Mechanical Engineering

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  • Topic: Dimension, Technical drawing, Engineering drawing
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  • Published : March 6, 2013
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P•A•R•T • 2

STANDARDS

Chapter

4
Drawing Interpretation

Patrick J. McCuistion, Ph.D
Ohio University
Athens, Ohio

Patrick J. McCuistion, Ph.D., Senior GDTP, is an associate professor of Industrial Technology at Ohio University. Dr. McCuistion taught for three years at Texas A&M University and previously worked in various engineering design, drafting, and checking positions at several manufacturing industries. He has provided instruction in geometric dimensioning and tolerancing and dimensional analysis to many industry, military, and educational institutions. He also has published one book, several articles, and given several academic presentations on those topics and dimensional management. Dr. McCuistion is an active member of several ASME/ANSI codes and standards subcommittees, including Y14 Main Committee, Y14.3 Multiview and Sectional View Drawings, Y14.5 Dimensioning and Tolerancing, Y14.11 Molded Part Drawings, Y14.35 Drawing Revisions, Y14.36 Surface Texture, and B89.3.6 Functional Gages.

4.1

Introduction

The engineering drawing is one of the most important communication tools that a company can possess. Drawings are not only art, but also legal documents. Engineering drawings are regularly used to prove the negligence of one party or another in a court of law. Their creation and maintenance are expensive and time consuming. For these reasons, the effort made in fully understanding them cannot be taken for granted. Engineering drawings require extensive thought and time to produce. Many companies are using three-dimensional (3-D) computer aided design databases to produce parts and are bypassing the traditional two-dimensional (2-D) drawings. In many ways, creating an engineering drawing is the same as a part production activity. The main difference between drawing production and part production is that the drawing serves many different functions in a company. Pricing uses it to calculate product costs. Purchasing uses it to order raw materials. Routing uses it to determine the sequence of machine tools used to produce the part. Tooling uses it to make production, inspection, and assembly fixtures. Production uses 4 -1

4-2

Chapter Four

the drawing information to make the parts. Inspection uses it to verify the parts have met the specifications. Assembly uses it to make sure the parts fit as specified. This chapter provides a short drawing history and then covers the main components of mechanical engineering drawings.

4.2

Drawing History

The earliest known technical drawing was created about 4000 BC. It is an etching of the plan view of a fortress. The first written evidence of technical drawings dates to 30 BC. It is an architectural treatise stating the need for architects to be skillful as they create drawings. The practice of drawing views of an object on projection planes (orthographic projection) was developed in the early part of the fifteenth century. Although none of Leonardo da Vinci’s surviving drawings show orthographic views, it is likely that he used the technique. His treatise on painting used the perspective projection theory. As a result of the industrial revolution, the number of people working for companies increased. This also increased the need for multiple copies of drawings. In 1876, the blueprinting machine was displayed at the bicentennial exposition in Philadelphia, PA. Although it was a messy process at first, it made multiple copies of large drawings possible. As drawings changed from an art form to a communication system, their creation also changed to a production activity. From about 1750, when Gaspard Monge developed descriptive geometry practices, to about 1900, most drawings were created using first-angle projection. Starting in the late nineteenth century, most companies in the United States switched to third-angle projection. Third-angle projection is considered a more logical or natural positioning of views.

While it is common practice for...
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