Mechanical drawing - Jones

As mechanical drawing has been a very popular subject among students in schools and shops, numerous text-books have been published on mechanical drafting practice. This book is added to the list because the publishers believe that there has been need for a treatise dealing more thoroughly with methods which are actually employed in well-managed drafting-rooms. Many books on mechanical drawing have covered such subjects as geometrical drawing problems, orthographic projection, the development of intersecting surfaces, etc., but the application of these principles and the real object of mechanical drawing as related to machine and tool manufacture has been dealt with vaguely, in many instances. The student has been taught certain details, but he has not been given a clear conception of the work of draftsmen and designers in the drafting-rooms of machine-building plants. This book presents the subject in a way that will enable the student to understand what the term "mechanical drawing" really means in its broadest sense, the essential features of modern drafting practice, and the difference between the mere representation of a design by a suitable drawing and the more valuable work of originating and developing the design itself.

A special effort has been made to secure a well-balanced treatise in which the various elements of mechanical drawing are dealt with according to their relative importance. For instance, little space is given to c, because making fancy letters in numerous styles is not the work of a draftsman in a well-managed drafting-room, although this subject has been greatly emphasized in many books. The aim has been to present methods which are in actual use rather than exercises in drawing which do not conform to the practice in manufacturing plants. An elaborate drawing of a bevel gear with all of the teeth accurately reproduced may be an attractive and impressive feature in a text-book on mechanical drawing, but it is misleading, because working drawings are not made in that way. This book, in its arrangement and scope, is based on the assumption that it is essential for the student of mechanical drawing - whether in school or in shop - to understand the purpose of drawings as applied to machine and tool construction, how various mechanical devices may be represented by means of drawings, the necessity of making drawings which completely and clearly show what they are supposed to, and the relation between drawing and designing. Special attention has been given to the dimensioning of drawings and to the importance of using printed instructions or any legitimate means of making a drawing entirely clear to the men in the shop.

In dealing with the numerous details of the draftsman's work, an effort has been made to present methods which are sanctioned by common usage and to explain the reasons for the more important variations in practice. To accomplish this the methods and systems of many of the representative drafting-rooms were studied, and much valuable information was also secured from articles pertaining to different features of drafting practice which have been published in Machinery

When any new or improved form of tool or machine is being developed, its general arrangement and the principle governing its operation or use may be quite clear in the mind of the inventor or originator, and he may proceed with the actual work of construction, guided only by a mental picture of the device. Many simple tools or appliances could be, and some are, produced in this direct way, but it is evident that such a method of procedure is greatly restricted. It is often easier for the originator of a new type of mechanism to build it with his own hands than to attempt, simply by verbal description, to give someone else a clear enough mental picture of the device to enable him to construct it. This direct method of construction is, of course, impracticable as applied to regular manufacturing. In the first place, it would be impossible to originate many of the more complicated mechanisms by simply forming a mental picture of them. The basic principle of the device and possibly its general arrangement might be entirely clear, but in order to determine the exact relation of the various parts when they are all properly proportioned and assembled, it is necessary to make a fairly accurate drawing. Such a drawing not only shows the arrangement of the mechanism as a whole, but greatly assists the designer, in many cases, in the development of the idea. Frequently the mental picture is distorted and, when an accurate drawing is made, it is apparent that changes are necessary either in the form and size of one or more parts or possibly in the entire arrangement of the mechanism.

General Uses of Drawings. - The method usually fol- lowed by inventors and designers in originating new or improved mechanical appliances is to make a drawing of whatever plan or idea is to be developed. When this has been done, a clear conception of the form and often of the practicability of the device represented by the drawing may be obtained not only by the originator of the idea but by others who understand drawings and are able to "read" them. Drawings, then, as applied to the manufacture of tools and machines, serve several important purposes. First, they assist in the development of a plan by enabling the inventor or designer to see clearly the relation of different parts to one another and whether or not the desired motion or effect may be obtained. Second, drawings make it possible for the originator of a plan to convey the idea to others readily. Third, they show to those who are actually to construct the device, the proportions of its different parts and their relation when properly assembled. Finally, drawings are useful as records of what has been done and make it possible to reproduce whatever tool or mechanism is represented on the drawing.

