1.
INTRODUCCIÓN
For many in the manufacturing sector, geometric dimensioning
and tolerancing (GD&T) is a new subject. During World War II,
the United States manufactured and shipped spare parts overseas
for the war effort. Many of these parts were made to
specifications but would not assemble. The military recognized
that producing parts that do not properly fit or function is a
serious problem since lives depend on equipment that functions
properly. After the war, a committee representing government,
industry, and education spent considerable time and effort
investigating this defective parts problem; this group needed to
find a way to insure that parts would properly fit and function
every time. The result was the development of GD&T.
Ultimately, the USASI Y14.5-1966 (United States of America
Standards Institute"predecessor to the American National
Standards Institute) document was produced on the basis of
earlier standards and industry practices. The following are
revisions to the standard:
■ ANSI Y14.5-1973 (American National Standards
Institute)
■ ANSI Y14.5M-1982
■ ASME Y14.5M-1994 (American Society of Mechanical
Engineers)
The 1994 revision is the current, authoritative reference
document that specifies the proper application of GD&T.
Most government contractors are now required to generate
drawings that are toleranced with GD&T. Because of tighter
tolerancing requirements, shorter time to production, and the
need to more accurately communicate design intent, many companies
other than military suppliers are recognizing the importance of
tolerancing their drawings with GD&T.
Conventional tolerancing methods have been in use since
the middle of the 1800s. These methods do a good job of
dimensioning and tolerancing size features and are still used in
that capacity today, but they do a poor job of locating and
orienting size features. GD&T is used extensively for
locating and orienting size features and for many other
tolerancing applications. Tolerancing with GD&T has a number
of advantages over conventional tolerancing methods; three
dramatic advantages are illustrated in this introduction.
The purpose of this introduction is to provide an
understanding of what GD&T is, why it was developed, when to
use it, and what advantages it has over conventional tolerancing
methods. With this understanding of GD&T, technical
practitioners will be more likely to effectively learn the skill
of tolerancing with GD&T. With this new skill, they will have
a greater understanding of how parts assemble, do a better job of
communicating design intent, and ultimately be able to make a
greater contribution to their companies" bottom line.
Chapter Objectives
After completing this chapter, you will be able to
■ Define GD&T
■ Explain when to use GD&T
■ Identify three advantages of GD&T over coordinate
tolerancing
What Is GD&T?
GD&T is a symbolic language. It is used to specify the
size, shape, form, orientation, and location of features on a
part. Features toleranced with GD&T reflect the actual
relationship between mating parts. Drawings with properly applied
geometric tolerancing provide the best opportunity for uniform
interpretation and cost-effective assembly. GD&T was created
to insure the proper assembly of mating parts, to improve
quality, and to reduce cost.
GD&T is a design tool. Before designers can properly apply
geometric tolerancing, they must carefully consider the fit and
function of each feature of every part. GD&T, in effect,
serves as a checklist to remind the designers to consider all
aspects of each feature. Properly applied geometric tolerancing
insures that every part will assemble every time. Geometric
tolerancing allows the designers to specify the maximum available
tolerance and, consequently, design the most economical
parts.
GD&T communicates design intent. This tolerancing scheme
identifies all applicable datums, which are reference surfaces,
and the features being controlled to these datums. A properly
toleranced drawing is not only a picture that communicates the
size and shape of the part, but it also tells a story that
explains the tolerance relationships
2. MARCO
TEÓRICO
2.1 DIMENSIONING AND TOLERANCING FOR
CAD/CAM
Data base models
Many designers feel that solid model drawings produced with
CAD/CAM programs do not need to be dimensioned or toleranced. The
method of producing a design and transmitting that information to
the manufacturing equipment is not the major cause of
irregularity in parts. Although these systems may eliminate some
human error, the major cause of part variation occurs as a
result of a variety of other sources, such as
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