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Dear Jim:
The security at airports is getting more
and more ridiculous every day. It’s the only place where it has become
socially acceptable for a perfect stranger to ask customers things like,
“Would it be alright if I stick my hands down your pants?” I
want to reply, “Only if you buy me dinner first.” I actually
had six security guards search my carry-on briefcase so thoroughly that
they found a mini-bar key I had lost over three years ago. And, of course,
I missed my flight, which they don’t seem concerned about at all.
“Take your shoes off. Undo your belt. Fold the front of your pants
down. Do you have an explosive device surgically implanted in your chest?
(It was a metal button on my Levi shirt that set their wand off).”
These are all things that happen regularly at my visits to the airport.
What’s next, “Stand on one foot and hop in a circle while you
sing You are My Sunshine?” And it’s not just me. They do it
to old people, children, anyone. We’ve all heard the story about
the new mother holding her baby who was made to drink her own breast milk
she had stored in her carry-on case. No wonder the airlines are all going
out of business. No one wants to go to the airport anymore. It’s
like volunteering to be molested. I say if that is the way it is going
to be, they ought to allow us to choose the guard that molests us.Have
a line of great looking men and women and let us pick out the one that
gets to shove their hands down our pants. That way, it would involve mutually
consenting adults and be more like prostitution, instead of rape.
Agitated and Aggravated in Akron
Dear Agitated:
I don’t know. I have had many writers say they have made lasting
relationships with airport security guards recently. They say it’s
as close to romance as they get. Some have enjoyed their searches so much
they want to go through airport security three or four times. They volunteer
(and sometimes shamelessly beg) to be the one searched. It used to be
you had to go to bars to meet people and have intimate encounters. Now
you just have to go to the nearest airport.
Dear Jim:
I’ve been reading a book lately called
“Bible Code 2” that claims the world will end in 2006. It seems
a mathematician has found a hidden code in the bible that he has decoded
and it says we are all doomed. The way current events are unfolding, it
seems like he’s right! So, my questions are these, “Why should
I care about my future, when I don’t have one? What difference does
it make if I eat a gallon of Ben and Jerry’s Cherry Garcia ice cream
and drink a gallon of Jack Daniel’s Sour Mash every day for the rest
of my short pitiful existence? Why not be fat and drunk? Who cares?”
We are in the END OF DAYS.
Depressed and Doomed in Detroit
Dear Depressed:
The reason is simple. Although both Cherry
Garcia ice cream and Jack Daniel’s Sour Mash are unquestionably the
best treats in life (other than being felt up at the airport), they don’t
compliment each other. Let me suggest enjoying both, but on alternating
days of the week.
Dear Jim:
I have been reading on the ASME website
that you are the chairman of Y14.43, the document on Dimensioning and
Tolerancing of Functional Gages. When will this document be available
for purchase, where can I get it, and what will it cover?
Starved for Knowledge in Norfolk
Dear Starved:
The Y14.43 standard on Dimensioning and
Tolerancing of Functional Gages has been approved and is currently being
edited by ASME. It will cover rules, options and recommendations on the
design, dimensioning and tolerancing of GO gages to verify compliance
with maximum material condition limits of size, NOGO gages to verify compliance
with least material condition limits of size, and Functional gages to
verify compliance with Geometric Tolerances. It will also cover rules
and options on the design, dimensioning and tolerancing of fixtures used
to stabilize a part and establish datum reference frames to be used to
measure geometric tolerances with variables data collectors such as Coordinate
Measurement Machines.
It will establish: 1) rules that have been in practice, but not formally
stated in a standard, 2) rules that have been stated in older, but now
retired, standards and 3) rules and options that have never been fully
understood or available until now. It will be a standard of substantial
length, filled with important data for anyone involved in the design or
use of gages and/or fixtures, or those who just want a better understanding
of Geometric Tolerances and measurement practices. It is a display of
the physical embodiment of the theory of Geometric Dimensioning and Tolerancing.
I have always believed that anyone who doesn’t have a thorough understanding
of gages and fixtures cannot have a thorough understanding of GD&T.
I am not certain of its exact date of publication. I would expect it by
(at the very latest) mid-2003, but it all depends on how long it takes
the ASME editors to complete the process of readying the standard for
presentation. When published, it will be available for purchase directly
from ASME. You may wish to keep track on their website at www.asme.org.
