Earl, let me offer 3 quotes.
The first I don't remember exactly, but everyone's heard it. (I say
"everyone", having told others any number of times "When I hear, 'Everyone
knows', I hear 'Not everyone knows'"). The one where Lord Kelvin says when
you can express your knowledge in numbers, you reached the stage of science
- or something like that.
"Understand variability", Deming.
"The key to success in engineering is understanding the variability in the
measurement process, and using that understanding to control the
variability in the manufacturing process", Hart. It could be said better.
Referring to your posting below, I would say that a process is in control
if it is consistent (stable) - its mean and variance are (to a chosen
statistical confidence) constant. In-control processes can produce lots of
bad product. Before determining a process is or is not in control, some
knowledge of the variability in the measuring process is needed.
Cpk is what I would call process performance index - a number. Cp is
process capability. Process must be in control before calculating these
indices. These indices depend on spec values, so are subject to
manipulation, especially if engineering, sales, management doesn't really
know what it wants.
Let me try to adopt the viewpoint of designers in the DFM picture. Is
manufacturing capable of providing technical guidance to designers with
regard to manufacturability, based on data and engineering rather than
speculation? In many cases, I suspect not. Maybe that is why it DFM seems
to take place so infrequently. If data and engineering are lacking, the
two groups may collaborate to generate them. Would that collaboration be
part of DFM? Lou Hart
-----Original Message-----
From: Earl Moon [SMTP:[log in to unmask]]
Sent: Monday, January 07, 2002 11:00 AM
To: [log in to unmask]
Subject: [TN] A SERIOUS QUALITY QUESTION
Kind folks,
Before I go off in the frozen tundra in my current MI location, to finish
my
book, I need some input on the following:
Process control is defined as using process capabilities
in an efficient, effective, consistent manner. To say a process is
in control means the process is operating without assignable cause.
If assignable cause is found, the process is out of control, and
must be corrected, redesigned, and re-characterized. If a process is
found to be in control it does not mean it is capable of meeting
product acceptance specifications. It simply means it is consistent ? good
or bad.
Process capabilities charts (Cpk) describe the relationship between process
control and product quality. When process capabilities are statistically
proven to provide specified product quality, they are capable of providing
product meeting customer/contract requirements.
The above factors should be considered at the design concept level. Then,
DFM/CE has a firm footing so a concept may be turned into a design capable
of being manufactured as a product meeting specified customer contract
requirements.
Just a bit wordy, as usual, but do these words have validity in today?s
?just do it? world. I mean, does what Juran, and others, have been
preaching
so long, have meaning for us in light of some of the previous discussions
concerning quality systems, ISO, too much restraint, not enough creative
freedom, and all the rest?
MoonMan
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