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Before I begin to answer technical questions in this newsletter, let
me tell you about my two new books. One was written in response to
a question which doesn’t appear in this newsletter. It was from a
gentleman who asked why there were no books comparing the ISO and
ASME dimensioning and tolerancing rules and symbology. Other than a
very short, very outdated pocket guide, he was right. There was
nothing I could find on the topic. So, I wrote an 80 page,
absolutely up-to-date book on that very subject. Since the book
references about 20 different standards and technical reports, it
will constantly be updated as new standards are issued. But as of
today, it is the latest word on Differences and Similarities
between ASME and ISO Dimensioning and Tolerancing Standards.
The other book, entitled Geometric Dimensioning and Tolerancing
in 2007, is 537 pages long and is meant to take the place of my
other textbook on GD&T, which was published in 1995. Although both
books are still current and comply with rules in the most current
ASME Y14.5M-1994 standard, the GD&T in 2007 textbook offers some
insights and information not contained in the other book. For
example, it quotes rules found ASME Y14.43-2003 on Dimensioning and
Tolerancing Principles for Gages and Fixtures, and ASME Y14.41-2003
on Digital Product Definition Data Practices. It also covers a lot
of information on tolerance stack-up analysis and statistical
tolerancing methods and specificity that are not found in my 1995
GD&T textbook
Below, I’ve included a single illustration from each of my new
books. The first is a charted comparison of controls commonly used
on round surfaces from
Geometric Dimensioning and Tolerancing in 2007.
The second is a chart of symbology from ISO 1101:2004 and explained
in
Differences and Similarities between ASME and ISO Dimensioning and
Tolerancing Standards.
Geometric Dimensioning
and Tolerancing in 2007, by James D. Meadows (537 pages).
Differences and
Similarities between ASME and ISO Dimensioning and Tolerancing
Standards by James D. Meadows (80 pages).
For further information on either of these books, please
see Online Store page.
Now, on to the technical questions.
Subject: Question on Reporting Position
Hello Jim,
We are having discussions with Chrysler Dimensional
Engineering group
(Contracted by Chrysler from QMC out of Auburn Hills) on how true
position should be reported.
Say we have a feature frame
(Position /
Diameter of 6mm at MMC / A-D / B at MMC /C).
We report this giving the actual and deviation from nominal in X, Y,
and Z
Nominal Actual Deviation
X - 175.67 177.83 2.16
Y - 89.02 90.05 1.03
Z - 444.62 444.89 .27
Then we also report the actual true position which is also
considered the
deviation as 0 would be best, but you can have up to 6
Nominal Actual
Diameter of 6 2.12
Chrysler is stating that this should be reported in two ways
1) deviation from nominal for position
2) radial position
Our problem is the feature control frame calls this feature a
diameter so
with the print being the bible in this instance how can we report
radial.
Any ideas or suggestions? Is there a standard in ASME Y14.5M-1994
that I am over looking for reporting? Is there an industry
standard?
Thanks in advance
Mark
Mark,
There is no
standard that dictates how you report deviations from true
position. You can report it in any way that best serves your
organization. At any rate, I don't see a problem with reporting the
allowed tolerance (which is stated, as you say, as a diameter in the
feature control frame) as either a radius or a diameter. As long as
everyone knows that you are either speaking in radii or in
diameters, there is no real difference. So, if you are allowed a
diameter of 6 mm, you can say you are allowed a radial deviation
from true position of 3 mm. If you are out of position a diameter
of 2.12 millimeters, you can say you are out of position a radius of
1.06 millimeters.
As long as
everyone agrees to report in either diameters or radii, it just
doesn't matter.
Jim
Subject: Question on Whether the Box Goes Around the
Number of Features When Using Basic Dimensions
Jim,
Our group has your VHS tapes of your Fundamentals of GD&T training
and they've been extremely helpful in teaching me the basics of
GD&T. One question that is causing some discussion within the group
is whether it is correct to put the 3x (for example) within the
boxed (basic) dimension (if three features all are the same distance
from a datum plane) or if it must stay outside the box? Or if it
even necessary as in the ASME Y14.5M-1994 standard on pg 107 they
aren't calling out the number of features (e.g three holes 6.4 up
from B) at a specific dimension.
Thanks for the website and any comment you're able to send our way.
Derek
Lawrence
Berkeley National Laboratory
Derek,
I usually get
this question about basic angles. For example, if I use 3X 120
degrees, does the 3X go into the box, or just the 120 degrees. The
answer is either. The Y14.5 standard shows examples of both. As a
personal preference, I usually leave the 3X outside of the box and
just put the dimension inside. But, since both are legal, I'm not
consistent with that preference.
