Most softwares allow you to do SPC in isolation but an integrated system to put all machines in the process would be better in my opinion.
    Just my opinion : I am against wall or paper charting.
    Cheers! Dominic
    Β 

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