How to read a drawing

Preface

The understanding of a drawing has appeared to many who are not familiar with this branch of education to be a very difficult study. It has been the writer's province to know that there are a vast number of mechanics who look upon drawings with awe, thinking that to become proficient in the understanding of drawings they first must have scholarly or technical knowledge. It is the purpose of this book to show that this is a very erroneous idea, it being a fact that this branch of education requires less study than almost any other branch of useful knowledge; and all the education necessary is, that the student be able to read and understand the English language.

As to its usefulness, I doubt if there is any branch of education that compares with it; for by drawings we can express our thoughts and desires more clearly and concisely, and they can be understood more readily, than by any other system known to the civilized world. In the commercial or industrial world no work of consequence is carried on without them, and, from the making of a special bolt to the building of a railroad, drawings are necessary.

Who then, realizing this, can afford to be without this knowledge? Surely not those whose wages are small, for their wages will always be small unless they acquire useful knowledge which will tend to better their conditions in life; surely not those in the higher stations of life, for unless they understand drawings they must depend solely upon some one else who has this knowledge, in order that their desires may be carried out. The manager, purchasing agent, superintendent, foreman, mechanic, clerk, shopman, or laborer, will find this knowledge to be a stepping-stone toward success.

In this book it has been my endeavor to explain the reading of a drawing without the use of technical terms; but where

4 PREFACE

it has been absolutely necessary to use such terms, their meaning has been fully explained.

I also wish the student to understand that, while I have divided the different classes of drawings under different headings, this work should be studied as a whole; as failure to understand thoroughly any part of it may result in a misunderstanding of it all.

The main principles of drawing have been thoroughly explained and sufficient examples given to enable any one to understand clearly any drawing; but I want to impress upon the student that, in order to master it, he must study it carefully and conscientiously, for, in the words of a great philosopher, ''Nothing great is accomplished without an effort."

In conclusion, I want to add that, while this book was written with the intention of helping many, if it helps a few I shall be repaid.

Contents

CHAPTER PAGE

I. Method of Representing Objects 7

II. Lines Used in Projection Drawing 12

CHAPTER I Method of Representing Objects

It is my intention to explain in this work the method by which an object may be described by a drawing.

The branch of drawing about to be explained is known as Projection Drawing, as projection drawing not only enables those who are familiar with it to know exactly what the object represented looks like, but also shows the object in such a manner that the surface or any line or angle of the object can be readily measured on the drawing.

There is, however, another branch of drawing, called Perspective Drawing, that is very useful in showing what an object looks like, but would be of very little use to those who wish to construct the object so represented.

Even those who are thoroughly familiar with perspective drawing would have great difficulty in measuring or determining the size of the object represented, for, while they might know positively that the two ends were of the same size or that the tops or bottoms were the same, yet, if they were to measure on the drawing either of the two ends, or the top or bottom of the object, they would find that they are there shown to be of different dimensions.

Perspective view.

This is clearly shown in Fig. i , it being a perspective drawing of a rectangular object, similar in shape to an ordinary brick.

This figure represents the rectangular shape as it actually appears to the eye when looked at from a certain position. In this drawing three sides of the object are shown and the other three sides are hidden, but in projection drawing only one side is usually shown, the other sides being hidden.

HOW OBJECTS ARE SHOWN BY PROJECTION DRAWING

Suppose we were to place a brick or some other rectangular object, with one of its largest sides down, upon the top of a table, or the floor of a room, or any other flat surface; then, if we were to stand so that we could look down upon it with the eye directly above it, the outline of the top of the object would appear to the eye as it is represented in Fig. 2. As we were then looking at the top of the object, this view would

be known as the top view, or Fig. 2. PLAN view, as it is sometimes

called.

Rule. — A drawing which represents an object as if it were resting upon the top of a table the floor of a room, or any other flat surface, the observer looking directly down upon it from above, is called a top view or plan view.

If we were to look directly at the side (long side) of the object, with the eye at the sam.e level as the top of the table upon which the object is resting, the outline of the side of the object would appear to the eye as it is represented in Fig. 3 . As we were then looking at the side of the object, this view would be known as the side view, or front view, or, as it is sometimes called, side elevation.

