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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