Now we come to a very important part of this book—the working drawing or shop drawing. As their name implies, these drawings are carefully drawn to scale, and are used by the mechanic or carpenter in making the actual tool or machine part. Every tool or machine part is made from a shop drawing, and hence, a knowledge of shop drawings is essential to every draftsman, or for that matter, to anyone else who wants to do mechanical drawing.

Since solids all have three dimensions, the maximum number of views possible is six—the top, the bottom and the four sides. A shop drawing is an accurate scale drawing which shows as many views of the object as is necessary to make the actual object.

Of course the number of views of an object that is necessary for its complete manufacture depends entirely upon the object. There can be any number from 1 to 6. There is no need to show more than one view of a ball, because all views will be alike. No matter which way you look at a ball, it will always have a circle for an outline, and it is the outline that counts in shop drawings. The same thing, naturally, is true of a cube. All six views of a cube will show a square outline; hence, for either a ball or a cube, only one drawing is necessary. On the other hand, a cylinder requires two drawings—the top and •the front. The outline of the top will be a circle; the outline of the front (there is really no front because there are no faces to the cylinder), will be a rectangle. A brick will require three drawings, as will almost all other objects. If you look around now at the objects in your room, you will see that they can be pictured in only three or four views—the top view, the side view, the front and the rear views. A table can easily be built from a shop drawing showing the top, the side and the front. The same is true of a chair, though here it may be necessary to show the back or rear view as well. In all the shop drawings considered here, we shall show only three views of the object in question.

Look down on the kitchen table. What do you see? You see a rectangle and nothing more. The top of the table is a rectangle; therefore, the top view is a rectangle. Now look at the end view or the side view. Here you see the two legs and a very thin rectangle which shows the thickness of the top of the table. The front view shows the same thing except that the legs are further apart. If you drew what you saw you would have a shop drawing similar to that shown in Figure 32. If you drew it to scale and put in all the dimensions, any carpenter could easily make the table from your drawing. The same is true for a chair or any other object. There must be a sufficient number of drawings showing different views of the same object to give all the information necessary to make that object.

Shop drawings, then, always consist of three or more drawings of the article to be made; they are generally made from what is known as "isometric drawings" which show the three sides of the object. You can see from Figure 33 that if this isometric is projected out on three planes, the outline of each view—the top, the side and the front—will show just what you would see if you looked "head on" at the object. The top view is just what you would see if you looked directly down on the object, and the front and side elevations are just what you would see if you looked directly at them. Study this figure very carefully; then look at the three drawings below it. They are the shop drawings taken from the isometric and drawn to the quarter-inch scale from the dimensions given on the isometric.

As isometric drawings serve the same purpose as perspectives right now we shall use the term "perspective" in place of the term "isometric" throughout this section. Later on we shall learn the difference between them.

Fig. 32

Now look at the four shop drawings (without dimensions) on page 56. They show four rough perspectives in the upper right corner of each box, as well as the three views obtained from each perspective. Imagine yourself looking directly down on the perspective in the first box. All you would see is a rectangle with a small rectangle to the right of it, as shown in the upper left drawing. Now imagine yourself looking directly at the side and compare what you see with the drawing of the side view. Do the same with the front view. Now check up on the other three boxes to make sure that the three views in each box are consistent with the perspective. Note that the side view is at the lower left, and extension lines from it act as guides for the front and top views. Study these four boxes very carefully and understand them thoroughly, for, while they are not drawn to any scale, they show the principle of shop drawing.

FILL IN THE MISSING LINES

On page 54 are shown four shop drawings, done from the four perspectives in the upper right-hand corners. There are lines missing in each of the drawings, and it is now up to you to fill in these missing lines correctly in order to convince yourself that you understand the views in shop drawing. Remember, the drawing at the top of the box is always the top view, the drawing at the left of the box is the front view, and the drawing to the right of the front view is the side or end view.

In filling in the missing lines you will have to observe the perspective drawing carefully. For example, take the two drawings shown opposite. They represent two shop drawings of the same thing. One contains missing lines (A), and the other is complete (B). Now look at A. If you study the perspective carefully you will see that line number 1 should be represented on the drawing of the top view. It is omitted, so you must put it in as shown on the top view drawing of B. Now look at the front view in A. Line number 2 is missing on this drawing, so it, too, must be put in, as shown in the same drawing of B. In the same way, line number 3 is missing in the end view of the drawing in A, so that it must be put in as shown in the end view in B.