Rule. — ^A drawing which represents an object when ob- - served directly from the side, with the line of vision upon the same level as the top of the table upon which the object is resting, is called the side view or side elevation.

Let us now turn the object around so that its small side,

METHOD OF REPRESENTING OBJECTS 9

or end, can be viewed from the same position that the large side was viewed from; then, viewing this small side or end in the same manner in which we viewed the large side, it will appear to the eye as it is represented in Fig. 4. As we were then looking at the end of the object, this view would be known as the end view or end elevation.

Fig. 3.

Fig. 4.

Rule. — ^A drawing which represents an object when observed directly from its end, with the line of vision upon the same level as the top of the table upon which the object is resting, is called an end view or end elevation.

The manner in which the top, side, or end of an object is shown has now been explained, but there is another view to be shown, namely the bottom view. ^^^- 5»

Let us now imagine that our object has been turned over so that it rests upon one of its sides; the bottom view would now be visible ; then, by viewing it in the same manner in which the side was viewed,

the bottom view would appear to the eye as it is represented in Fig. 5. As we w^ere then looking at the bottom of the object, this view would be known as the bottom view.

It may be well to state here that sometimes the end or the side view of an object could be considered the front or the rear of an object ; when this occurs, the view shown would be known as front view or front elevation, or rear VIEW or rear elevation.

In Fig. 6 the different views are placed in their proper positions — that is, top view at the top of the drawing, end views at each end of front view, side view or front view in the centre, bottom view at the bottom, and view of rear side at the extreme right-hand side of the drawing.

For convenience, I have repeated Fig. i on this page, it being a perspective drawing of Fig. 6; also the corners of the object are marked with letters of the alphabet; these letters correspond with the letters in Fig. 6. This was done in order that the different drawings may be readily compared.

In Fig. 6 the letters A, B, C, D represent the Top View or Plan View of the object shown in Fig. i ; D, C, H, G repre-

Different views.

sent the Side View or vSide Elevation, or, as it is sometimes called, the Front View or Front Elevation; A, D, E, H and C, B, G, F represent the ends of the object, which would be known as the End View or End Elevation; H, G, E, F represent the bottom view, and A, B, E, F represent the Rear View or Rear Elevation of the object.

As there are six sides to our object, it will be noticed that in Fig. 6 all the six views are shown ; all of these views are not absolutely necessary in this case, as the object represented is very simple in construction; but all the views are shown in

METHOD OF REPRESENTING OBJECTS order that the student may see the manner in which all the surface views of any objects may be represented by projection drawing. In order that the student shall clearly understand why these different views are shown in this manner, I give another illustration in Fig. 7.

Fig. 7 shows the different views seen in Fig. 6, joined together. In order that this illustration may be thoroughly understood, I wish to have the student cut out of cardboard a figure similar to the one shown by the heavy lines of Fig. 7, then, by bending the cardboard where the lines are dotted, the cardboard will form the figure of a rectangular object similar to the one shown in Fig. I . This then will give the student a practical example of why the views are shown by the method used in Fig. 6.

Fig. 7.

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Cardboard diagram.

To be able to understand how to read a drawing, the whole secret lies in being able to see in the mind what the object represented looks like ; for in projection drawing the object is not represented as it is shown by a photograph, for in a projection drawing you see only one view at a time, and the student must remember that the view shown represents the outline, or profile, of the object ; and every line that represents the form or shape of the object is shown exactly as it would appear to the eye if the eye was at a point directly facing the object.

Therefore, study carefully every word of this chapter, for if the student clearly understands all that is contained in it he will have mastered the fundamental principles of how drawings are read and the balance of this study will be found to be very simple.

CHAPTER II Lines used in Projection Drawing

There are several kinds of lines used in projection drawing, the ones commonly used are as follows :

The full line

The dotted line

The dash and dotted line

The Hght full line .

The full line is most employed, it being used to draw the outline and all parts of an object which can be seen with the eye.

The dotted line consists of a series of very short dashes, and is used to show that part of an object that is not visible to the eye; for instance, in Chapter I, page lo. Fig. i, the edge or outline of the object marked by the letters B and F, E and F, and F and G cannot be seen by the eye ; thus these edges are shown by the dotted lines. This rule of showing invisible parts of the object by a dotted line is always used in projection drawing.

