Locking Control Limits and SPC Sample Sizes (AIAG/Automotive Requirements)

dave.d

Starting to get Involved
#1
Does anyone know of automotive OEM requirements on when to lock control limits and/or requirements of the number of samples required prior to calculating control limits?

We have been discussing the phenomenon of a stable process with control limits narrowing to the point of a nonconformance occurring due to a point being out of control. I've seen reference to "locking" control limits and am wondering if anyone is familiar with either IATF/AIAG or automotive OEM requirements on how/when this is done.

Thank you,

Dave
 
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Steve Prevette

Deming Disciple
Leader
Super Moderator
#3
I would definitely NOT recommend "locking" the control limits. They would then simply become specification limits and would not be based upon the current data. If the variation has been reducing with time due to improvements (yea!) you should follow the data and narrow the control limits if you understand what the change was that cause the variation to reduce. That is an important point Dr. Wheeler pushes - you do want to be sure you know WHY the process changed before you change the control limits. In my experience, even if you do not know, you eventually need to "follow the data".

If a process has been improving, with improved data and control limits in your SPC trending, you would still want to examine why you got an out of control (but perhaps in specification) result. Graded approach is acceptable - but the idea still is to head off a problem BEFORE you end up making out of specification components.
 
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