vertical line in the left side view represents theback face. Again, there is a line that appears asa visible line in the right side view and as ahidden line in the left side view.In the back view, the block is shown reversed,so that the cutaway part, which appears to theright in the front view, appears to the left in theback view. Similarly, the right-hand vertical linein the front view represents the right side of theblock, while the right-hand vertical line in the backview represents the left side.As a general observation, it is helpful in viewanalysis to note that in the top, bottom, and sideviews, the line that represents the front face ofthe block faces toward the front view of the block.Similarly, in the back view, the line that representsthe left side faces toward the left side view of theblock. This applies to third-angle projection only.A point that constitutes a corner on anobject is sometimes numbered for purposes ofidentification in various views of the object. Ina particular view of an object, a corner pointnumber may be visible, or it may be hidden, asshown in figure 5-18. In the upper left corner ofthe figure, there is an oblique projection of ablock, with a corner numbered 2. You can see thatthis corner is visible in top, back, and left sideviews, but hidden in bottom, front, and right sideviews.The rule for numbering is that for a hiddencorner point, the number is placed within theoutline, and for a visible corner point, outsidethe outline. You can see how the rule has beenfollowed in figure 5-18.A multi-view projection should contain onlyas many views as are required to describe theobject fully. If you refer back to figure 5-17,you can see at once that the back viewdoes not convey any information that isnot available in the front view; the backview is therefore superfluous and should beomitted. The same applies to the bottom view,which conveys no information not availablein the top view. Likewise, the left side viewconveys no information not available in the rightside view.You have the choice of omitting either the topor bottom view and either the right side or leftside view. One general rule in this instance is thata top view is preferable to a bottom view and aright side view, to a left side view; another ruleis that a view with a visible line is preferable toa view with the same line shown as a hidden line.Both rules apply here to eliminate the bottom andthe left side views. All you need here is athree-view projection showing the top, front, andright side views.It is often the case that a two-view projectionis all that is required. The view at the top of figure5-19 shows a single-view projection of an object.It is obvious that a top view of this object tellsyou everything you need to know except thethickness; a right side view tells you everythingyou need to know except the length; and a frontview tells you everything you need to knowexcept the width. All you need to do, then, is toselect a particular view and couple it with anotherview that gives you the dimension that is missingin the first view.There are three possible two-dimensionalprojections of the object shown in A, B, and C.In the selection of one of these three, everythingelse being equal, the balance of the drawing wouldbe the deciding factor. Either A or B appearsbetter balanced than C, and between A and B,A would look better on a long oblong sheet ofpaper, and B, better on a shorter oblong sheet.The object shown in figure 5-19 has adefinitely designated top and front; it follows thatthe right and left sides are also definitelydesignated. This is the case with many objects;you have no choice, for example, with regard tothe top, bottom, front, and back of a house.Figure 5-19.-Two-view multi-view projections.5-11
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