Technical Drawing

Ata Atun

 

Table of Contents

Preface

 

iii

CHAPTER 1

Introduction

1

CHAPTER2

Drawing Instruments and their use

5

CHAPTER3

Lettering

22

CHAPTER 4

Lines

38

 

Geometry of Straight Lines

46

CHAPTER 5

Scale Drawing

57

 

Dimensions

58

 

Development of Surface

65

CHAPTER 6

Shape Description

72

 

Selection of Views

77

 

Projecting the views

78

CHAPTER 7

Pictorial Drawing

82

 

Dimetric Projection

82

 

Trimetric Projection

83

 

Isometric Projection

83

 

Oblique Projection

87

 

Perdpective Drawing

88

 

Freehand Pictorial Sketching

93

 

Cross Section

94

Special Course for Students in

CIVIL ENGINEERING Department                                                                        102

ELECTRICAL ENGINEERING Department                                                         122

MECHANICAL ENGINEERING Department                                                        131

References

144

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CONTENTS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 1

 

 

Introduction

 

In beginninig the study of engineering drawing (or engineering graphics, as it is now coming to be called), you are embarking upon a rewarding educational experience and one that will be of real value in your future career. When you have become proficient in it, you will have at your command a method of communication used in all branches  of technical industry, a language unequaled for accurate description of physical objects.

 

The importance  of this graphic language can be seen by comparing it with word languages.  All who attend elementary and high school study the language of their country and learn to read, write, and speak it with some degree of skill. In high school and college most students study a foreign language. These word languages are highly developed systems of communication. Nevertheless, any word language is inadequate for describing the size, shape, and relationship of  physical  objects.

 

 Study the photograph at the opening of this chapter  and  then  try to describe it verbally so that someone who has not seen it can form an accurate and complete mental picture. It is almost impossible to do this.

 

Even a picture such as the one above, although possibly easier to describe, presents almost insurmountable problem. Furthermore, in trying to describe either picture, you may want to  use  pencil and paper to sketch all or a part in an attempt to make the word description more complete, meaningful, and accurate, or tend to use your hands, gesturing to aid in explaining shape and relationship. From this we can see that a word language is often without resources for accurate and rapid communication of shape and size and the relationships of components.

 

Engineering is applied science, and communication of physical facts must be complete and accurate. Quantitative relationships are expressed mathematically. The written word completes many descriptions. But whenever machines and structures are designed, described, and built, graphic representation is necessary. Although the works of artists (or photography and other methods of reproduction) would provide pictorial representation, they cannot serve as engineering descriptions.  Shaded  pictorial drawings and photographs are used for special purposes, but the great bulk of engineering drawings are made in line only, with arranged in a logical system of projection. To these views, dimensions and special notes giving operations and other directions for manufacture are added. This is the language of engineering  drawing, which can be defined as the graphic representation of physical objects  and  relationships.

 

As the foundation upon which all designing and subsequent manufacture are based, engineering graphics is one of the most important single branches  of  study in a technical school. Every engineering student must know how to make and how to read drawings. The subject in all types of engineering practice, and should be understood by all connected with, or interested in, technical industry. All designs and directions for manufacture are prepared by draftsmen, professional writers of the language, but even one who may never make drawings must be able to read and understand them or be professionally illeterate. Thorough training in engineering graphics is particularly important for the engineer because he is responsible for and specifies the drawings required in his work and must therefore be able to interpret every detail for correctness and completeness.

 

 

Our object is to study the language of engineering graphics so that we can write it, expressing ourselves clearly to one familiar with it, and read it readily when written by another. To do this, we must know its basic theory and composition, and be familiar with its accepted conventions and abbreviations. Since its principles are essentially the same throughout the world, a person who has been trained in the practices of one nation can readily adapt himself to the practices of another.

 

This language is entirely graphic and written, and is interpreted by acquiring a visual knowledge of the object represented. A student's success with it will be indicated not alone by his skill in execution, but also by his ability to interpret lines and symbols and to visualize clearly in space.

