Sounds like the process Capable but the data is not centered about the
target which will have an impact on Cpk.  Example - a machine ft/min
spec is 15-25 and the machine is running 20-25 - Process Capability is
good but Cpk is low.  This machine could be running 30-35 still with a
good Process Capability but with a negative index.

Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED 
           76 Technology Drive - POB 1890 
              Brattleboro, Vermont 05302
                Voice - 802.257.4571 ext 37 
                    Fax - 802.257.0011
                       <http://www.vtcircuits.com/> 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ryan Grant
Sent: Wednesday, March 28, 2007 11:39 AM
To: [log in to unmask]
Subject: [TN] Explanation of Cpk (Ppk)

Hi all,

I have a supplier that is wondering why we rejected a shipment for
finding a critical dimension as having a Cpk of 0.65 (Ok, technically,
we are calculating Ppk).  None of the 45 measurements were actually out
of spec (although one measurement just missed exceeding the upper limit
by 0.002mm).  Now they want to know why we are rejecting for low Cpk.
Sigh...  

Rather than go through the tedium of carefully explaining what a
distribution is, what it means, how to calculate it and what the
calculation implies, etc., does anyone know of some websites I can point
them to that can explain this?

Any help is appreciated.

Thanks,
Ryan Grant

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