Chapter 1: Introduction – Machine Drawing with AutoCAD

Chapter 1

Introduction

Chapter Outline

Drawing has played a crucial role in the progress of mankind from the ancient age of cave dwellers to the modern world of Skylab and Space Shuttle. Early efforts to improve the physical means of our well being gave impetus to the beginning of the “art of engineering”. Without drafting, it would have been impossible for man to develop and keep record of the vast wealth of engineering knowledge that has been gathered till date. As a result, man's progress would have been much slower.

In ancient times, men used to draw pictures to communicate their ideas to others and at the same time to record these ideas so that they may not be lost. Some of these drawings still exist. They are basically crude sketches made on clay. The detailed plans of early buildings were drawn on parchment or papyrus. In fact, people of Mesopotamia used drafting as a mode of communication as early as 2200 B.C. Historians have recovered a headless statue of Gudea, a ruler and engineer who reigned over Mesopotamia around 2000 B.C. The inscription on the statue illustrated the design aspects of a building. The plan view of a temple drawn on a stone is also placed on Gudea's lap.

The earliest form of writing, used by Egyptians and known as hieroglyphics, was in picture form. The plans for the pyramids and other structures of Egypt were first drawn on papyrus, slabs of limestone or on wood before they were given actual shape. However, in the classical period, engineers of the Roman empire first realised the importance of making good drawings. In fact, during this period, many detailed drawings of buildings, bridges, aquaducts, temples, underground drainage system and roadways were prepared for subsequent construction.

The word GRAPHICS means illustration of ideas by lines or by engraving. A drawing is a visual or graphic representation of a real object. Therefore, drafting can be thought of as a graphic language that uses pictures drawn by lines to communicate thoughts and ideas. Man developed the art of drawing along two distinct forms, depending on the purpose for which they were to be used. Artistic drawing, having different forms, mainly deals with the expression of ideas which may or may not be real objects. A technical drawing of any object, on the other hand, always expresses ideas or thoughts that have a bearing and basis on the physical form of that object.

A language formed of and developed from written/oral words has some inherent drawbacks. The shape and size of a fairly complicated object cannot be completely expressed even by the most advanced word language. But it can be described adequately by a proper engineering drawing or drafting. For this reason, drawing is referred to as the ‘Language of Engineers’. It is a marvellous medium of expression and hence a powerful language. It does not change its complexion, like spoken languages, with the crossing of the boundary of a country or even within a country like India. The practice or implementation is more or less same over the entire world. Unlike artistic drawing, this medium of expression is unique as it can be understood without any confusion by American, Russian, European, or Indian engineers.

HISTORY OF ENGINEERING DRAWING

The methods followed in engineering drafting have been standardised over the last century. But this has not been achieved suddenly. All real objects have three dimensions. A drawing paper, on the other hand, has only two dimensions. Men struggled for centuries with the problem of converting the views of three-dimensional objects into two-dimensional drawings, yet representing the original object accurately without losing any data. The major problem was to show the dimensions of length, width, and height in drawings. A giant step forward in this regard was made by the genius of the 15th century, Leonardo da Vinci. He used drawing both as a tool of scientific investigation and as an expression of artistic imagination and changed forever the art of drawing. He was one of the first artists to use sketches to work out his artistic and architectural compositions. He spent much time on anatomical drawing and drafting and design of architectural plans, machine guns, movable bridges and so on. He also drew plans for aircraft, helicopter, and even a parachute. Leonardo explored the technique of perspective drawing which painters use to create an illusion of depth on a flat surface. In his study for Adoration of the Magi (Fig. 1.1), Leonardo carefully drew all the lines needed to create perspective before sketching in all the figures. Look carefully and you may find the horizon line, orthogonals, and vanishing point. The concept of vanishing point (Fig. 1.2), that is, the optical illusion of parallel lines seeming to converge at a point, was evolved during this period.

 

FIG. 1.1   Leonardo da Vinci's Adoration of the Magi

 

FIG. 1.2   Vanishing point in perspective drawing

 

The Industrial Revolution took place in the 18th century. Many new machines and industrial structures were developed and the importance of sketches and engineering drafting became evident. In fact, in this era, drafting was identified as the universal language for engineers and many useful symbols and drawing conventions were incorporated as standard practice.

Americans contributed in the progress of engineering drawing by developing different kinds of instruments to produce accurate technical drawings. George Washington and his officers employed parallel rulers, dividers, ruling pens and so on during the drawing of maps and buildings. Later, Thomas Jefferson also created a number of house plans based on which houses were built subsequently.

Although the need for generating drawings became obvious, representation of planes were accomplished mostly in a visualised manner. With the progress of engineering practices in various diversified fields, there was great need for developing methods that would ensure accurate and easily comprehensible graphical representations. During the period of Napoleon, the rules and construction methodology for such representations were invented, and subsequently developed to an astonishing level by the French mathematician Gaspard Monge, the inventor of descriptive geometry. The method of parallel projection (Fig. 1.2), invented by Monge was, and is still accepted and followed as the principal method for making engineering drawings. For a long time, his work was considered so valuable that it was guarded as a military secret. Monge could publish his work only at the end of the 18th century when the prohibition for publishing his book was lifted by the French government.

