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Dear Readers:
I haven't written a newsletter in quite
a while. I have been preoccupied writing another book. The new book is
finished now and I have conducted several courses based on it, so I have
been adding to the original text and refining the material. That also
seems to be complete now. The book is on Tolerance Stack-Up Analysis and
I wrote it because of a conversation I had with a former student who is
now the expert in Geometric Tolerancing for his company.
He contacted me a while ago saying that
he wanted to verify an answer that he, six other experts and a computer
program had given him on whether or not parts in an assembly would interfere
or not. He said they had all arrived at the same answer with some of the
numerical answers differing, but with the same basic conclusion. "The
parts won't fit together if they span their tolerances. They interfere.
On this we are certain. We just wanted one more verification." So,
I looked at their numbers, and given their premise, they were right. The
parts would interfere. But the premise lacked logic. In fact, it defied
all possible logic. "These parts fit together just fine." I
said.
"No." he said. "You don't
understand. I have done the calculations. My six experts have done the
calculations. The computer program has done the calculations. We all agree
that there is no way these parts, if produced at their tolerance limits,
will assemble."
In time, I was able to point out the error
in their premise. It took a while to get him to listen closely enough
for him to see it. When he paused for a moment and uttered an expletive,
I realized he had gotten the point. It was an eye opener for me as well.
For decades now, even though I was taught Tolerance Stack-Up Analysis
by one of the world's experts on the topic, I had always shunned writing
on the subject and teaching full blown courses on it, because I considered
it boring. It was simply a few math formulas, assigning positive and negative
values to numbers, charting and graphing out the results and arriving
at the answer. And whether that answer was a minimum gap, maximum gap,
minimum interference, maximum interference, overall housing requirements
or whatever the unknown happened to be, it seemed uninteresting to me.
It seemed to lack logic and analytical reasoning and challenge. But what
I discovered in this short discussion with my former student was that
these were the exact missing elements that made them go wrong in their
analysis.
"You used the clearance between all mating features
to make the parts interfere." I said. "You took the clearance
between features like the screws mounted in their threaded holes and the
clearance holes they are supposed to fit into and pushed them until, in
your calculations, they crashed through walls, and you called this interference.
Not only is that illogical, it is impossible. It would require the screws
to be long enough that you could pass the screws through their clearance
holes, mount the screws into their threaded holes, then raise the part
that contained the clearance holes up out of the cavity that it fits into.
Then you would have to push the part with the clearance holes to one side
until the screws were up against the side of their clearance holes. Then,
in this position, you would have to try to slide the part with the clearance
holes back down inside the cavity. And yes, at that point it would interfere.
But that isn't how people use clearance between a hole and a screw. They
use it to move the part around until it fits, not until it interferes."
It was at this point that it hit me. In Geometric Dimensioning
and Tolerancing, our calculations and part definitions are meant to say,
"Even if the parts are made as bad as they can be made, we have deduced
that they will still fit together." What we sometimes forget is this
premise is based on the assembler using the optimum assembly conditions.
The assembler would not try to push the parts through the walls of the
mating part and would not attempt to use clearance to push the parts around
until they were made to interfere.
But, in fact, that is exactly what traditional Tolerance
Stack-Up Analysis does. It not only assumes the worst case in terms of
how the parts will be produced (at the extremes of their tolerances--as
GD&T does), but it also assumes that the parts will be assembled in
the worst possible (and in many cases--impossible) ways. It was like a
building falling on me. It was a basic difference in the ways traditional
GD&T looks at assembly conditions and the way Tolerance Stack-Up Analysis
looks at them.
At that point, Tolerance Stack-Up Analysis became interesting
to me. Because, if you assume the worst assembly conditions without logic,
judgment or experience, the amount of things that can go wrong are almost
infinite. And that is exactly what these Tolerance Analysts and computer
programs on Tolerance Stack-Up Analysis were trying to predict. And the
more I looked at it, the more impossible, but interesting, it became.
You may not be able to predict every little thing that could go wrong,
but you would need to employ a ton of logic, judgment and knowledge about
manufacturing, inspection and assembly to augment the mathematics we commonly
use to even make a good guess.
So, I wrote a book. And in this book it teaches all
the things my teacher taught me all those years ago about Tolerance Stack-Up
Analysis. But it also goes much farther. It shows how to use these techniques
with some of the new and difficult concepts of Geometric Dimensioning
and Tolerancing, and it also shows how to consider what would happen if
they measured a part in one orientation to get it to pass inspection,
but attempted to assemble it in another orientation. And it shows how
this can compound itself over a series of parts.
At first, I showed what a traditional Tol. Stack would
calculate. Then I introduced logic to determine what was likely, or even
possible, that the traditional Tolerance Stack-Up Analysis approach did
not consider. What if there were several geometric controls on a part,
how would one determine which would be pertinent for a calculating a particular
unknown? What if a datum feature rocked, and an inspector rocked it one
way to inspect the part, but the assembler rocked it another to assemble?
And worse, what if the inspector did what the Y14.5 standard tells them
to do, rock it to an optimum position, say shim it up on either side of
the rock point (which may or may not be in the middle)? Then the assembler
gets it and just rocks it over one way to try to assemble it. And what
would the effect of all this rocking, say vertical rocking, have on the
horizontal housing requirements on an infinite series of parts, rocking
and rocking until the entire assembly curled completely around on itself
creating not a vertical stack of parts but, instead, a circular stack
of parts? To calculate this, one would have to deduce that it was possible
to have differing approaches between inspection and assembly and both
different than the person who applied the tolerances in the first place
anticipated. For this calculation you would need logic, analytical reasoning,
knowledge of the rules of dimensioning and tolerancing, and the techniques
of manufacturing, inspection and assembly.
