iSixSigma

Capability Vs Performance

Six Sigma – iSixSigma Forums Old Forums General Capability Vs Performance

  • This topic has 6 replies, 6 voices, and was last updated 18 years ago by bjb.
Viewing 7 posts - 1 through 7 (of 7 total)
  • Author
    Posts
  • #36769

    Silva
    Member

    I am a manufacturing process development engineer. When I design a process, I always evaluate Cpk (sigma is evaluated by Rbar/d2). If I achieve a capability value >2, I conclude that my objective is achieved.
    But, subsequent to pilot lot production runs, I find that performance based indices, Pp (sigma is sample std. deviation) is between 0.8 to 1.2. I believe that it is the performance problem not the capability problem. If the process is controlled better sample std. deviation will be considerable less & better performance capability will be obtained.
    I am unable to convince my colleagues on this point. How do I approach the problem? Urgent help appreciated.
    Silva

    0
    #106896

    Mike Carnell
    Participant

    Silva,
    It isn’t about hitting a number as much as it is about the conditions you hit the number under. Anyone can build “lab queens.” That doesn’t mean you can manufacture them. If you get to 2.0 by hand selecting input parts and having maintenance techs running equipment rather than operators and any other “simulated control” then your 2.0 number doesn’t have anything to do with reality. It can be a nice number to have for selecting projects. It is the capability of your technology not your process. It is ok if you are selling technology but most of us sell products from a process.
    If you have ever been a supplier to an Automotive OEM then you know there is (or at least used to be) mating ritual that is done around DV and PV testing. It makes people feel good but it is a process that has become so bastardized that it amounts to sqat in terms of turning the process on.
    Lets see a difference between short term capability and long term capability. It does sound vaguely familiar. How big was that shift in capability? You said it got worse?
    Just my opinion.
    Good luck.

    0
    #106967

    S
    Member

    define DV and PV please….not familiar with that acronym… 
     
    Thanks..

    0
    #106973

    Savage
    Participant

    DV = Design Verification
    PV = Process Verification

    0
    #106976

    Mike Carnell
    Participant

    Matt,
    Thank you for answering that question. I missed it.
    Regards

    0
    #107046

    Chris Butterworth
    Participant

    Silva,
    If you claim to have a 2.0 capable process and production runs it at 0.8 then it is a 0.8 capable process. In manufacturing, there are many sources of variation that are often not captured in a pilot run. This is clearly the case.
    You stated that “If the process is controlled better sample std. deviation will be considerable less & better performance capability will be obtained.” This is true but it doesn’t help.You should dig into the process and learn about the sources of variation and help your Production team to get that process back to where it needs to be. You’ll get better at your job as well.
    Chris
     

    0
    #107094

    bjb
    Participant

    I guess I looked at it simplistically.
    —Capability— is how good  the current process could potentially operate (what the process is capable of achieving).  It looks at only a small amount of variation over the short term.  In order to achieve the same capability over the long term, you must put in controls to ensure that no additional sources of variation are introduced over the long term (multiple or new work cells, new operators, environmental changes, procedural differences, raw material changes, tooling changes, processing errors, etc.)
    —Performance– is how the current process is actually operating over a longer period of time (the actual performance of the process).  I looks at the total amount of variation over that time period.  The gap between the capability and performance are all the new sources of variation not captured in the short term capability study.
    There are more technical explanations, but this provides a framework for application of Cpk and Ppk.
    Thanks…Brad
     
     
     
     
     
     
     
     
     
     

    0
Viewing 7 posts - 1 through 7 (of 7 total)

The forum ‘General’ is closed to new topics and replies.