object as seen from F, erect a vertical plane at AD, and project an elevation on this plane; now turn the plane into the position AD', and we have the elevation shown by fig. 30. Fig. 31 is an elevation projected on the plane BC, the observer being at G. Figs. 30 and 31 may be projected similarly to fig. 29, as shown in figs. 32, 33, 34. Fig. 34 is a projection on the vertical plane AD, which is turned into a horizontal position, giving the elevation as shown. The same elevation is obtained in figs. 30 and 34, the only difference being one of position; both methods are employed, convenience generally deciding which is to be adopted; the former, however, is the one chiefly employed. The circular dotted lines shown in the figures need not be put in your drawings after becoming familiar with the principles laid down. 16. The elevations obtained in figs. 30, 31, are the same, both in form and position, as would be obtained if we were supposed to be looking at fig. 29 in the directions H and K respectively, and saw it projected on the vertical planes at AD", BC". From these remarks the student will understand that the position of the object is between himself and its projection; and when one elevation is projected from another, as fig. 30 from fig. 29, the given elevation and the projection are in the same order as before, calling the given elevation the object; for example, if the observer is at H, fig. 29, looking towards the figure, and wishes to draw the elevation as seen by him in that position, the elevation must be placed in the position occupied by fig. 30; similarly an elevation in the direction K would be placed as fig. 31. In practice it is sometimes inconvenient, owing to the size of one of the elevations, to follow out this order correctly; in such cases figs. 30 and 31 change places; this should, however, be considered only as an exception. The student is recommended to follow out strictly the order indicated, as it will save him much time and trouble, and make the study of projection more natural to him, and will, at the same time, remove one source of error which he is liable to fall into. Fig. 28 is a plan; fig. 29 a front-elevation; figs. 30 and 31 end or side-elevations. The term "view" is often used instead of "elevation." CHAPTER VI. THE examples in this chapter may be used as drawing exercises, the figures to be drawn full or half size. 17. We will now apply the foregoing principles to the representation of various pieces of machinery, &c. Nuts. Figs. 35 to 37, Plate III., are elevations and plan of an ordinary hexagonal nut for a bolt 1" diameter, as represented in scale-drawings; the elevations are not quite correct, but are good approximations. Fig. 35 is the plan; fig. 36 the front-elevation; fig. 37 an end-elevation. The construction lines clearly indicate how each view is obtained. In figs. 36 and 37 the curves a'b' are considered as arcs of circles; their true form will be considered in the following figures. The greatest and least diameters of the nut are taken from a table of Whitworth Standards, a copy of which, with a few additions, is here given (see Table I.). The thickness of the nut (t) is equal to the diameter of the bolt. We shall not give dimensions of nuts in the details to follow, but refer the student to the table given. For ordinary scale drawings, sufficient accuracy will be obtained in drawing nuts if we take the following as a rule for nuts for bolts under 11" diameter, viz., make the diameter across the angles = two diameters of the bolt. In figs. 38-41 we show the nut given in the preceding figures drawn full size; in this example the nut is turned on the top face and chamfered; there are two common ways of chamfering, that forming a conical outline, and the one shown, which has a spherical outline, the radius TABLE I. radius r. of the sphere equalling in figs. 38, 39, 41. By a a'b', b'a' are also equal; but as those marked a'b' are inclined faces are all equal, it therefore follows that the curves of the nut; the curve b'a' is part of this circle. The six will be a circle of radius r' and the greatest diameter made by the vertical plane vs1, containing the face a'b'c'd', &c., would touch the inner surface of a hollow sphere of cal outline we mean that the chamfered surfaces d'e', b'f', All sections of the sphere being circles, that spheri A TABLE OF THE SIZES * OF WHITWORTH STANDARD HEXAGONAL NUTS. *There are intermediate sizes which are not enumerated in the Table. |