It is only by carefully studying the perspective and imagining yourself looking "head on" at the top, front and side, that you will be able to tell what lines are necessary in the shop drawings. In the drawings on the opposite page there are a number of lines omitted, and it requires a study of the perspective in each case to determine what these lines are and where they ought to be placed on the shop drawings. If you do these correctly you will have no trouble in sketching in the shop drawings from the perspectives on page 57. And you will be familiar enough with the method of shop drawings to draw them up yourself, to scale, with all dimensions marked on them.

Remember this extremely important point: a shop drawing is an accurate line-representation of the thing to be made. It is not a pretty picture to be hung on the wall. Every line, every curve, every dimension and, in fact, every drop of ink that touches that drawing must mean something! All shop drawings must be checked for accuracy before they are blueprinted and given to the contractor or mechanic or carpenter. On the accuracy of the shop drawing depends the accuracy of the finished tool or machine part.

After you have become thoroughly familiar with the principle of shop drawings as illustrated on page 56 and have correctly filled in the missing lines in the drawings on page 54, sketch in the correct working drawings on page 57. It is not necessary to draw these views accurately; all that is necessary is to make sure that you can draw, even roughly, the top, side and front views of the four perspectives. If you can do this correctly, you will have gone a long way on the road to making shop drawings. Answers on page 94.

LAYOUT OF A SHOP DRAWING

On page 55 is shown the layout of a shop drawing. The principle involved is the same as given previously, except that three views instead of one are usually shown. The side elevation is always in the lower left-hand section of the paper; directly above it is the top elevation, and directly to the right of it is the front elevation. The illustration shows four stages in the development of a shop drawing. Box 1 shows the center of the paper and the center of the circle which appears in the top view. Note that the side and front elevations are blocked out roughly. In Box 2 the circle is inked in and the top elevation is blocked in by means of circular arcs extended from the front elevation as indicated. Box 3 shows all vertical and horizontal lines inked in. Box 4 gives the completed shop drawing with all pencil lines erased.

Note that the upper right-hand space is blank. This is for the perspective drawing which completes the shop drawing. After dimensioning this shop drawing carefully and lettering in all notes pertinent to the drawing, the mechanic has a complete description of the job. He sees the drawing as it appears in perspective, as it appears from the front, side and top, and he has all the dimensions with which to work. In the case in point, there is no need for the other three views since they would merely be two rectangles and a circle. But it may happen that in some more complicated machine parts these other three views differ. In such cases it is necessary to show six views in two separate drawings.

In more complicated shop drawings it is frequently necessary to show sections through the drawing. This will be discussed later under the heading Sections, but for the time being it is only necessary to know the principles of laying out working or shop drawings, and the principles of dimensioning them correctly.

In this part we will learn how to transform perspectives, or isometrics, into mechanical or shop drawings. The examples given here have to do entirely with this transformation. Later on, under the heading Isometrics, we shall learn how to reverse the process and make isometric drawings from working and shop drawings. Both these operations are extremely important to the draftsman. He must understand them thoroughly. When he is given a job he must be able to visualize it both from a standpoint of three dimensions and from the standpoint of each of its six elevations. Every architect in drawing sets of plans is really doing this very thing in a different way. The plans he draws are really sections through the building, showing walls and window spaces. When he has completed his plans he draws the front and side elevations—four of them all together, since it is unnecessary to draw the bottom and roof elevations. The architect also makes perspective drawings of the house for the purpose of giving his client a clear idea of what the finished job will look like. In the case of the mechanic, the perspective drawing takes the form of an isometric which shows clearly the three dimensions, and gives the mechanic or carpenter a view of all three faces of the object which is to be manufactured.

ACCURATE SHOP DRAWINGS

Now that we understand the principle of shop drawings and know how to lay them out, the next and most important step is to draw them accurately to some fixed scale—usually full or half-size. If you examine the shop drawings shown on pages 60 and 61, you will see the completed job with all the dimensions marked on it and everything done accurately to scale. Check each of these five shop drawings very carefully and be absolutely sure that you understand them.

It cannot be said too often: if you understand how to do shop drawings and can do them neatly, accurately and quickly, you have won more than half the battle of mechanical drawing and can consider yourself a draftsman. You must put in a lot of time and effort in order to master the technique of shop drawing. And right here is where you should do it—not later on! After you have carefully studied and checked the shop drawings on page 60, cover them up, leaving only the perspective exposed to view; then draw them for yourself to a different scale. After you have done this successfully, turn to page 59 and make shop drawings of each of the perspectives shown there. Draw these to scale and mark all dimensions. After you have drawn these nine shop drawings (there will be twenty-seven in all), check each one carefully. If you find that you have made a mistake, do the drawing over until you have satisfied yourself that it is perfect.

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