Using Cpk index for Accept / Reject Decisions

L

lday38

MY company has been using the Cpk value on the points plotted on their control charts to make accept/ reject decisions on their product when a poitn goes outside the limits.
However, the spc electronic file can have a months of data in it, the Cpk being reflected is through that period, not through the current production lot or subgroup. It is my understanding that Ford want you to sort when the Cpk vlaue is under 1.33.
I should also mention they monitor only twice a shift. I also
have set modified reject limits ( 4std dev) away form the min and max specification to get them to buy into spc. ( the old thing yyyyou are cutting our tolerances, the whole quality thing is new to this comapny) They were trained but in practice it is differant.
Can anyone suggest a workable way to meet the automitive requirments with this senario?
 
S

Sam

Cpk can only be used to make decisions about the process; not the parts.
However, if the data is several months old you can't make a reliable decision about the process.
You are correct, Ford requires 100% sort of product when the Cpk is below 1.33. Also read the notes atg the bottom page 72 of the QS standard.
 
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