The Work of the Draftsman. - In the manufacture of various kinds of mechanical tools and equipment, the work to be performed may be divided into four branches: (1) Originating entirely or in part the general type of device to be constructed and the principle governing its operation; (2) designing the mechanism in accordance with established mechanical principles and in such a way that the different parts will be strong enough to resist any stresses to which they may be subjected; (3) making drawings such as are needed in the actual work of construction; and, (4) making, fitting, and assembling the different parts. In the study of mechanical drawing, it is essential to understand the relation of these four branches of work to one another, because a draftsman may simply make drawings according to the ideas of others, or he may have more or less to do with originating the plan. Some draftsmen are also able to determine the proportions of different parts and many of them control, to some extent, the method of manufacturing. In a restricted sense, a draftsman makes drawings of appliances originated by an inventor or designer. The designer may be an inventor or vice versa, and he is always a draftsman and is capable of making mechanical drawings. The draftsman, however, is not necessarily a designer and may know little or nothing about the principles governing the design of machinery or tools.

It is evident, then, that a man is valuable in the designing department in proportion to his ability to originate, design, and develop useful and practical appliances. It is also apparent that the name "draftsman" has a broad meaning and may include anyone, from a man who can make a drawing to scale from a free-hand sketch to a man who can design as well as draw  complicated automatic machine. The first one is a draftsman pure and simple, while the second one is a designing draftsman, who can create. Properly designated, the first man is a draftsman, while the second is a designer. This distinction, however, is not usually made except in salary, and anyone working on drawings (not tracings) is known by the general term of draftsman. If the work is restricted to the making of tracings, the one doing it is commonly known as a tracer. A designer must be a specialist, because it is impossible for one man to know how to design mechanical devices for any and all purposes, and there is no known rational method of design which can be studied in the same way as one might study mathematics or physics.

What the Draftsman Should Know. - In taking up the study of mechanical drawing, it is important to know what is involved in becoming a designer or draftsman who, instead of simply making drawings of the plans of others, is capable of original work. To begin with, the ability to originate or improve plans and designs may be developed by studying what others have accomplished. It frequently happens that the principle governing the operation of one device may be applied to some other mechanism which is used for an entirely different purpose. In this way, the original idea is made more useful and of greater value because it is utilized for more than one purpose. While this is not original work in the exact meaning of the term, the fact is that very few mechanical appliances are absolutely originated by one man; moreover, it may not always be advisable in machine design to attempt to be entirely original, but rather to apply what is definitely known to be sound in theory and practice. This does not mean that the inventor or designer should not think for himself nor that he should deliberately appropriate the ideas of others, but simply that one should proceed cautiously when attempting to improve or change entirely some commonly accepted method or principle which has been thoroughly tested in practice.

The draftsman whose work is not confined merely to drawing lines on paper, must have a knowledge of mechanical laws, the various well-known methods of transmitting and modifying motion, and how to proportion parts of tools and machines so that they will resist the stresses to which they are subjected. Many worthless designs have been the direct result of ignorance of fundamental mechanical principles. Another requirement is a knowledge of the art of drawing, which is the principal subject dealt with in this book. While a draftsman, to be successful, must know more than how to make mechanical drawings, nevertheless this is an important part of his work, because drawings which do not clearly represent the object drawn are a source of trouble and are liable to cause serious mistakes. Delays in the pattern shop and machine shop are often due to poor drawings which are lacking either in dimensions, in the arrangement and number of the views, or in some other respect.

