Projection 49

The end view gives at once a much clearer idea of the object than either plan or elevation. What is marked A° on the elevation, and A' on the plan, in the profile view at AP is shown to be a slot. By reference to A', it will be seen that the slot does not extend the entire length of the block, and is rounded at the end. The vertical clotted line e°-f° represents the left-hand end of the slot A. In comparison with A, BP repre sents a narrower slot of less depth, and one which extends the whole length of the piece_ This latter statement accounts for the fact that B" is left open at the top, while AP is closed. The part lettered C" and Cv is found on the pro file view to be a rounded projection appearing at CP.

Assuming, for the sake of illustration, that the plan and elevation are drawn, and that the end view is to be constructed from them, it would be done as follows: Draw first a center line on the plan; then, at any convenient distance to the right of the elevation, draw a vertical line for the center line of the end view. Since vertical heights will correspond on the end view with those of the elevation, a T square line through lower edge of D will give the level of base of the required view; and a second T-square line through the lower edge of Cv will locate the flat part of the top of the block.

Next, the width of the end view must be the same as that of the plan; hence the distance from the center line G-H to point 1 is taken, and laid off on the end view from J-K along the line L-M to the left, giving point 1; and point 8 is located in the same way to the right of J-K. Vertical lines drawn through 1 and 8 will give the general outline of the end view. Then, measuring for the remaining points, and laying off from the center line each time, points 2, 3, 4, 5, etc. are located, those on the near side of G-H being measured to the left of J-K, and those on the further side to the right.

Next the depths of the slots as shown on the elevation are projected over with the T-square to the end view, and AP and Bp completed. Having located on the end view point 6 as a center, the half-circle is drawn with the radius 6-7, taken from the plan. The upper edge of the shallow groove Dv is projected across to J-K, becoming the highest point of the curve. The radius of the curve being given, its center is found on J-K, and the curve drawn-in as the arc of a circle, thus completing the end view.

67. Let it be required to draw a wooden chest of given length, breadth, and depth, and to show the cover opened through 150 degrees. Evi

dently this is a case which requires the end view to be drawn first. This end view is first drawn from the given dimensions at A, Fig. 57, showing the cover opened at the given angle. The ends of the chest are supposed to be nailed to the bottom and sides, which are represented in view A by the dotted lines.

The front elevation, B, is taken looking at A in the direction of the arrow. This elevation will show the length and height of the chest, and will show the cover in its real length, but narrower than its true width. The different heights are projected over with the T-square from A to B. In a view of A in the direction of the arrows, 1-2, the extreme left-hand edge of the cover, will not be seen; hence this is drawn with the dotted line 1-2 in view B. The plan, C, shows the length and width of the chest, the extreme width of the chest and cover together being equal to the distance e-f of the end view A.

In this view also, the cover appears in less than its real width. Note carefully that the cover is shown less in width in elevation than in plan. This is simply because the cover is more nearly horizontal than ver tical; hence, in looking from above, the cover appears more nearly fiat-wise, and therefore nearly in its real width; while, viewed from the front, the cover is seen more nearly edgewise, and thus shows narrower.

This affords an exercise in reading a drawing —that is, in obtaining from the various views an exact understanding of the different parts of the object represented, and also of the object as a whole. This figure, and also Figs. 51 to 55, illustrate an important principle.

68. This principle may be stated as follows: The size of the projection of any given surface on a plane is determined by the angle which the surface makes with the plane of projection. If the surface is actually parallel to the plane of the drawing, then it will appear in its true size; if the surface is perpendicular to the plane, it will be seen edgewise simply as a line.

69. Suppose that a square prism is to be drawn standing on one corner, and in an oblique position so that its long edges are inclined at 30 degrees with the horizontal, and that the prism inclines upward to the right and away from the draftsman. To draw the object in this oblique position, in which none of the faces and none of the edges will he parallel to either plane, requires first the construction of two simpler positions.