7 Deadly Sins of
Dimensioning and Tolerancing
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many on- and off-line quality control tools in their zeal to make
it right the first time. These tools have their place, but
the quest for quality must begin at the drawing board. Engineering
drawings communicate part makeup and function to suppliers, manufacturing,
and customers. For clear, unambiguous drawings, geometric dimensioning
and tolerancing (GD&T) rules govern size, location, orientation,
and form expressions for each part surface. GD&T leaves
no room for ambiguity; it assumes the person reading the drawing
has no knowledge of part function.
rules are violated frequently. These errors have appeared so often,
and for so long, they are accepted without question by many drawing makers
and users. Violations, however, are dangerous and expensive because
they introduce ambiguity, multiple interpretations, and guesswork into
the manufacturing process. Before a company can eliminate errors,
it must be able to recognize them. These errors or deadly sins generally
fall into seven categories.
Improper use of the word "thru"
This is a dangerous word, unless it is used with absolute precision. It
means one thing, and one thing only; namely, "all the way through."
If through is not the intent, it is best to specify a depth.
use of the word "central"
The question to ask before using this word is: Central to what? It
may relate to any of several diameters. When using the word
central, include a frame of reference or the dimension will just
float on the drawing.
tight titleblock tolerances
To produce prints quickly, some designers use tight title block
tolerances so fewer exceptions have to be thought through and noted
elsewhere on the print. Manufacturing eventually bears the brunt
of these false goals. People work harder than necessary, time
and effort are wasted, and more deviations are asked for. Engineering
time is consumed processing print-change requests.
4. Use of
Many situations handled using notes would be better handled using
geometric tolerances to eliminate any ambiguity.
A dimensionfor example, from a hole center line to an undefined
vertical linecannot be measured during part inspection. With
GD&T, the dimension would be from the hole center line to a
These usually occur when there are many basic dimensions. A
tolerancefor instance, for locationshould be included
with every basic dimension.
This seldom happens with principal or obvious dimensions, but can
when components are not thoroughly documented. Missing dimensions
are particularly dangerous because inspectors are trained to inspect
what is on the drawing, not what is missing. Missing dimensions
may go undetected for years until there is a warranty problem or
cause many problems. For instance, if a company uses drawings
to decide what machines will be needed and what production rates
will be, they may calculate product costs incorrectly. Violations
can also extend product cycle time by causing manufacturing to tool
up with the incorrect equipment. Errors may force manufacturing
to guess at the designer's intent, and a finished product may function
delays, and sub-optimal products are all expensive. Eliminating
tolerancing errors can help a company decrease scrap, rework, changes,
confusion, and downtime. To eliminate GD&Ts seven deadly
sins, a company needs a combination of training, feedback, and quality
audits. Everyone involved with prints should be trained to be conscious
of common errors. Each part feature should be defined in terms of
size, location, orientation, and form. Vigilance by trained
people will help companies catch dimensioning errors before they
cause real damage.
feedback system, whether a report, memo, or meeting, will allow
drawings and problems to be discussed and solved. Dimensioning
errors often are not caught because companies do not communicate
effectively. Designers send drawings out, and no one reports
back about detected errors.
on drawings in use, independent quality audits, similar to those
used in manufacturing, could find and reject bad drawings. Engineering
should retrieve for zero defects on all drawings, the way manufacturing
does for parts.
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.
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
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 firstname.lastname@example.org