Dear Jim:
What is the difference between the philosophy
of Geometric Dimensioning and Tolerancing as practiced and taught in most
companies and the philosophy of GD&T as it is scrutinized in Tolerance
Stack-Up Analysis?
Dear Writer:
The difference is an interesting one. In
regular GD&T, one assumes that the parts will (or could) be made to
their extreme conditions in manufacturing (consuming all of their assigned
tolerances) but will be assembled in the most optimal way possible. And
in that one way (if statistical tolerancing has not been used), the parts
will function and assemble.
In Tolerance Stack-Up Analysis, it is assumed the parts will (or could)
be made to their extreme conditions in manufacturing (consuming all of
their assigned tolerances) and will be (or could be) assembled in the
absolute worst way possible. It makes you consider whether or not tolerances
should be adjusted to compensate for these assembly blunders. One consideration
may be that the part is rocked one way on a convex or rough surface to
inspect it and rocked an entirely different way during assembly. It gets
very pessimistic and can get very trigonometric, depending on how far
the analyst wishes to take it. It is a fascinating way to better understand
plus and minus and geometric tolerances. It makes you examine with an
entirely different perspective the geometric “soup” of sizes,
shapes, angles and locations when all are stirred together on one part
or in one assembly. Then it even takes an odd turn by presenting Statistical
Tolerancing as an alternative to arithmetic tolerancing, and in doing
so, almost reverses the logic with which it began.
Statistical Tolerancing says that manufacturing probably won’t use
all of the assigned tolerances, so it calculates the amount likely to
be consumed and shows one how to increase the tolerances so that all of
the functionally available tolerance will now be the amount likely to
be consumed.
And, it does this while still maintaining the pessimistic
possibility that the assembler could still assemble the parts in a way
that is not optimal. If you have never looked at tolerances in a Tolerance
Stack-Up Analysis approach, I think you are in for an eye opener when
you get into it. If you are interested, take a look at the book description
of my latest book entitled Tolerance Stack-Up Analysis shown on this website.
The topics covered in this book are also the topics I cover in on-site
courses that I do on the subject and in courses open to the public that
I do for the University of Wisconsin at Milwaukee.
Dear Jim:
How do I tie different patterns of holes
together in such a way that makes people gage them with the same gage
or inspect them in one setup.
Dear Writer:
There is a rule in the Y14.5 standard that
says all you have to do is locate them from the same datums. These datums
must be used on all applicable patterns of holes (or shafts) in exactly
the same order with the same material condition symbols used or implied
after the datums. If they are, all patterns will automatically be considered
one pattern of holes (or shafts) and must be inspected with the same gage
or in the same setup. The local note SIM REQT (for Simultaneous Requirement)
may be used as a clarifying redundancy beneath or next to the feature
control frame.
By that same token, if you want to disassociate patterns from one another
that use the same datum reference frame, allowing them to be inspected
with different gages or in different setups (rocked differently on irregular
datum features such as rough or convex surfaces or shifted in different
directions to be allowed because of datum features referenced at MMC or
LMC), the local note SEP REQT (for Separate Requirement) must be used
near the feature control frame.
Be careful, though, because the Simultaneous Requirement rule only applies
to regular controls such as position and profile (capable of location),
but not in the lower level control (feature relating tolerance zone framework)
of a Composite Tolerancing control (two levels of control sharing one
position or profile symbol with the tightest tolerance in the bottom or
lower level control). Even in a composite control, though, the “Simultaneous
(Gaging) Requirement Rule” as it is known, does apply to the upper
(pattern locating tolerance zone framework) control that contains the
larger cost saving tolerance. If you want it to also apply to the lower
level (feature relating tolerance zone framework) with the tighter, more
functional tolerance, you must write a local note to the right of the
lower level that states “SIM REQT”.
Dear Jim:
Can controls like Perpendicularity, Angularity
or Parallelism locate holes, shafts and surfaces if enough datum references
are used?
Dear Writer:
No. They are capable of controlling angles
only if used on axes or centerplanes of holes, shafts, slots or tabs.