 

FIG. 1.3   Monge's projection method

CHANGE IN DRAFTING ENVIRONMENT

Nowadays, drafting is unquestionably the most precise method of graphically recording and communicating engineering concepts. With the advancement of technology, in the last 50 years, there has been a great change in the drafting room environment.

For many years, technical drawings were produced on paper using a pencil or pen. The paper was mounted on a smooth board, known as a drafting board, and the lines were guided with drafting instruments like T-square, triangle, divider, and compass. Hand-guided lettering machines and templates enhanced the quality and speed of executing the process of accurate and artistic lettering. Subsequent introduction of drafting machines and electric erasers improved the precision and speed of the drawing process. However, the method of manual drawing has several inherent disadvantages. Modifying a manual drawing is a slow, tedious process, as is the copying or tracing of details from one part of a drawing to another or from one drawing to another. To bring about a minor change in a repetitive type of drawing, this method of drawing is found to be very inefficient as each one has to be drawn afresh. If a drawing contains too many standard symbols or components (nuts, bolts, and so on), each one of them has to be produced separately and the process becomes time consuming. With change in technology, when everything is getting faster day by day, this type of slow process is totally unacceptable.

The last two decades have witnessed the application of computers in almost every sphere of human activities due to the rapid and stupendous advancement in computer hardware and software. The advanced graphical display system is definitely one of the most fascinating devices that computer technology has produced. Equipped with such an advanced graphical display technique along with sophisticated drafting softwares, a computer can be used nowadays as a highly efficient drafting device to draw and design complicated buildings and structures and view them in true perspective. A variety of other objects, for example, machine components, integral circuits, automobile, aircraft, and the like can be drawn and displayed on the computer screen. With the advancement in computer-driven plotter, it is possible to plot a drawing automatically.

It is simple to write a graphical program by specifying a sequence of operations that produces a list of instructions for the display processor. Thus it is possible to change the picture in a dynamic fashion by a mere change in the database. One can copy a part of or the entire drawing, regenerate it at any desired location, remove a portion of the drawing or change its position without much difficulty. The sharing of repeatedly used symbols are very simple; even the symbols may be individually scaled and rotated as per requirement. With the help of computer graphics it is now possible to generate realistic views of a solid object, removing the hidden lines and surfaces. Tasks like clipping, hiding lines and surface removal were extremely time consuming even some years back. But, with the help of efficient algorithms alongwith fast computers, this type of work can be executed very quickly. All such developments in the graphical display abilities of components have culminated in a new type of virtual representation of physical attributes and form of an object called Solid Models. With the help of the solid modelling technique, a modern day engineer can give birth to an almost real life model of an object much before its actual commercial production. For visual representation and also for the necessary engineering analysis to optimise the design process, solid models are invariably created and used, saving a lot of money and other valuable resources. Mention may be made here of the very significant and extensive use of Computer Aided Design (CAD) softwares in the automobile, aeronautical, and naval architecture industry where visualisation of full scale models with simulation and virtual walkthrough are possible. The Boeing Corporation, in recent years, has not marketed a single model which was not completely prototyped in a computer before actually manufacturing even one single component. In civil, structural, and architectural fields of engineering, CAD has played a major cost-saving role by enabling the building up of models and Finite Element (FE) Analysis techniques.

Considering this tremendous importance of modelling, almost all computer drafting software packages nowadays include Solid Modelling. As a result, in today's computerised environment, the word drafting stands not only for the traditional two-dimensional drafting but also for Solid Modelling. Several computer drafting packages are now available in the market that are fast, easy to use, and relatively inexpensive. Thus keeping pace with technology, computer-aided drafting has become very popular throughout the world and, in the near future, it may completely replace manual drafting.

CAD AND COMPUTER-AIDED DRAFTING

Engineering design constitutes the core of the possible solutions to the technological changes faced by the engineers. Design is a sequence of operations performed to achieve a goal. In its first phase, the design model is conceptualised to meet the functional requirements. Next, a preliminary design is made to select suitable configuration and proportions of the components. The designer may use sketches to communicate his ideas at the conceptual level of design process. But during preliminary design, drawings are required to convey precise information about the form, appearance, structural specifications, kinematic behaviours and so on of the model. Then comes the analysis of the design model and evaluation of its performance. If possible, modifications may be undertaken to reach an optimum solution. The design process ends with the preparation of a detailed manufacturing drawing to enable the production of the designed model.

It is well understood that engineering graphics play a dominant role in all the phases and steps outlined above in the process of engineering design. It may also be understood that a complete design process needs many other inputs than engineering graphics. Let us examine the role of computers in general and CAD software in particular in this regard.

Computer-Aided Design is an interactive process, with graphics playing a very important role. A smooth exchange of information between the designer and the computer paves the way for a good and efficient CAD system. Designers of engineering systems always like to put down their ideas graphically (spatially) in the form of sketches. This necessitates the use of Interactive Computer Graphics Techniques which provide capabilities to model design ideas. Graphics-based information exchange, capability of conveying more information in less time and pictorial representation help the designer immensely to transform his intuitive thinking/imaginative capability into better and correct design decisions. Thus, Interactive Computer Graphics is one of the most important software components of a CAD system.