Why Draftsmen Should Understand Manufacturing Methods. - A fourth requirement in connection with the work of designing is a knowledge of manufacturing methods. Other things being equal, the draftsman excels who is capable of designing parts which are as simple and free from complications as possible and which are, therefore, cheaper to manufacture. The competent designer not only thinks of the operation of a tool or mechanism, but carefully considers the work of the patternmaker, molder, machinist, and tool- maker. It is much easier, of course, to draw lines on paper than to form the parts which the lines represent, in wood, iron, and steel; yet this simple fact is often disregarded and many designs and inventions have been discarded because the cost of manufacturing was unnecessarily high. Since a simple change on a drawing will often greatly reduce the work in the pattern shop, the designer should understand the principles of patternmaking and molding. A knowledge of machine shop practice is even more important, and for this reason experienced machinists and toolmakers who take up drafting work find that their shop training is invaluable, especially when designing tools, jigs, or parts which require considerable machine work. The draftsman who has not had actual experience in the shop should consult with machinists, patternmakers or others who may be able to supply valuable information. When making special tools, such as jigs, milling fixtures, etc., for reducing the cost of the work, the foreman or workman should not only be consulted, but should usually have a deciding voice as to what should be made. The men who use the tools often know better than the draftsman what is needed.

The machinist and shop foreman, being constantly with the work, know which are the expensive operations, when there is difficulty in fitting, where the clearances are so small as often to become interferences, and other facts of importance in developing a good design. A method that has been used with excellent results is for each foreman and responsible workman to have a blank stub-book with the pages numbered and provided with suitable printed headings. In this book all suggestions are written and the perforated leaves are removed and sent to the chief draftsman, the stub being kept as a memorandum for the shop man. These leaves are sorted when received, and those requiring immediate attention are investigated; others are filed under the respective machines until another lot is to be built, when the suggestions are considered collectively. The workmen are not only at liberty to use these books, but are held accountable if they allow troubles to exist on the machines they build, and do not report them. In this way, advantage is taken of the mechanical knowledge and the ideas stored up in the minds of the men who are actually doing the work.

Mechanical Drawings and Their Application. - A mechanical drawing is a representation on paper of a machine, ma- chine part, tool, or other object used in the mechanical industries, that is to be made from metal, wood, or other material, and it may show either the form or shape to which a machine part is to be made, or the relation of a number of parts to each other in order to indicate how these parts are to be assembled; hence, a mechanical drawing may be used to represent anything from a locomotive to a small machine screw. These drawings are generally made on a drawing-board by means of a T-square, triangles, compasses, scales, etc., but sometimes they are made without the use of any instruments except a pen or pencil, in which case they are generally known as "free-hand drawings" or "sketches.'' All instruments,  appliances, and accessories necessary for the production of a mechanical drawing are generally known by the general term "drawing instruments,'' except the various kinds of paper, cloth, etc., upon which the drawing is made, and the pencils, ink, and erasers, which are grouped under the head of "drawing materials."

Classification of Mechanical Drawings. - Mechanical drawings may be classified under two main headings, outline drawings and working drawings. Outline drawings merely show the general appearance and over-all dimensions of machines and devices and are used mainly in catalogues and for representing the general features of a machine to prospective purchasers. Working drawings are used in machine shops and pattern shops in the building of machines and tools. Working drawings may be of two kinds. The first class, known as general or assembly drawings, show all the parts of a machine or mechanism, in their proper position and relation to each other. The principal dimensions may or may not be given on drawings of this kind, which are used by the assemblers and erectors in the fitting and assembling of machine parts that have already been made in other departments of the shop. The second class of working drawings, known as detail drawings give all the dimensions and complete information as to the form of separate machine parts or of sections composed of several parts. The dimensions on working drawings indicate the sizes of all parts requiring machining operations, and such drawings should also contain complete information regarding the material from which parts are to be made and the treatment they are to undergo - such as hardening, case hardening, etc. The assembly drawings are generally made to a much smaller size or scale than that of the actual machine or tool, while the detail drawings are made either full size, or to as large a scale as possible.