If they are used on planar surfaces (as surface controls), they are capable
of controlling flatness, straightness and angles.
Dear Jim:
If perpendicularity, parallelism, angularity
or position is used to control holes, shafts, slots or tabs, what is the
inspector supposed to find the angle or location of? Is it the median
points of all opposed elements on the surface, or is it something else?
Dear Writer:
It is something else. All of these controls,
if used on features of size (holes, shafts, slots, tabs, etc.), fall under
the concept of “mating size”. This concept is that the maximum
inscribed cylinder for holes and the minimum circumscribed cylinder for
shafts generate an axis. This axis is perfect but can be simulated through
the use of inspection aides such as gage pins or collets.
Technically,
all of these features of size generate a middle that is the perfect representation
of the largest gage pin that can be inserted into a hole, the largest
gage block that can be inserted into a slot, the smallest enclosing cylinder
that can be fit around a shaft or the smallest distance between two parallel
planes that can enclose a tab. Whether this middle is an axis or a centerplane,
this is what the inspector is trying to verify is inside of the tolerance
zone generated by a tolerance of position, perpendicularity, angularity
or parallelism of a feature of size. Now the logical extension of that
rule is the bad news that when an inspector uses a probe to try to inspect
these types of features and the probe contacts the actual feature surface
(hole, shaft, tab or slot) instead of the substitute for the feature (gage
pin, gage block, etc.), the results may deviate from acceptable limits
of inspection uncertainty if the feature being inspected is not well formed.
In other words, to a certain degree it is not being inspected in compliance
with the theory of the concept of mating size. On the other hand, if the
hole, shaft, slot or tab is very well shaped (very cylindrical in the
case of holes and shafts), there should be no appreciable difference in
the inspection results.
Dear Jim:
I love my job! But lately, I have begun
to worry that my boss doesn’t love me. At first there were signs
that he wasn’t happy with my work. I used to work very hard, but
nothing I did satisfied him.
He was always giving me hints. At first,
he would come into my office and fling reports I had turned in high up
into the air and then just leave. If I wrote new reports, he would shred
them. If I wrote more, he would get so frustrated that if I bent over
my desk, he would walk up behind me, and kick me square in the butt. So,
after a while, I stopped doing any work, figuring that if I didn’t
do anything, there wouldn’t be anything for him to get upset about.
But on successive days he caught me learning to juggle and to play the
ukulele in my office and he just couldn’t stop yelling. So, I began
spending a lot of time in one of the stalls in the Men’s Room. I
made it very comfortable by padding the seat with Styrofoam, installing
bookshelves and a cup holder. I would sit, read, drink coffee, eat sandwiches,
make calls on my cell phone and talk to people in the other stalls. I
was able to avoid seeing my boss for about a month. The guy was like a
camel. He never used the restroom. Then one day I was in there having
lunch and reading the company newspaper and I saw a job just like mine
was listed. I’m still thinking about applying for it. The next day
one of my co-workers happened into an adjoining stall and when he recognized
my voice singing along with my Walkman, he invited me to a Going Away
party. When I got there, I found out it was for me! I said, “But
I’m not going anywhere.” And they said, “That’s what
you think.” What knuckleheads!
My vacation is 3 months away! On my
way back to the Men’s room, I noticed someone was changing the lock
on my office door. Do you think anything is wrong here? Should I come
out of the restroom now?
Dear Writer:
You can come out now. It sounds like the
problem is solved.
Mr. Jim Meadows:
In your February 1999 newsletter, you mentioned
gage guidelines (also addressed in greater detail in your 1998 textbook
entitled Measurement of Geometric Tolerances in Manufacturing). In the
guidelines, you state that it is recommended to only add material to the
GO or Functional Gage when tolerancing the gage. What if you are measuring
a hole greater than 5 inches? At this size, I would believe, slip and
interference fits would need to be considered. What would happen if for
a given diameter, one needs .002 of an inch clearance for a tight sliding
fit? Meanwhile, one toleranced the gage pin at –.000 and +.001 at
the max end. This would allow any hole that measures +.001 to + .002 to
pass when it actually should fail. Thus, gaging policy #1 would be incorrect
because you have passed a bad part. In addition, would one not also have
to consider sliding fits on the min spec side as well when dealing with
large diameters?