 

In the remainder of this chapter we shall introduce briefly the various aspects of engineering drawing that will be discussed at length later. It is hoped that this preview will serve as a broad perspective against which the student will see each topic, as it is studied, in relation to the whole. Since our subject is a graphic language, illustrations are helpful in presenting even this introductory material; Figures are used both to clarify the text and to carry the presentation forward.

 

 

ESSENTIALS OF DRAFTING:

 

LINES AND LETTERING. Drawings are made up of lines that represent the surfaces, edges, and contours of objects. Symbols, dimensional sizes, and word notes are added to these lines, collectively making a complete description. Proficiency in the methods of drawing straight lines, circles, and curves, either freehand or with instruments, and the ability to letter word statements are fundamental to writing the graphic language. Furthermore, lines are connected according to the geometry of the object represented making it necessary to know the geometry of plane and solid figures and to understand how to combine circles, straight lines, and curves to represent separate views of many geometric combinations.

 

METHODS OF EXPRESSION. There are two fundamental methods of writing the graphic language: freehand and with instruments.

 

Freehand drawing is done by sketching the lines with no instruments other than pencils and erasers. It is an excellent method during the learning process because of its speed and because at this stage the study of projection is more important than exactness of delineation. Freehand drawings are much used commercially for preliminary designing and for some finished work. Instrument drawing is the standard method of expression. Most drawings are made "to scale," with instruments used to draw straight lines, circles, and curves concisely and accurately. Training in both freehand and instrument work is necessary for the engineer so that he will develop competence in writing the graphic language and ability to judge work done under his direction.

 

METHODS OF SHAPE DESCRIPTION.

 

Delineation of the shape of a part, assembly, or structure is the primary element of graphic communication. Since there are many purposes for which drawings are made, the engineer must select, from the different methods of describing shape, the one best suited to the situation at hand. Shape is described by projection, that is, by the process of causing an image to be formed by rays of sight taken in a particular direction from an object to a picture plane.

 

Following projective theory, two methods of representation are used: orthographic views and pictorial views.

 

For the great bulk of engineering work, the orthographic system is used, and this method, with its variations and the necessary symbols and abbreviations, constitutes an important part of this book. In the orthographic system, separate views arranged according to the projective theory are made to show clearly all details of the object represented. The figures that follow illustrate the fundamental types of orthographic drawings and orthographic views.

 

"Pictorial representation" designates the methods of projection resulting in a view that show the object approximately as it would be seen by the eye. Pictorial representation is often used for presentation drawings, text, operation, and some working drawings.

 

There are three main divisions of pictorial projection: axonometric, oblique, and perspective. Theoretically, axonometric projection is projection in which only one plane is used, the object being turned so that three faces show. The main axonometric positions are isometric, dimetric, and trimetric.

 

Oblique projection is a pictorial method used principally for objects with circular or curved features only on one face or on parallel faces; and for such objects the oblique is easy to draw and dimension. Perspective projection gives a result identical with what the eye or a single - lens camera would record.

 

 

METHODS OF SIZE DESCRIPTION.

 

After delineation of shape, size is the second element of graphic communication, completing the representation of the object. Size is shown by "dimensions," which state linear distances, diameters, radii, and other necessary magnitudes.

 

 

 

BASIC MACHINE ELEMENTS.

 

 Many machine elements occur repeatedly in all kinds of engineering work. Familiarity with these elements is necessary so that dimensioning and specifications on the drawings will be correct. The material that follows introduces basic machine elements and shop processes, so illustrating the principles of engineering drawing laid out the previous sections.

 

 

References

 

1.   F.W. Dodge Corp., AManual of Essential Architectural Data, New York, 1954

2.   Thomas B. Dwyer: Draughtmanship, London, Rhodec International, 1975

3.   American Concrete Institute, ACI Standart 318-71, 301-72, Michigan, ACI, 1975

4.   İnkilap ve Aka Kitap Evi, Yazma Teknigi, Istanbul, 1964