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(c) Fourth view is redundant drawing takes very little longer than an ordinary one. True, constructing the ellipses takes time, but, on the other hand, there is only one view to draw. However, its main use is as a 'pictorial' to help workers, in the pattern shop especially, to visualise the shape of the object to be made.

How many views?

We have just remarked that an oblique projection of any sort can show in one view information which may take three or more in normal orthographic representation. This immediately raises the question "How many views are necessary in the normal engi-

Fig. 25 An example of the type of drawing used in workshops in the 1880's. Even then, separate drawings of individual parts were preferred. ("Machine Drawing" - Jones & Jones, 1907)

simple "student exercise; see how many of the missing views you can find.

simple "student exercise; see how many of the missing views you can find.

neering drawing?". There can be no rule. There must be enough views to ensure that there is no risk of misinterpretation. In Fig. 24 two views at (a) should suffice, but three are needed at (b). On the other hand, the additional view at (c) is redundant; it conveys no additional information, and wastes space. However, on a complex object it may be necessary to add such redundant views because otherwise the number of dimensions on the necessary ones may be overcrowded and it can be said that too many views are better than too few. Fig. 25 is such an example, in the style of drawing commonly used about 100 years ago. There is sufficient information to allow all the parts to be made, but the risk of confusion is considerable. Today, of course, each part would be laid out on a separate drawing, with an 'arrangement' drawing for the erectors. At the other extreme, Fig. 26 is an exercise given to students in a drawing class: (b) and (c) are plan views and (a) a row of elevations. Each has one example of the elevations. Make a sketch and see how many different objects these could represent (some students found more than six). Fig. 26A is a case where a third view is not essential, but might be helpful. As we shall see later, in many cases a partial view, of just one aspect of the object, may ease visualisation as well as simplify the dimensioning. It is not a bad idea either when dealing with a component which is an unusual shape to set a thumbnail sketch in one corner of the drawing to give a pictorial view. Fig. 27 is an example; the parts

Fig. 26a A case where two views are sufficient but a third might help to interpret the shape.

of the fabrication were fully detailed, but this sketch (in trimetric) helps to show the shape as well as clarifying the order of assembly.

Section 3

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