Please give me any suggestions that you
might have on this subject.
Thank you,
Big is Different
Dear Big:
First of all, let me say that these policies
and many variations on each theme (given in my textbooks and the soon
to be released Y14.43 standard on Dimensioning and Tolerancing of Functional
Gages) allow each company to use the policy that best fits any situation
they may run into. This is not a dictatorial situation wherein you must
always choose the same approach for every situation but allows you to
decide for a given situation what the best gaging policy would be, then
to write that into the inspection plan for the part being considered.
Having said that, in the situation you present using gage policy #1 (Absolute
Gaging),
I do not see how the gage pin using 5%-10% of the part tolerance on the
plus side of the “size” you begin with and zero tolerance on
the minus side could possibly allow a bad part to pass inspection. This
type of tolerancing policy infringes (denies) the part being inspected
a small portion of the tolerance range toward one end of its extreme (the
mating end). So, instead of buying a bad hole, it would actually run the
risk of rejecting a barely good hole that was made almost too small. This
would ensure that no hole that was a line fit in the assembly (at the
extreme end of its smallest size) would pass inspection (even though technically
it would meet the specification). This gaging explanation applies only
at the GO gage (checking MMC) or Functional gage (checking virtual condition-MMC
concept on geometric tolerances) end of the business. Let’s not forget
there is also a requirement for checking least material condition (NOGO
gage use or simulation) to make certain the hole is not too big. This
inspection of LMC is an entirely separately verifiable requirement
.
So, following the Absolute Gaging policy as it is called in the Y14.43
Dimensioning and Tolerancing of Functional Gages standard runs no risk
of buying a bad part (no matter how big or small the hole is that is being
gaged).
The only reasons I can think of that might
make you believe there could be a problem here are:
-
that you are thinking that the gage must begin its
size at the absolute extreme (which is not true--remember that 5% of
the 5%-10% tolerance rule is wear allowance and can be added to the
size of the gage pin before the other 5% is applied as size tolerance)
or
-
that one gage is supposed to do all jobs (also not true). Remember that
there would be one gage to inspect the LMC, one gage to inspect MMC
and another to inspect the Virtual Condition generated by the geometric
tolerance referenced in the feature control frame at MMC. Each gage
(if toleranced using the Absolute Gaging Policy) would be toleranced
in such a manner as to deny the part feature being gaged some (5%-10%)
of its assigned tolerance range. And in doing so, could reject a borderline
technically good part, but not accept a borderline (or otherwise) bad
part.
Again, having said that, you and your company
are free at any time to decide to use another tolerance amount, a different
distribution of the tolerance, another starting size for the gage pin
or even another policy. It’s your company and your product. Who knows
more about its needs than you?
Remember though, it is the opinion of the Y14.43 committee that the policies
wherein gage holes are allowed to be toleranced on the plus side and gage
pins are toleranced on the minus side of the boundary (MMC or Virtual
Condition-MMC Concept) you are inspecting, could possibly accept a bad
part.
The Optimistic Gaging Policy doesn’t deny the part a portion of its
assigned tolerance. In fact, it adds tolerance to the part by going outside
the part tolerance for the gage tolerance. It says gage holes are toleranced
on the plus side and gage pins are toleranced on the minus side of the
boundary being gaged (MMC or Virtual Condition-MMC Concept).
The Tolerant Gaging Policy both adds and subtracts tolerance from gage
holes and gage shafts around the boundary being gaged. This means the
gage may be made at a size that either accepts a small portion of technically
bad parts or rejects a small portion of technically good parts (and no
one knows which until the gage is produced and measured).
If you choose against the Absolute Gaging Policy (where only good parts
are accepted) the Optimistic and Tolerant gaging policies are open to
you, but don’t seem like the best or safest way to go in almost all
situations to the Y14.43 committee.
You also seem to think that size matters. For example, a hole measuring
greater than 5 inches, as you explain it, should follow a different set
of rules than anything smaller. I cannot see how that 5 inch hole size
takes on any special need for a different rule system. But again, the
choice is yours. Y14.43 and the gaging standards community merely shows
you the tools available in hopes you will make a decision for your products
that is best for your customers.
I hope this helps.
James D. Meadows
Chairman Y14.43
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