GD&T: Minding Mechanical Specifications Pays Off in Production
OEM Mechanicals
When you specify
electrical components on a drawing, you use electrical specs.
Mechanical components need their own spec system that is repeatable,
clear and not overly restrictive.
The recent revision of the American Geometric Dimensioning and
Tolerancing (GD&T) Standard, ASME Y14.5M-1994, is fact becoming
the spec system of choice for mechanical components by today's
OEMs.
Two reasons for this preference are:
- Geometric tolerancing is the key to specifying mechanical parts
that can be measured repeatedly.
- The latest revision of Y14.5 is ninety percent compatible with
the GD&T sections of the ISO standards, bringing OEMs more
in step with the global manufacturing scene.
An International Language
GD&T is an international language used on drawings to accurately
describe a part. The language consists of a well defined set
of symbols, rules, definitions, and conventions that can be used to
describe the size, form, orientation, and location of part features.
GD&T is an exact language that enables designers to “say what
they mean” on a drawing, thus improving product designs. Production
uses the language to interpret the design intent, and inspection looks
to the language to determine set up. By providing uniformity
in drawing specifications and interpretation, GD&T reduces controversy,
guesswork, and assumptions throughout the manufacturing process.
Cost Benefits
Because of the lack of studies demonstrating the cost benefits of
GD&T, many upper management people, especially those without
engineering backgrounds, lack the understanding needed to estimate
actual savings. Consequently, they may be reluctant to fund GD&T
training programs. The benefits of using GD&T to specify
mechanical parts can readily be demonstrated when we compare GD&T
to coordinate dimensioning, which has been in use for over 150 years.
Here are seven advantages that add up to significant savings
in production:
- The
design philosophy of GD&T is that of functional dimensioning,
which means that a part is defined by how it functions in the
final product. Instead of copying a tolerance from an existing
drawing, the designer bases the tolerance on part function, this
allows the maximum amount of tolerance to produce the part.
When properly applied, functional dimensioning can often double
or triple the amount of tolerance on many component dimensions,
which reduces manufacturing costs. With coordinate dimensioning,
tolerance zones are not related to functional requirements.
Problems can result when designers assign tight tolerances because
they are not focused on determining a functional tolerance.
- GD&T
allows round tolerance zones. In Figure
1, Arrow A points to a GD&T symbol which specifies a round
tolerance zone (for the mount holes). The zones specified
by coordinate dimensioning (see Figure 2, Arrow A). Round
tolerance zones allow for 57% more tolerance than square zones,
resulting in more usable parts. By allowing more tolerance
on parts, the process will be more capable, reducing manufacturing
costs.
- In
addition to the tolerance gained from using round zones, GD&T
allows a “bonus” tolerance under certain conditions.
This bonus tolerance is gained by using the MMC (Maximum Material
Condition) modifier, as indicated by Arrow B in Figure 1.
The MMC modifier allows a hole to have additional tolerance when
it is produced larger than its minimum size. This is a win-win
situation for the OEM because engineering can be assured that
the part will assemble when the holes are the smallest, and manufacturing
can have additional tolerance when the holes are larger than their
minimum size. In coordinate tolerancing, the tolerance zone
is always fixed in size (Figure 2, Arrow A), at all hole conditions.
This results in a number of functional parts being scrapped and
a more stringent condition for manufacturing. With bonus
tolerance, parts that are more functional are used, and more tolerance
is allowed for production, resulting in lower operating costs.
-
GD&T's datum system clearly communicates one set up for inspection. Datums
are theoretical planes, points, or axes, and are simulated by
the inspection equipment. The symbol used to specify a datum
feature is shown on Figure 1, Arrows C. These symbols denoted
which part surfaces touch the gaging equipment during inspection.
Datum features are selected on the basis of part function and
assembly requirements; they are often the features that mount
and locate the part in its assembly. Datum reference letters
are specified (see Figure 1, Arrows D) inside the geometric controls
and denote the sequence in which the part surfaces contact the
gaging equipment. This sequence is needed in order to have
multiple inspectors set up the part in an identical manner.
In coordinate dimensioning datums are implied, allowing choices
for set up when inspecting the part. Different inspectors
may get different results; some good parts may be scrapped and
some bad parts may be accepted.
- GD&T
reduces assembly problems. Since the
inspection process with GD&T ensures the at parts will assembly
properly, assembly methods no longer need to be addressed by the
guy on the assembly line with a two-by-four and a hammer.
The inspection process with GD&T ensures that OEMs can use
competitive souring or obtain multiple sources for the same part,
resulting in increased profitability.
- In
the area of inspection, GD&T supports
the use of SPC. GD&T's Datum system provides the
repeatable part measurements that are necessary for making a meaningful
SPC chart. With coordinate tolerancing, SPC data may include
assumptions, which reduce the accuracy of the data. The use of
the profile control is another example of how GD&T supports
SPC. The profile control helps in two ways:
- It
establishes a mathematically defined tolerance zone, and it
relates the measurement to datums. When coordinate tolerancing
is used, the precise tolerance zone definition simply doesn't
exist. For example, try to define the size and location
of the radius (Arrow B) in Figure 2.
