Earnest T. Child.

In our last talk the subject of sections was taken up, and a few of the conventional lines used in showing sections of various materials were given. The proper use of sections is of such great importance that further consideration is necessary, and the student will do well to study the matter of sectional representation very carefully. This is necessary in order that a clear understanding of the subject may be acquired. The best method for attaining proficiency is by constant practice, and this point should be borne in mind by the student who hopes to become a professional draughtsman. One may look at drawings and read books on the subject, but not until he actually learns to use draughting instruments properly, by careful and continuous practice, will he begin to advance towards the desired end.

It is not meant by this that every spare moment should be devoted to practice ; but a certain time, say two or three evenings a week, should be set aside by the student and devoted to it. The writer, although a professional draughtsman, devoted two evenings a week for seven consecutive winters to various classes of drawing which were not included in his regular work. And this time was very profitably spent. Students who live in cities have a great advantage, as it is possible to attend evening drawing schools, where they may receive instruction from teachers who have a large experience ; but there are many who are not so fortunate, and it is this latter and larger class which it is intended that these talks on drawing may reach and help. As stated above, it is continuous effort which shows results in the long run, just as falling water, drop by drop, will wear away a stone.

But to return to the study of sections. It is often more convenient to show a piece of mechanism in outline, when it is necessary that a section should be made of some particular part. Instead of making a separate section showing this particular detail, it may be shown by dotting the section lining of the part in question. Another method is to merely hatch the outline of the part which is to be shown as sectional. This is shown more clearly in Fig. 14, showing piston. Sections when shown this way are called " dotted sections," and may be used to good advantage when pencilling drawings from which two distinct tracings are to be taken, one showing outline, the other section.

When finished, drawings are made on paper for record work. Sections are often shown in colors, which greatly adds to the clearness of the drawing. Unfortunately, drawings of this class are of little real value for anything except record, as they cannot be given to the workmen, who would certainly deface them,and they cannot be blue-printed. The result is that, in a majority of instances, the tracing constitutes the sole record, and line sectioning has to be resorted to. Tracings possess a great advantage over paper drawings, as they may be blue-printed any number of times, and being thinner, require less room in the files. This may seem a minor matter ; but when a manufacturing concern has ten or fifteen thousand drawings on file it amounts to considerable. The proper choice of location for a section is all important, and requires much study and more judgment. As already explained, the function of a section is to add clearness to the drawing, which without it would be more or less confused. The section, or cutting plane, should be so located as to show all the details which need explanation in the clearest manner. In order to do this, it is often necessary to omit certain details of the mechanism which may be beyond the section, but which, if shown, will only confuse the drawing, without helping in any way. For instance, in showing the section of a gear, only the parts which are continuous should be in section, as the hub and the rim. The teeth and arms should be shown in outline. It will not be necessary to dot all the teeth to the gear. This will only add confusion, and increase the labor of making the drawing. If it is desired to show in section a circular flange with a number of bolts through it, all the bolts being equidistant from the center, it will not be necessary to show all the bolts. Only two should be shown, and they should be located in the section at the proper distance from the center, whether they happen to come exactly so in the section, or not. It is not at all necessary that a section be continuous. If it is found to add to the clearness of the drawing to show one part in section and another part in outline, it may be done, or it is allowable to change the plane of a section in different parts of the same object. When this is done, it is customary to show the line of the section by letters, and mark the drawing section on A B C D, or by any other letters which may be readily seen. If it is necessary to show a section of a piece of mechanism which, while it is symmetrical, may not be cut evenly by a section, as, for instance, a cylindrical piece having an odd number of hubs or ears, it will be found expedient to depart from strictly conventional lines, and either show a complete section which is balanced, both sides being equal, or better to simply show a half section through one lug to the center. If a strictly conventional section were shown, it would be misleading rather than helpful, and the value of the sections would be lost. This is a point which cannot be brought out too forcibly. The section is primarily used to add clearness to the drawing, and should be used only when it is necessary to avoid confusion. Never lose sight of the fact that your drawings must be kept as simple as possible. Every line means extra work, and sections require more time than any other part of the work; therefore they should be used only when necessary. It will not be well to go to extremes and try to dispense with sections, for, when necessary, they would be neglected. The proper use of sections cannot be taught in a minute ; it must be acquired by practice and judgment. The latter is acquired by the former, so they really go hand in hand. Fig. 14, already referred to, illustrates two distinct methods of showing sections. The detail of piston and rod is really a side and end elevation, but dotted sectioning has been used, and it will be seen that both classes of dotted sections add clearness to the drawing. The type shown in the lower half of the piston is preferable in that it may be more readily accomplished. The detail of crosshead illustrates the ordinary method of sectioning, so far as the upper part of the work is concerned. This also illustrates a method of saving extra drawings in that the upper part is shown in section, and the lower part is in elevation. This is often done and is allowable when an object is perfectly symmetrical. This method adds to the clearness of the drawing, as it shows at a glance the external appearance as well as the sectional arrangements. If it is found expedient to use this method, it is well to show an elevation of the object on the same sheet with the section, thus giving a clearer conception of the object to be shown. Fig. 15 shows the fly-wheel for the 4x4 engine. It will be seen that the same general remarks are appropriate for showing this section as for a gear; that is, only the parts which are continuous should be in section. Even if the wheel were turned so that the section line would come through the spokes, they would not be shown in section, as this would give the impression that they were continuous, forming a web instead of being only six in number, as happens to be the case. It will also be allowable to show the keyway on the center line, if thought necessary, but it is preferable to keep to projection as much as possible in this particular case.

The last two illustrations, Figs. 14 and 15, have been of cast iron so far as the sectional work was concerned, and consequently the section lines have been full and evenly spaced.

In Fig. 16, showing details of valve and stern, stuffing boxes and plug, is an illustration of the method employed for showing composition, or brass, using the conventional line given in Fig. 9-C. In showing the valve and stern, only one view is necessary, with the exception of the lower end, where the knuckle joint for connecting to the eccentric strap is shown. This has to be shown by two views, as it will be seen that it is not of regular shape, while the remainder of the detail is cylindrical. The valve plug is threaded into the top of the steam chest and guides the end of the valve stem. The stuffing boxes are used to prevent the escape of steam from the cylinder and steam chest, while permitting the rods to slide with comparative freedom.

These details should be carefully copied, and it will be well to enlarge them, making the drawings on a double standard sheet, thus gaining experience in transferring from one scale to another. This will also bring the drawings more nearly to the size and scale required in actual practice.