I've never seen a method for calculating the traditional Cp and Cpk for
product attribute data. One book I have says that you take the average
output (yield) and compare with "management expectations". It goes on
state that "if the level is unacceptable, further analysis and action
must be directed toward the process itself (management responsibility)."
Their parentheses, not mine.
Now that I've stopped rolling on the floor laughing and wiped the tears
from my eyes, I'll continue. If you subscribe to the theory that SPC is
a means to end, a tool by which to improve processes and capabilities,
putting up the control chart, tracking yields and responding to the out
of controls is probably adequate to accomplish your goal.
If you have to have a Cpk number, why not reverse engineer it? After
all, the Cpk value is only telling you what the expected yield of the
process is. As an example, a process capability of 1.00 means that you
have an expected defect level of 1,350 ppm on a one-sided specification
(which is applicable to a yield attribute). Thus if your attribute
control chart showed you had an average yield of 99.865%, your Cpk would
be 1.00.
----------
From: Hinners Hans CIV WRALC/LYPME
[SMTP:[log in to unmask]]
Sent: Thursday, September 09, 1999 2:55 PM
To: [log in to unmask]
Subject: Re: [TN] Process capability
Eric and all,
How about tracking the yield through the process/machine
over time -
per shift, daily, weekly, whatever makes sense? Example, I have
95% yield
so I know for every 100 scheduled/produced I can expect, on
average, to get
95 good ones to go to the next step. I can't tell you which
ones are good
until I measure/inspect something.
I think of process capability (or the index), Cp or Cpk,
as a means
to compare the ability of a process/machine to produce something
within
tolerance. It tells me whether I will go broke trying to make
something.
Hans
~~~~~~~~
Hans M. Hinners
WR-ALC/LYPME Bldg.
640
Materials Engineer
380 Second
Street, Suite 104
Manufacturing Eng. Sec. Robins
AFB GA
31098-1638
912-926-1970 (Voice) 468 - 1970 (DSN) 912-926-7974
(Fax)
[log in to unmask]
>
> What is the best way to determine the process capability of a
machine in
> terms of defective units produced? How can I say that a
certain machine is
> capable only of producing a certain no of good units everytime
it is being
> run? Any inputs will be highly appreciated.
>
> Eric
> FAAI
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