-
Because the datum system and profile control allows SPC data
to be more accurate, needless changes in the manufacturing
process are avoided, regarding the OEM with time and cost
savings.
- GD&T
is supported by national and international standards. ASME
Y14.5M-1994 and a series of ISO standards rigorously document
the interpretation of each GD&T symbol and concept.
On the other hand, coordinate tolerancing is like folklore; it's
not well documented, even though it has been around for 150 years.
Producing parts to GD&T's documented standards assures the
OEM that parts will be accepted by the customer. Fewer replacement
parts will be needed and recalls can be avoided, saving time and
money.
The
following summarizes the above comparison between GD&T and coordinate
tolerancing:
With coordinate
dimensioning, tolerance zones are not related to functional requirements.
Problems can result when designers assign tight tolerances because
they are not focused on determining a functional tolerance. (Click
on the figure for larger view.)
Errors can be Costly
In order for OEMs to reap all the benefits of GD&T, it must
be used correctly. Using GD&T incorrectly is likely to
result in errors on drawings, and such errors can be costly. For
example, if a company uses drawings to decide which machines will
be needed and what production rates will be, they may calculate
product costs incorrectly. Misuse of GD&T can extend product-cycle
time by causing manufacturing to tool up with the incorrect equipment. Errors
may also force manufacturing to guess at the designer's intent and
the finished product may function poorly. The more proficient
a company is at using GD&T, the more cost-efficient it will
be.
CAD Systems and GD&T Training
Many of today's CAD systems have GD&T capabilities, but
the operator must still understand GD&T in order to use it effectively. While
companies routinely send employees for three to six weeks of CAD
training, they sometimes only allow two to three days for GD&T
training. Without the proper instruction in GD&T, an OEM
will be unable to make full use of their investment in the CAD system.
Training Requirements
Just like learning a foreign language, GD&T requires learning,
application and practice. Everyone involved with part drawings
should be trained in GD&T so that they can recognize common
errors. Many of today's engineering programs only offers a
brief exposure to GD&T, with little emphasis on application.
(Click on the table for a larger view.)
There are three levels of GD&T training: Fundamentals,
Advanced Concepts, and Tolerance Stacks:
- The
fundamentals of GD&T include the purposes and objectives of
GD&T, its symbols, rules, and terminology.
- Geometric
tolerancing advanced concepts include how to select datums and
how to solve real world problems.
- Tolerance
stacks includes using geometric tolerances in stacks to establish
part tolerance and analyze product designs.
Design engineers
need all three levels of training, consisting of sixty to eighty
hours of interactive classroom sessions with testing, and six months
of on-the-job training, followed by occasional refresher courses. Machine
programmers, ship supervisors, and quality engineers need training
in the fundamentals and advanced concepts for a total of forty to
sixty hours of instruction, within six month period. Sales,
purchasing, and administration typically need twenty four hours
of fundamentals training.
There are different ways that an OEM can get the necessary training. Instructors
can come to the site, or an OEM can appoint their own moderator
who will schedule training sessions via interactive videos. The
third option is team self-study, a concept pioneered by the author
in which individual team members use self-study materials to learn
GD&T on a prescribed schedule and hold team meetings for discussion
and testing.
Some community colleges offer excellent vocational education courses
in GD&T, giving local OEMs a fourth training option.
Implementation
Once training has taken place, the next step is for the designers
to use GD&T on mechanical part drawings. Independent quality
audits, similar to those used in manufacturing, can find and reject
bad drawings. Engineering should strive for zero defects on
all drawings, the way manufacturing does for parts. A regular
system of feedback, whether by report, memo, or meeting, allows
problems to be discussed and solved.
Certification Available
Certification can provide OEMs with a way to verify that their
second manufactures have the needed GD&T skills. A certification
program called the Geometric Dimensioning and Tolerancing Professionals
(GDTP) [is available] from the ASME.
Giving Your Company the Edge
With proper training and implementation, OEMs can use GD&T
to reduce scrap, increase the percentage of usable parts, simplify
inspection and assembly, replace fewer parts, avoid recalls, and
increase efficiency. Minding mechanical specs on your drawings
can give your company the edge over the competition in today's cost
competitive OEM marketplace.
Try
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
how to correctly apply and interpret GD&T.
Where to
find out more about GD&T
Effective Training
Inc. is a world leader in the field of geometric tolerancing. ETI
president, 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.
Onsite
workshops
include GD&T fundamentals
and advanced concepts;
tolerance stacks; statistical
tolerance stacks; an ISO/ASME
comparison; a GD&T
overview; and solid model
tolerancing.
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 info@etinews.com
today.
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