The Language of GD&T
Excerpt from article
in Quality Magazine, May 1997.
Anyone who has marveled
at Leonardo Da Vinci's stunning renderings of his own designs for flying
machines and submarines knows that design drawings have existed for as
long as man has had product ideas. While today's industrial part
drawing may not have the artistic impact of a Da Vinci masterpiece, the
fact remains that properly rendered engineering drawings represent the
quality professional's incorruptible standard against which to measure
In 1935, with the publication of the American Standards Association's
"American Drawing and Drafting Room Practices," the first recognized standard
for engineering drawings was established. Steady progress was made,
in the United States and abroad, throughout the World War II years to
define and specify the symbols and terms used in functional dimensioning.
to 1997. In this era of tighter and tighter part tolerances, CAD-generated
drawings, and digital transfer of parts data---and when many university
engineering curricula no longer include any kind of drafting coursequality
professionals could be forgiven for wondering if there was still a reason
to learn this special international language of engineering drawings.
However, many are finding out that a working knowledge of geometric dimensioning
and tolerancing (GD&T) is necessary if they want to truly understand
the designer's intent, and therefore plan accurate and appropriate inspection
of the product the drawing represents. As part tolerances get tighter
and tighter, it's more important than ever to establish a realistic, agreed-upon
part drawing that subsequent inspections can be measured against.
If that describes your inspection situation, experts suggest it's a smart
idea to brush up on your knowledge of this precise language of terms and
symbols, as well as ASME Y14.5M-1994 (American Society of Mechanical Engineers,
New York), the applicable standard.
What GD&T is
is a precise mathematical language that describes the size, form, orientation,
and location of part features. It’s also a design-dimensioning philosophy
that encourages designers to define a part based on how it functions in
the final product.
Through the use of functional dimensioning, tolerances are assigned to
a part by the designer based on the part's functional requirements, often
resulting in a larger tolerance for manufacturing. This eliminates
problems that result when a designer assigns arbitrary, or too tight,
tolerances to a part in a drawing because he or she doesn't know how to
determine a reasonable, functional tolerance.
"Two people involved in every design need to understand GD&Tthe
designer and the inspector," said consultant Alex
Krulikowski of Effective Training Inc., Westland, MI. Krulikowski
is the author of a self-study workbook based on the revised ASME standard.
"CAD drawing systems do an outstanding job of displaying nominal geometries,
but the designer still needs to put in allowable variation, and GD&T
symbols are the way those tolerances are expressed on the drawing," said
Krulikowski. He added that, ideally, there is a product team participating
in the design process, including those involved in assembly, manufacturing,
and quality. GD&T can provide uniformity in the specification and
interpretation of the drawing, eliminating guesswork and erroneous assumptions,
and ensuring that professionals in design, production, and inspection
are all working in the same "language."
"There should be a quality/inspection plan for every CAD design, and inspection
specifics should be notated in the drawing," he said. Krulikowski added
that the drawing could specify such quality information as the following:
- How will the part
- How frequently
will the part be inspected?
- What tools will
be used? Hard (functional) gages? Coordinate-measuring machines
- If CMMs are to
be used for inspection, how many data points will be taken?
- What is the reliability
and reproducibility of the gages to be used?
- How much may the
part deflect during inspection?
- Will the part
be clamped or fixtured during inspection? If so, where and with how
Brushing up on your GD&T knowledge is important for another reason--the
looming problem involving automated inspection equipment that does not properly
measure parts in conformance with ASME Y14.5M-1994. Without adequate
knowledge of functional dimensioning, it will be harder for the quality
professional to pinpoint measurement discrepancies during inspection.
The basic discrepancy between functional dimensioning and automatic inspection
is that the above-mentioned Standard assumes that parts are measured with
hard, or functional, gages-simple go/no-go plug and ring gages, for example. When
today's CMMs, which measure points rather than surfaces, are used to inspect
parts that were dimensioned and documented to conform to ASME Y14.5M-1994,
problems may result.
Most CMM software, with a couple of rare and expensive exceptions, does
not compute basic measurements, such as hole size. Instead, the software
uses least-squares/best-fit mathematical substitutions to perform inspections. This
discrepancy could lead to CMM inspection passing bad parts and failing good
ones, even when performing basic measurements such as flatness, perpendicularity,
According to Richard Jennings, vice president and general manager of software,
supplier Icamp, Bolton, CT, this discrepancy between design and inspection
methods didn't matter nearly so much a few years ago, when part tolerances
were not so tight.
"With today's tighter tolerances, you can use the analogy of water-skiing
in a manmade reservoir with a bunch of tree stumps at the bottom," Jennings
said. "When the reservoir is full, you don't even know the stumps are
there. During a drought, the falling water level can make the stumps
a real hazard. Tighter tolerances are exposing these measurement/inspection
Jennings' company provides gage-emulation software that can collect the
data points generated by CMM inspection, and perform GD&T-based measurements. This
type of software translation can solve many problems, or, as Jennings puts
it, "referee the disagreement between measurement processes."
Several types of dedicated gage-emulation software packages are also available
from inspection-equipment vendors. According to Krulikowski, a major
thrust for CAD software developers in the next few years will probably be
automatic tolerancing in solid modeling programs, which will also help designers
and inspectors stay on the same page.
Still, it behooves the quality professional to know both the intricacies
of inspection equipment, and the dimensional-tolerancing Philosophy
followed by the designer and reflected in the part drawing--the better
to help find a transition method when problems arise. End
the GD&T Potential Savings Calculator
The calculator is a tool that helps companies understand the amount
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find out more about GD&T
Inc. is a world leader in the field of geometric tolerancing. ETI
founder, Alex Krulikowski is an expert on geometric tolerancing,
with a degree in industrial vocational education and over 30 years
of industry experience. He has taught GD&T to thousands of students
through classroom seminars, and to countless others through his
books, self-study workbooks, videos, and CD-ROMs.
ETI provides expert
GD&T training with an emphasis on practical, on-the-job application.
include GD&T fundamentals
and advanced concepts;
tolerance stacks; statistical
tolerance stacks; an ISO/ASME
comparison; a GD&T
overview; and solid model
Online training is
also available at their ETI Learning
Center. ETI's GD&T Trainer is a complete course in GD&T
fundamentals available in single-user, multi-user or LAN software.
With proper training
and implementation, GD&T will help your company reduce scrap,
increase the percentage of usable parts, simplify inspection and
assembly, replace fewer parts, avoid recalls, and increase efficiency.
Geometric dimensioning and tolerancing can give your company the
edge over the competition in today's cost competitive marketplace,
and Effective Training can provide the training and materials you
need to reap those benefits.
If you’d like to discuss
how geometric tolerancing will benefit your company, call 800-886-0909
or email email@example.com