Besides this, software tools like Knowledge Based Expert System, Database Management System, Alternate Programming Language and, above all, analytical tools like Finite Element Modeling and Optimisation Techniques provide the designer a complete and robust CAD system for his design process activities.

A little explanation about the selective usage of the word CAD would not be out of context here. Usually the word CAD stands for Computer-Aided Design. But when we use CAD in the context of drawing, it becomes more representative of Computer-Aided Drafting than Computer-Aided Design. Design is much an all-embracing concept and though it is obviously true that Computer-Aided Drafting plays a major role in almost each and every stage of a Computer-Aided Design process, it must not be confused with the wholesome nature of a complete CAD system. A Computer-Aided Drafting package is basically a computer graphics system which is used at every stage of a design process from primary design to analysis, evaluation, detailed design, and eventually drafting.

With so much advancement in its capabilities and broad spectrum of usage in the engineering industry, it is no wonder that almost all the universities in India have included courses on CAD in general and AutoCAD in particular, in their engineering syllabi.

The purpose of this book is to provide a text book and a reference source for students and professional engineers who have an interest in the field of Computer-Aided Drafting basically in mechanical engineering. There are many Computer-Aided Drafting packages available in the market. AutoCAD, being the most powerful PC-based software and relatively inexpensive, has been chosen as the platform for this book.

Throughout the 16 chapters of this book, an attempt has been made to explain the underlying principles and to provide insights into the applications of various types of machine components that are used in day-to-day engineering practices. At the same time, the knowledge base required for drafting of those components using AutoCAD has also been imparted. The book has been conceptualised in such a fashion that as you learn machine drawing, you get to know AutoCAD as well without any extra effort and without going through any other book on AutoCAD.

The book really starts from Chapter 2 in which basic concepts of orthographic projection drawing are explained with several illustrations. Different methods of projection drawing (along with the existing drawing conventions) are described. The chapter ends with a number of example problems for orthographic drawing practice.

Chapters 3 and 4 give an overview of the AutoCAD package along with details of the essential AutoCAD commands necessary for 2-D drawing. Most of the commands are covered with appropriate illustrations. Since dimension is an important part of any drawing and there are various conventions being followed for dimensioning, it is dealt with separately in Chapter 4. Each of these two chapters conclude with a worked-out drawing of a machine component and its dimensioning in detail. Chapter 5 focuses on concepts of sectional drawing and how to implement them in the drawing using AutoCAD.

In Chapters 6 and 7, different types of fastening devices, such as threaded fasteners, keys, cotter joints, and so on are described and their working principles are explained. The use of the Block command to build up AutoCAD library functions for frequently used machine parts are dealt with in Chapter 6.

Chapter 8 covers an important concept—Paper Space and Model Space. This is an essential feature provided by AutoCAD either for plotting or for creating orthographic views from its solid model.

As discussed earlier, the recent trend is to develop an object(s) in 3-D environment as it offers several advantages in regard to analysis, machining, and so on. Therefore, in Chapter 9, an attempt has been made to convey the techniques of creating complicated objects in solid models. Since AutoCAD provides a large number of commands for this purpose, only the essential commands are explained in detail. Detailed example problems are worked out so that even complicated objects can be created following the given procedures.

Chapters 10 to 12 describe various machine elements, such as pulleys, couplings, clutches, pipe joints, and so on. In order to highlight the working principle of each component, appropriate solid model views of the object are also presented along with orthographic projection views. In the chapter on pipe joints, an example of piping drawing in 3-D is presented to show the state of the art being practised in recent times.

Different types of valves and their usages are explained in Chapter 13. The orthographic drawings of valves are not always good enough for understanding the details and intricacy of the components. However, a 3-D view of the object in a cut section may throw enough light to have a clear concept about it. Therefore, various solid model views are also shown in this chapter along with usual drawings to explain the different features of valves.

Chapter 14 deals with gears. Gears are a common machine element. Therefore, special attention has been paid in this chapter to include different types of gears, and their tooth profiles, being used in industry. The chapter also highlights how the drawing of gear pairs becomes very easy, yet accurate, by means of Computer-Aided Drafting. A few typical problems have been worked out in steps with vivid illustrations.

Any production drawing contains some symbols to communicate additional information regarding surface quality, manufacturing methods, and applicable standards. In Chapter 15, these features are explained in detail. Emphasis has been laid on the usage of AutoCAD to incorporate all these features in the drawing. The book ends with Chapter 16 in which some complicated yet common machine components are described and their working principles explained with suitable drawings in 2-D/3-D environment.

While preparing the material for the main chapters, we decided to put some advanced topics, which may be skipped at the initial level but are necessary for advanced users and for reference purposes, in the Appendices (16).

We have not covered the database management aspect of AutoCAD, which, despite its tremendous importance in commercial design and manufacturing processes, is not much related to engineering drafting practices.

Thus, a humble attempt has been made in this book to provide a comprehensive and practical overview of the subject of Machine Drawing using one of the most widely used mechanical CAD software as the drafting platform.