The dash and dotted line is used to indicate centre lines of an object, or to show from what points a section has been taken. The use of this line will be fully explained later.

The light full line is used for showing dimensions on a drawing (that is, the distance from one point to another) ;

sometimes, however, the dash and dotted line ( — )

is used for this purpsoe, as it serves to prevent the dimension lines from being mistaken for the full line which is used to show the outlines of the object, but, as a general rule, most dimensions are shown by the light full line. The student will soon learn to distinguish the different lines, so that no mistakes or confusion will result. Any other lines not shown here will be fully explained when the necessity for using them arises.

In Chapter I (page lo) it has been stated that all the views shown were not absolutely necessary, as the object was very simple in construction, and they were given merely to show to the student the proper location of the different views by which difficult objects may be represented. In this chapter it will be explained why all these views were not necessary.

LINES USED IN PROJECTION DRAWING

In Fig. 8 this same object is shown by only three views, — namely, top view, end view, and side view. In this figure dimensions and dimension lines are shown, which will help to explain why these three views are sufficient to show the object properly. At each end of all the dimension lines is

placed an arrow point ( >) . This symbol is employed to show

from what points the dimensions are taken; the dimension lines are light full lines about one-half the thickness of the outlines of the object.

In this drawing it will be noticed that two small marks ('') are placed after the figures. These two marks are used to denote inches ; when only one is used (') it denotes feet; thus I '-2'' is read one foot and two inches; ic'-3>^" is read ten feet and three and one-half inches. This method is universally used to represent feet and inches on all drawings; thus, the top view of Fig. 8 shows the object to be nine inches long by four

and one-half inches wide, the side or front view shows the object to be of the same length and two and one-half inches deep, the end view also shows the same dimension for the width that is shown in the top view. While this latter dimension is not absolutely necessary, it helps the student to see at a glance that it is an end view, as the dimension showing the width of the top is the same dimension as the width in the end view.

It will be seen now that these three views are all that is necessary to properly show the construction of the object, as the top view shows the length and width, the side or front view the depth, and the end view the shape of the ends. As these views show the object complete, no others are necessary, for when the top of an object is shown the bottom is always understood to be the same unless some other view shows that the bottom is different ; also when one side or end of an object

Fig

. 8.

^4

1^

N\f is shown the other side or end is regarded as the same unless some other views show that they are different. This perhaps can be more clearly understood by referring to Fig. 9.

Fig. 9 shows an object similar to the one pictured in Fig. 8,

except that the bottom

^^^- 9- corner of the front of the

object has been cut off.

This cut-off is shown by the top view and front view to extend the entire length of the object.

The top view and side view show that the cut begins i>^" back from the front and Y^" up from the bottom; in the top view the exact point where this cut starts, not being visible to the eye, is shown by a dotted line. In the front view the point where the cut starts being visible, it appears as a full line. The end view shows the shape of the object very clearly.

This brings me to the point where I desire emphatically to impress upon the student the necessity of looking at all the views of the object that are shown before determining w^hat the object really looks like. For example, in Fig. 10 the top view and side or front view are the same as the top and

side views of Fig. 8. The end view, however, shows that the shape of the object is different, it having a wedge-shape instead of the rectangular shape shown in Fig. 8. Thus it will be seen that all views of the object must be taken into consideration before determining what the object represented really looks like.

Fig

. 10.

■ ""^1

Chapter III

Fig. II.

Views Needed

In all practical projection drawings only those views are shown which will actually describe the object; for instance, in Fig. II is represented a cylinder. The lines marked A B and C D are known as centre lines. In this drawing only two views are necessary to describe the object properly. The upper view shows that the top of the object is circular in shape, and the dimension 2" shows the diameter. Now, if we know that the top is circular and that the object is a solid body, all that remains for us to know is the depth, for every solid circular body must have a depth. By looking at the lower view, which is projected from the centre lines A B, we see that the object is 6" deep. Thus the views which are shown are stifhcient to describe this object properly.

There is one other view which we must take into consideration, and that is called the Sectional View, or section of an object. This view is often of great help in determining exactly what an object looks like at a point where it can not be shown prominently by either top, bottom, side, or end views.