A working drawing must convey to the eye of the workman a clear idea of what the designer wants made. It should be so complete that, when it passes into the shop, no further questions or explanations will be necessary; hence, a complete working drawing contains all the necessary information as to materials, treatment, limits, fits, finish, etc., that the shop man requires.

Mechanical drawings do not represent an object in the form of a picture - that is, they do not show the object in the same way as it would appear to the eye of the observer. A drawing made to appear exactly as it would when viewed from a certain point is known as a perspective drawing, and the mechanical draftsman is seldom required to represent machines or machine parts in this way. He conveys his ideas by much simpler and better methods than this and, in making working drawings, uses what is known as orthographic projection, or simply projection. Many mechanical drawings do not appear to the untrained eye to represent the true form of an object - that is, they do not always look like the object, as would a picture for instance, - because certain methods of representing machine parts have been adopted, by means of which drawings can be made much more rapidly and the exact form and dimensions can be indicated more accurately than if the true perspective form were reproduced. As an example, screw threads are not ordinarily drawn in the way in which they actually appear to the eye, but a much easier and quicker way of representing them is used.

General Views and Detail Drawings. - When it is desired to make a drawing of a machine already constructed, each part is measured and sketched separately, and all necessary dimensions are placed upon the sketch; then these parts are assembled, so to speak, in the form of a general drawing. On the other hand, if it is desired to design a machine, a general drawing with the parts in place is made, and then the dimensions of the various parts are determined and the extent to which they must be machined by turning, planing, milling, drilling, reaming, tapping, etc. The different parts or small units consisting of several parts, or of a great many parts in some cases, are drawn separately, at least in sufficient detail to show clearly what is wanted.

In the general views, outlines are drawn of such details as are thought essential to clearness. If the machine, tool, or other device is not complicated and consists of a relatively small number of parts, a general drawing may be sufficient, but if there are a great many separate parts, separate working drawings of these details are necessary. If all the details of a complicated design were drawn on the general view, there would be so many lines that it would be difficult, if not impossible, to show clearly the form and size of each part. The smaller detail drawings are also much more convenient for shop use.

A mechanical drawing should show clearly the form and dimensions of the part it represents. Every line should stand for some definite thing; and when lines cannot express the ideas in a direct and unmistakable manner, abbreviations, symbols, and printed notes should be used so that the patternmakers or machinists who are to use the drawings will be able to work without other instructions. Each note should consist of concise sentences and should be placed close to the part to which it refers, in order that it may be easily read and understood. The different views should be arranged so that they dearly represent the object drawn, and a reasonable degree of neatness in the drawing of lines and in lettering is also desirable.

Why a Knowledge of Mechanical Drawing is Essential. - Every man engaged in the mechanical trades who has aspirations toward advancement must, at least, learn how to read or understand mechanical drawings; and in order to obtain a complete knowledge of the reading of mechanical drawings, it is necessary to know, in a general way, how to make them. A man who cannot make accurate and understandable sketches according to the methods of mechanical drawing is seldom able to read any except the simplest drawings; hence, the importance of the study of mechanical drawing. Drawing may be called a universal language, and the mechanic who cannot read drawings or blueprints is handicapped in his trade almost as much as if he could not read or write. The knowledge of how to make sketches and drawings is also an exceedingly useful accomplishment to a man who, as foreman or superintendent, has to direct the activities of others. The importance of understanding the principles of mechanical drawing, therefore, is apparent and it should be clearly understood that not only draftsmen must understand these principles, but every mechanic who wants to read drawings rapidly and accurately.

In order to read a working drawing, it is necessary that one be familiar with the conventional methods commonly used to represent parts, material, finish, etc., and that one understand in what respects mechanical drawings differ from perspective drawings or photographs, which represent the object as it appears to the eye. To the inexperienced, a working drawing may appear like a conglomeration of lines which do not represent clearly what they are intended to show, but the man who understands such drawings will have a mental picture of the object drawn.


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