PART II.

MACHINE AND ENGINE DRAWING AND DESIGN.

SECTION XIII

INTRODUCTION.

ENGINEERING drawing in its generally accepted sense really consists of two distinct parts, the first being the mechanical act of merely drawing an object in accordance with certain recognised laws of projection; the second, that of designing and deciding the proportions and form of the object by considering the strength of the materials of which it is made, the forces which will act upon it, and the work which it will be called upon to do.

It is scarcely possible to become a good designer without possessing some powers of draughtsmanship, but it is quite easy to be an excellent draughtsman without knowing anything of the principles of design and construction. For instance, a student who has acquired a knowledge of practical geometry, and who, therefore, knows the meaning and uses of "plans,' "elevations," "sections," "interpenetrations," and "developments," could, if given sufficient sketches and measurements, make complete drawings of the most complicated machine, and yet be quite ignorant of the manner in which the shape and proportions of the machine had been arrived at.

But such a condition of mere mechanical working is above all things to be avoided, and the objects of the following pages is to show in an elementary manner, the general principles of theory and practice which guide the design and construction of engineering machines and structures. In order that they may be intelligently approached, the student should endeavour to understand the following remarks on drawing and design.

(1) Drawings.-Engineering drawings may be divided into two classes :-I. "Working drawings," which show all the details

of separate parts, so that the parts can be made from them. II. "General-arrangement drawings," showing the complete machine, and the relation and position of the different parts. Usually the designer first makes a rough general-arrangement drawing, in order to obtain the leading sizes of the different parts, after which a complete working drawing of each part is made and sent into the workshops, in order that the part may be at once proceeded with; and, finally, a finished general - arrangement drawing is completed.

(2) Scale of Drawings. In working drawings the scale should be as large as possible-3", 4", or 6" to 1' being usual proportions. All small parts should be drawn full size. Generalarrangement drawings are made to smaller scales, such as 1", ", 1", 1", or 3" to 1'. The scale adopted depends to some extent upon the size of the drawing papers, the usual sizes being Double Elephant, 40′′ × 27′′; Atlas, 34" x 26"; Imperial, 30′′ × 22′′; Half-Imperial, 22′′ × 15′′. In making scale drawings, lengths should be marked off by placing the edge of the scale along the line, and marking the line at the required lengths, not by first measuring the length on the scale with a pair of dividers, and then transferring to the drawing.

Always write the scale of a drawing prominently upon it (see p. 14).

(3) Number of Views in Drawings.-The chief desirable feature in all engineering drawings is clearness, and the number of views and sections must be enough to obtain this; anything further is a waste of time. The student must, therefore, decide the number of views necessary as a first step, since that obviously affects the scale of the drawing. Two views, at least, are invariably required; either one elevation and a plan, or one front and one end elevation; but, in the majority of cases, it is necessary to show three views-two elevations and one plan. With complicated parts or general arrangements, a larger number of views may be necessary in order to show all the details. When an object is symmetrical about a centre line, it is sometimes sufficient to show one-half of the view; also, when one part only of an object requires an additional view, it is only necessary to project that part, and not the whole object. For example, the construction of the inclined bearing in Fig. 157 is shown much better and more quickly by the plan of the cover drawn above the elevation, than by a geometrically correct plan, which would show the cover and nuts foreshortened, owing to their inclination.

(4) Sections. It is only necessary to draw parts in section where their construction cannot be clearly shown without doing

So. In many cases it is sufficient to show the parts by dotted lines, or it may be better to show a part only in section and the remainder by dotted lines. When an object is symmetrical about a centre line, it is usual to show one-half only in section, as seen in Figs. 1546, 164. The section may end exactly at the centre line, as in Fig. 1546, or it may extend beyond it, as in Fig. 153. The end of the section part is shown by black lines.

Objects which are completely solid, such as bolts, rods, spindles, and shafts, are not shown in section, although, strictly speaking, they are cut by the section plane as much as the other parts. Therefore, in drawing the section of a stuffing box, as in Fig. 163, or of a valve, as in Fig. 172, the rod, spindle, and the studs are not sectioned. In the same way the arms of a wheel (Fig. 174), the guides of a valve (Figs. 173, 175), and the nuts of bolts and screws are not shown in section. Nothing would be gained by sectioning such parts, and a drawing is much clearer by leaving them unsectioned.

(5) Dotted Lines.-The student will have seen from preceding examples that dotted lines are used to show parts of an object that are not really seen from the point at which the object is viewed. In engineering drawings it is often necessary to make a free use of dotted lines, but they should always be avoided when they do not really add to the clearness of a drawing. In such sectional views, as of the stuffing box in Fig. 1636, and of the shaft-coupling in Fig. 137, it would be strictly accurate to show by dotted lines the edges of the flanges on the back of the object, but as clearness is not generally gained by so doing, they are better omitted.

(6) Order of Drawing Centre Lines. - It seems to be a constant difficulty with a student to know where to begin a drawing. Few things look more careless than the different views of a drawing crowded together or near one edge of the paper, when there is plenty of room for a good space between the views and for an equal border all round. The order of drawing should be as follows:

I. Decide the number of views and the scale.

II. Find approximately the space each view will take up and the position of the chief centre lines.

III. Pencil in the chief centre lines of the different views in such a position as to give equal space between the views and an equal border all round when the drawing is finished.

IV. Draw the leading part in all the views, and then add the remaining parts in the order of their relation to the leading part and to each other. In each case show one part in all the views before commencing another part, so that the different views are