Fig. 12 represents an object similar in shape to a spool. The top view shows that O

the object at its ends, or at the top and bottom, is circular, the outer circle denoting the extreme diameter; the intermediate circle is that portion which is not visible to the eye when looking down upon it from above, and the inner circle denotes the size of the hole which runs through the object. The elevation, or front view, shows the outlines of the object as they appear when looking at the object as though it were resting on one of its fiat ends. The dotted lines indicate the hole which runs through the object, but which is not visible to the eye.

Fig. 13 represents the same object as above described, but instead of a side view or side elevation a section has been taken through the centre line A B, — that is, the spool is imagined to be cut in half lengthwise, and that part which is toward the front removed; then it is looked at in the same manner as if it were a front view or front elevation, but in order to distinguish it from any of these views the section lines are employed. These lines are simply light lines drawn at an angle inside of all the outlines.

The student should by this time be familiar with the method by which simple objects are shown by projection drawings, and also with the main principles b}^ which any object may be shown. However, there are many other minor points which shall be fully described as we proceed.

CHAPTER IV The Fig. 14

fJ.A

Universally Used Structural Shapes

following chapter is designed with the purpose of briefly showing and describing the various structural shapes which are universally used in the construction of buildings, bridges, etc.

Fig. 14 represents the top, end, and front views of what is known as an I Beam. This shape derives its name from the fact that the end view resembles the ca})ital letter I.

The end view of Fig. 14 shows the shape of the I beam, the front view represents the depth and length of the beam, and the top view shows the length of the beam and the width of the flange. In the end view the top and bottom portions of the figure are known as the flanges, and that portion between the top and bottom flanges is known as the web. As will be seen, the front view or length is represented by two upper and two lower lines, being projected from the sharp corners of theflangeshownin the end view.

I beams are rolled to various sizes and weights; these sizes and weights are given in hand-books which are published by manufacturers of this material. I beams are generally known by the depth of the beam and weight per foot.

Lers/c^TH

Fig. 14 represents a 12" I beam weighing 31}^ pounds per foot ; this is generally written 1 2 " I 3 1)^ lbs. The outlines of the top view indicate the top flange, and the two dotted lines represent the web. The method of projecting the top view or flange from the end view is illustrated in Fig. 14, it being shown in a perpendicular position, but, as this method of indicating the top view takes up considerable space on a drawing, the form generally used is that seen in Fig. 15. Here,

/. £/VCj T/f the top view is drawn immediately above the front view in a horizontal position, instead of a perpendicular position as is shown in Fig. 14.

Fig. 16 represents what is known as a channel. This shape or section is similar to the I beam, except the flanges, which do not extend over the web on one side. This shape is also rolled to various weights and sizes; the size shown in Fig. 16 is 10" deep and weighs 15 pounds to each foot; this is generally written 10" C 15 lbs. The end view of Fig. 16 shows the shape of this section.

The front view is projected from the end view in the same manner as is shown in Figs. 14 and 15.

The top view represents the top flange. The first line directly above the front view vshows the outer edge of the

UNIVERSALLY USED STRUCTURAL SHAPES 19

flange, the dotted line indicates the face of the web, and the next full line represents the back of the channel.

Fig. 17 represents an angle. This shape or section is also rolled to various sizes and weights. It is, however, generally known by the size of its legs, — i.e., the distance from the corner

^tANne or back of the angle to the extreme edge of the leg, and the thickness of the metal of which the angle is composed.

Fig. 17 shows a 3" x 3" x %" angle, which is generally written 3" X 3" X %" L. The top view is above the front view, and is represented in the same manner as the top view of the

5'

Le/vc^-r// channel shown in Fig. 16, but we distinguish it from a channel by the end view, which clearly shows the shape of the angle. In this illustration we have a practical example of why all the views shown must be taken into consideration before we know positively what the object represented really is.

Fig. 1 8 represents a T bar or T shape. This shape is known by the size of the legs and the weight per Hneal foot. Fig. i8 shows a 4" X 4" X 13.7 lbs. T. As the thickness of the metal varies in the legs of the T, the weight per foot is given. This shape gets its name from the fact that the end view resembles the capital letter T.

