We have already seen the importance of perspectives, isometrics and shop drawings. Let us now consider a new and equally important subject known as SECTIONS.

A section of any object, such as a machine or machine part or a tool or anything else, is what we would see if we could slice or saw that object into two parts at the point where the section is to be taken. Everyone is familiar with the appearance of a baseball, but few of us know what a section through the ball looks like. From all outside appearances, the ball could be hollow and we would not know the difference. The same is true for every other object which is not transparent. We have no way of knowing what the interior looks like until we slice or cut it down the middle, or any other way, for that matter. You can readily see that sections are just as important to the tool designer or the engineer or architect or carpenter as the various views of the object. In designing complicated machinery, sectional views, taken at various points, are essential; no architect's plans are complete without detailed sectional views through interior mouldings and window trim. A knowledge of sections is just as important, therefore, to the draftsman as a knowledge of working drawings or isometrics.

In dealing with sections we must use a considerable amount of imagination. Sometimes the problem is very easy and sometimes it is extremely difficult. There is no set rule for determining what a section will look like; you just have to use your imagination and experience. To take some very simple examples: any section of a sphere will always be a circle, no matter where it is cut, but any section of a cube will not necessarily be a square. If a cube is cut slantingly, the resulting section will be a rectangle and not a square. If a pear is cut vertically down the middle, the section will be pear-shaped; but if it is cut horizontally, the section will be a circle, the diameter of which will depend upon where you cut the pear. A right cylinder, cut vertically, will be a rectangle; cut horizontally, it will be a circle, and cut on the slant it will be an ellipse. A right cone, cut horizontally, will be a circle; cut slantingly it will be an ellipse; cut parallel to its slanting side, it will be a parabola; cut vertically, not through the middle, it will be a hyperbola. This has already been demonstrated in the conic sections part of the chapter on geometrical figures.

Now look at page 90. Here we see four different objects: a, b, c, and d. Object a is a box which, if cut vertically through the width, will produce section A-A. If it is cut vertically through the length, it will produce Section B-B; if cut horizontally, it will produce section C-C. Study these three sections closely and note the differences in them. Refer to the box to check up on the correctness of the sections. The cross lines always indicate sectional lines—parts that are cut through. If the box were as thick as indicated, the sections would be just what you see. Object b shows a hollow cylinder cut in the same three ways. Why are sections B-B and C-C the same? Now study objects c and d to see why the sections of these objects look the way they are shown. The best way to visualize sectional views is to imagine yourself sawing the object clear through the place designated. If you continually bear this in mind you will have no trouble understanding sections.

Figure 42 shows a few machine parts with sections cut out of them. This is another way of showing sections. These are very simple isometrics with parts of them shown in section. They are perhaps the clearest kind of section drawing, and are used to a great extent in large factories turning out complicated machine parts. In many cases, these sectional isometrics are air-brushed to give the final appearance of a photograph, with parts cut out to show the structure more clearly. They are sometimes known as "explosion drawings," and most of them are enormously complicated and difficult to do. Many of them are done by specialists who require several weeks of steady work to complete a single drawing.

In Figure 41 we see still another way of showing sections. Here is a gadget that looks like an ink bottle; all sections are vertical and are taken at the same point; a shows the gadget as it appears in elevation, while b shows it completely cut through. Note the difference in the direction of the cross lines between the bottle cap and the section of the bottle itself. This must always be indicated when two different materials are shown in section in contact with each other. In c we see half-section and half-elevation and in d we see the same gadget in isometric section.

It would be well to practice drawing sections of different objects free-hand, just to prove to yourself that you understand the basic principles. Examine some of the molding and trim in your home, and then try to imagine sawing them horizontally. Try to draw the cross sections, or if you happen to be reasonably near a lumber mill or carpenter shop, drop in and ask to see some stock trim. Examine the trim and then note its cross section. This will give you an excellent working knowledge of sections which will be particularly valuable to you if you go to work in an architect's office,

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