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1.33 CPK for Reman Parts

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  • #34646

    Mike Hurst
    Participant

    My question is, should a remanufacturing plant be held to a 1.33 min.CPK or is there some other stat. that would be better to use.
    A lot of parts at our plant are cleaned and gage parts. In other words, if the part is gaged and falles within new print specs. it is sent on down the line,so the measurements may be all over the the place within the specs.
                                                                    Mike                  
     

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    #95775

    asa
    Participant

    cpk is not a good measure because it is always inflated(more sub group size more better cpk). Use ppk and cpm and use variable data.
    with cpk 1.33 you can have 0.65% out of spec parts

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    #95776

    Statman
    Member

    You said:
    “cpk is not a good measure because it is always inflated(more sub group size more better cpk)…….  with cpk 1.33 you can have 0.65% out of spec parts”
    What in the ____ are you talking about?
    Statman

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    #95777

    Anonymous
    Guest

    Many processes, such as furnace process, only last fifteen runs or so, before they have to be cleaned. When tubes are pulled for cleaning, the spike thermal couples are disturbed, so the usual practice is to re-set the furnace. If you do this correctly, one often finds that Cpk = Ppk.  Many processes exhibit this behaviour.

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    #95780

    asa
    Participant
    #95782

    Statman
    Member

    Interesting set of articles.
     
    You not have noticed the little caveat in their discussion of the dangers of Cpk.  If not I pasted it below:
     
    “(Yes, for this example, we have ignored the first cardinal rule: Before one looks at Cpk, the process must be in control.)”
     
    This of course invalids any thing the author is saying about Cpk.  Makes you kind of wonder what his point is.
     
    Also, I can’t find any thing in the articles that tells me that a larger subgroup size will inflate (over estimate) the capability or that a 1.33 Cpk will have .65% defective.
     
    Statman

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    #95786

    asa
    Participant

    only mathmetics
    cpk=1.33 Zst =4 in worst case scenario Zlt=2.5
    from Z table performance will be 0.62% out.
    ofcourse based on:
    “(Yes, for this example, we have ignored the first cardinal rule: Before one looks at Cpk, the process must be in control.)”
     

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    #95789

    Statman
    Member

    Who’s mathematics?  Do you really believe that parameters estimated from a process in statistical control will be off by that much?
    You have been drinkin too much from the well of Dr Harry
    So what is Ppk in worse case? 
    Statman

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    #95795

    asa
    Participant

    how can you estimate ppk. it is the actual performance of process and depends on individual s.d. on long term basis. that is why i was saying don’t rely solely on cpk better to check cpm and ppk.
    i am new on this forum who Dr. Hary? and what he says?
    by the way i am not BB and corrections will be appreciated.

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    #95900

    N14diesel
    Participant

    I am a BB in a reman plant for a major diesel engine manufacturer and I understand what you are asking. At a remanufacturing location, you are inspecting cores that have been returned to your facility and as long as the feature being measured falls within an acceptable specification range the part is deemed good. For example, we resurface cylinder heads at our facility. When we receive the head cores, the thickness is generally at the nominal specification. After resurfacing, the thickness of most of the cores is less than nominal but still within specification. In addition, some cores will fall out as scrap because too much material must be taken off to remove surface damage. If a CpK study was done on this particular feature, the results would be much less than 1.33; however, the process is not a “broken” process. We do perform CpK studies on features such as surface finish, valve guide bore diameters, valve seat insert diameters, etc. As a general rule of thumb, if you are targeting a particular specification value, a CpK study is probably applicable. If you are performing a machining process and merely hoping that the finished product falls somewhere in between specifications when the damage is removed, CpK is probably not applicable. Cpk is also useful for monitoring process parameters such as wash solution temperature, chemical concentration, coolant concentration, etc. I’ve found that in a reman. plant the most important aspects of any process are the gage R&R (if gages are being used) and the attribute R&R (if visual inspection is employed). It is gaging capability that determines if you’re making alpha or beta inspection errors and possibly passing on defects to your customers.

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