And maybe not last, and certainly
not least, one would need knowledge of Proportions, Trigonometry and Algebra.
I asked myself what computer program had I ever seen demonstrated that
could do all of these things. And the answer was none of them. So, I wrote
a book that addressed them all.
But please don't get me wrong. The analysis still needs
you. This book shows what to consider, how to do the calculations, how
logic comes into play and it suggests strongly never to blindly trust
what a computer program or a strictly mathematical approach tells you
is true. It's like music without lyrics. It needs both words and music,
or in this case, logic and mathematics to be complete. And maybe, just
maybe with both, we get an answer that is close, an answer that almost
considers all things that can possibly go wrong. But in the end, we must
remember, it is an approximation, a fine guess, a beautifully calculated
and logically deduced unverifiable supposition.
Just consider this, without all of these components:
the calculations you perform are anemic, without substance. You may as
well take a wild fling at a dartboard with its variety of random numbers
and pray you land on the correct answer. So, remember, words and music,
logic and mathematics. One needs the other. And for you connoisseurs of
long drum solos, who think words are unnecessary pauses between the wondrous
thunder, I may be wrong about words and music, but logic and mathematics
is a sure thing.
For those who want to share in the results of my studies
in the area of Tolerance Stack-Up Analysis, see the section on this website
under Books, On-Site
Training and/or Public Workshops.
Dear
ASTE:
We make products that are unique. We don't make hundreds
of thousands of parts that have to interchangeably fit into any machine
we make. Therefore, it not only seems to me that we don't need Geometric
Dimensioning and Tolerancing, but we don't need tolerances at all. When
we get a part in from one of our vendors, we just grind it until it fits
into the assembly.
What benefit does GD&T have for us?
Satisfied in Syracuse
Dear Satisfied:
Just how much time is spent grinding things to fit?
Wouldn't it be more efficient if you were able to just take the parts
as they come into your facility and assemble them? And what if one of
your machines breaks down when your customers are using them? How can
you ship them a replacement part? It seems to me that although having
parts that fit interchangeably offers many benefits for those making thousands
of the same thing, it would beat the heck out of all those "grind
to fit" situations you are so satisfied with even if you do make
only one of each.
Dear
ASTE:
My feet are sore and my head feels like it wants to
bust wide open and spray its watermelon seeds all over the wall. I just
can't take it anymore. My boss acts like I'm his slave. He calls me at
home and orders me to come back to work at all hours. He beeps me and
calls me on my cell phone. In this new age of technology, there just isn't
any way to hide from him. I don't even get enough time to get a good night's
sleep. He has no life and he wants to make certain anyone working for
him has none as well. I'm at work so much I don't even remember what my
kids look like. My dog bit me last night when I finally got home at two
in the morning. My wife keeps getting my name wrong. I think when I finally
get to spend time at home, everyone there will have moved out. What the
heck can I do?
Troubled in Tucson
Dear Troubled:
How should I know? If I had any answers to the problems
that go with balancing work and having a happy home life, I wouldn't live
in hotels. Ask someone else. I'm a consultant.
Dear
ASTE:
I believe that cell phones mark the end of civilization
as we know it. People may as well walk around with these things surgically
implanted in their craniums. They drive and talk, eat and talk, talk and
talk. Talk, talk, talk! Is it really necessary to call someone from the
video store and describe every single movie in the store? Do you need
to call from the bookstore and read the book jacket to everyone in the
house? If you go to lunch with someone, shouldn't you expect to talk to
him or her, instead of watching them dial number after number and discuss
any inane thought that jumps into their big, old, empty head? These things
ring in training classes, in theaters, in malls, in restaurants, and even
in elementary, middle and high school classrooms. And no matter who is
being disturbed and interrupted by them, these people are compelled to
answer these ringing or chirping irritants and say earth-shattering things
like, "Not much. What's up wit chu?" These people are so desperate
to make everyone around them believe others are in need of their contact
that they could be standing next to a real person and refuse to talk to
them unless they could call them on the cell phone. It's pitiful and irritating!
I was standing in line behind several people waiting
to rent a car at the Dallas Airport and the guy at the counter could not
tear the customer there away from his phone conversation long enough to
get the information he needed to get the line moving. A huge southern
boy that was behind the "talker" put his hand on his shoulder
and offered to shove the phone up his…well never mind. But suffice
it to say the rest of us began to spontaneously applaud.
On one of my recent flights, a flight attendant had
to rip a cell phone from the ear of a passenger, just so we could take
off. He absolutely would not do it on his own.
What's wrong with everyone? Have they lost their collective
minds? No one cares very much that you are in the car, three blocks from
home, now two blocks from home, now one, now pulling up into the driveway,
now opening the door. My God! Give the rest of us a break! I can't take
it anymore!
Dear Reader:
I'm wit chu. I've had to take people by the arm and
lead them out of seminars I've conducted, and they were so wrapped up
in their cell phone conversations that they didn't even realize they were
being led out and talking louder than my lecture. And, furthermore, they
acted surprised and insulted when they found they were temporarily ejected
just so the rest of us could get on with our trivial seminar. And yes,
they were actually saying things like, "Not much. What's up wit chu?"
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