Hi Michael, Eric and all,
Now that I've stopped rolling on the floor laughing and wiped the tears
from my eyes, I'll continue.
Laughing is better than crying.
Do they an English translation for that book?
A SPC book used the word 'management', Oh boy.
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 aren't going to make money using SPC why use it?
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.
Here's how I understand it,
A high capability process should detect all process mean changes which
produce out of spec. product.
Otherwise, you will sell the customer stuff that doesn't meet her
requirements and even worse not know which ones are bad.
Cpk measures the distance between the process mean and both the upper and
lower specification limit on a mean chart at half width.
Cpk = lesser of (Upper Specification Limit - Process Mean)/3 Sigma or
(Process Mean - Lower Specification Limit)/3 Sigma
I think this makes the assumption that the random variations range only six
standard deviations (Sigma) but it's been a while.
Questions for Eric,
Which processes are we talking about here?
Are you already using SPC?
Is the process in control?
What are your spec. limits or are you trying to establish them?
Hans
Time to go home and get ready for the Hurricane!
~~~~~~~~
Hans Michel 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]
----------
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
~~~~~~~~
>
> 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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