The top view is projected from the end view by the same method as that used in showing the projection of the I beam, as illustrated in Fig. 14, but for convenience, as explained before and as is shown in Fig. 15, this view is placed directly above the front view. This brings me to the point where I must explain that the different views of an object may be placed in any position to suit the convenience of the draftsman, but those which are not in their regular position (see Fig. 6) should be so marked as to show which view they represent. However, a view is sometimes placed out of its regular position without being so marked where it is apparent for various reasons that it could not be taken for any view other than the one intended.

Fig. 19 represents what is known as a Z bar, which derives its name from the fact that the end view resembles the capital letter Z. This section or shape is generally known by the depth (4"). width of the flange (sKe'O' and thickness of metal (Kb'O- This is generally written 4" x ^y^i' x Ke" Z bar. The sizes and weights of all Z bars are shown in detail in any of the hand-books published by the manufacturers of this product.

UNIVERSALLY USED STRUCTURAL SHAPES 21

The views are shown by the same method used in the other illustrations. While the above illustrations by no means cover all the different shapes manufactured or used in the construction of buildings, bridges, etc., yet I have shown the

I. ^A/a -TH ^

M

H, -t main shapes that are used for general construction purposes, and in the following chapters will be shown some of the methods by which these shapes may be assembled to form the various members used in construction work.

CHAPTER V Scales used in Drawing

In making drawings and likewise in the understanding of drawings, there is a factor which we must take into consideration, and that is what is known as a scale. A scale is an instrument which is used to measure distances on drawings.

The student of course knows that it is impossible to make drawings of the full size of very large objects; so where it is desired to make a drawing smaller than the object, a scale is used to measure distances instead of the standard ruler. For example, a drawing one-fourth of the natural size of an object one foot long would be made only 3" long, because 3" are one-fourth of one foot; therefore, on the drawing 3'' would be equal to one foot or 12'^ This is known in drawings as a scale of 3" to the foot, or, in other words, a scale which is made or graduated to 3'' to the foot is simply a ruler by which 3" are equal to 12'' of the standard rule.

A drawing, however, may be made to any scale desired: for instance, if it were made one-fourth size, it would be known to be to the scale of 3" to the foot; if made one-eighth size, it would be known as iK" to the foot.

K2 size would be equal to i" to the foot. Ke size would be equal to K" to the foot. ]4i size would be equal to %" to the foot. K2 size would be equal to %" to the foot. jis size would be equal to %" to the foot.

When a drawing is well made, the scale to which it is drawn is usually marked under the title of the drawing, and is generally written in the following manner: 3''=i'-o" or i"=i'-o", etc.

Scales are usually made 6 inches and 12 inches long, and in shape are either triangular or flat. A scale 6 inches long is shown slightly reduced in Fig. 20. One edge is divided or graduated the same as the standard ruler, — that is, into inches,

SCALES USED IN DRAWINGS half inches, quarter inches, eighths of an inch, and sixteenths of an inch. This scale is used in the same manner as the ordinary ruler, or for making full-sized drawings.

The other edge contains two scales, — namely, i" to the foot and Vi to the foot. These sizes are indicated at the ends of the scale in large figures. In the inch scale, starting from the left end of the scale and reading toward the right, this being the scale of i" to the foot, each inch is equal to one foot or 12 standard inches.

The first (standard) inch is divided first into four equal parts, each of which represents one-fourth foot or three inches; for convenience in reading the scale, these divisions are marked with the figures 3,6, and 9 and represent respectively that number of inches. By this then we see that we can measure in inches any part of a foot. Each of these inches is divided into four parts, each part representing one-fourth of one inch, and, if so desired, each one-fourth inch may be again divided to represent one-eighth of an inch. Thus it will be seen that with a scale which is so graduated that \" is equal to 12", or I 'o" , we are able to measure inches or any part of an inch down to one-eighth of an inch.

Now, starting at, and on the same line as zero (o), each inch reading toward the right represents a foot. Thus, if we wish to measure 3' 4}^", it would be the distance between the points A-B in Fig. 20, or 5' iiK" would be the distance between the points C and D in the same figure.