The answer is
the same for dimensions other than angles. And, as you mentioned,
if the surfaces are coplanar and lines are extended from one surface
to another (to show they are not offset), it is unnecessary to even
include the number of surfaces. Redundancies such as stating the
number of surfaces, even when it is evident, aren't forbidden or
even frowned on by ASME standards. Clarity is the key goal for all
drawings and whatever you can do to make the drawing the most easily
interpretable is usually the best practice.
Labeling
datum features that include multiple coplanar or coaxial surfaces or
giving geometric tolerances to multiple surfaces such as these (as
in assigning profile to control coplanar surfaces or runout to
control coaxial surfaces) can be confusing unless the number of
surfaces is stated or multiple leader lines are used.
I used to
conduct training at the Berkeley Space and Science Labs years ago.
It was always interesting work and great people to work with.
Jim
Subject: GD & T Question on Composite Position
Tolerancing
Jim,
I was a
student at one of the GD&T classes you teach at UW-Milwaukee. I
attended about a year ago. An interpretation of a customer print
has come up which has all of us at MKC discussing. If you would be
so kind to give us your interpretation, it would be greatly
appreciated.
We have a situation where composite position tolerancing is being
used. The upper true position segment is not in question. The
lower position, being a more restrictive tolerance, refers to datum
-A- only which includes perpendicularity of two holes to datum -A-.
Are the two holes in question considered a pattern when their basic
dimensions are only independently dimensioned from datums -B- and
-C- with no basic between them?
The question is: There is no datum reference in this frame from
datums -B- and -C-; and since the two holes are defined
independently from datums -B- and -C-; do they remain unlocked or is
a basic implied resulting in a pattern requirement for
perpendicularity and position to each other?
Jim, we know the intent of the design which is a pattern. We just
want to confirm the GD&T does not convey the intent.
Thanks.
Gary
Gary,
As long as a
basic dimension is calculable between the holes within the pattern
from the other basic dimensions on the drawing, the drawing is
correct and the lower segment of the composite position control with
the tighter tolerance applies to the distance between the holes.
Hope this
helps.
Jim
Subject: Question Regarding Showing Inches and
Millimeters on the Same Drawing
Jim,
Long time no talk.
I’m a former student of yours at 3M. I have a question about
tolerancing on a
print that I can't find the answer in any of my books.
When English and Metric tolerances are listed, is there a correct
method
for doing so?
Example: d
.033 ± .002 (0.84 ± 0.05)
For each dimension, we put the metric dimension in (Parenthesis) and
sometimes [Brackets]. Is this OK?
Also, do we need to repeat the Diameter sign in the metric callout?
Redundant? And to get picky, is there a spacing procedure as in
between the diameter sign and the size?
These are some questions I'm getting. If you could shed some light
on this one way or another, I would appreciate it.
PS. Is ASME Y14.5M-1994 still the current standard?
Thanks for your help.
Regards,
Paul
Paul,
In prior
versions of the Y14.5 standard, they showed methods, like the one
you list, to allow both inches and millimeters on the same feature.
Even though the one not shown in brackets took precedence, there
were legal problems with companies meeting one of the tolerances,
but not the other (since, with rounding, both numbers were not
exactly equivalent). So, in recent versions of the standards, this
practice is not shown.
As far as the
rules, if you decide to do it anyway, the second number would be
assumed to have the same shape tolerance zone as the first, so
repeating the diameter sign is not necessary.
Since these
practices have not been allowed per Y14.5 since prior to 1982, I've
attached a couple of illustrations of how they looked when they were
allowed from a very old Y14.5 standard.
Yes, ASME
Y14.5M-1994 is the current dimensioning and tolerancing standard.
There are other more recent standards that augment that standard
though, like ASME Y14.41-2003 Digital Modeling Definition Data
Practices and ASME Y14.43-2003 Dimensioning and Tolerancing
Practices for Gages and Fixtures.
Hope this
helps.
Jim
Illustration from ANSI Y14.5M-1982,
Appendix D Former Practices
Illustration from ANSI Y14.5M-1982,
Appendix D Former Practices
Subject: Question on Assigning Datum Feature Symbols
Jim,
Another question for you if you have time.
About datum
selection: We have a pattern of holes on a plastic connector. It is
not critical to the outside of the part. Can I use the pattern
centerlines in each direction as the datums? Legally to ASME
Y14.5M-1994, some say it is not a feature size, so I can't. See
attached sketch for more info. (See attached file: Datums.doc)
Thanks for your help. Please reply to all with your response.
Other 3M'ers need this info as I might be away.
Regards,
Paul
Paul,
No, that's
not legal. As the others have said, it isn't a feature of size.
However, you can use one, two or all of the rectangular hole widths
as a single datum feature (one) or a datum feature pattern (more
than one). In my yellow textbook, on page 148, I show an example of
rectangular hole widths being used to construct a datum reference
frame. There are also examples of pattern datums being used on page
488.
Hope this
helps.
Jim
Illustration
from yellow textbook page 148 below:
Illustration
from yellow textbook page 488 below:
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