GD&T Improves Inspection
Excerpt from article
in Quality Magazine, November 2002.
A working knowledge
of geometric dimensioning and tolerancing (GD&T) is necessary
if engineers want to truly understand the designer’s intent, and
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.
GD&T 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 does not know how to determine
a reasonable, functional tolerance.
“Two people involved
in every design need to understand GD&T—the designer and the
inspector,” says consultant Alex Krulikowski
of Effective Training Inc., (Westland, MI.) Krulikowski is the author
of a self-study
workbook based on the revised ASME standard, Y14.5M-1994.
design) 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,” says Krulikowski. He adds 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 says. Krulikowski adds that the drawing can
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
Where to find out
According to the experts,
dimensioning rules are violated frequently. In fact, Effective Training
has developed a list of the seven
deadly sins of GD&T. These are: incorrect use of the word
“thru,” incorrect use of the word “central,” unnecessarily tight
titleblock tolerances, use of esoteric notes, imaginary dimensions,
dimensions without tolerances and missing dimensions.
“These errors have appeared
so often, and for so long, they are accepted without question by
many drawing makers and users,” according to Effective Training’s
Web-site. “Violations, however, are dangerous and expensive because
they introduce ambiguity, multiple interpretations and guesswork
into the manufacturing process.” End of excerpt.
the GD&T Potential Savings Calculator
The calculator is a tool that helps companies understand the amount
of unnecessary expenditures each year due to employees not knowing
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find out more about GD&T
Effective Training 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