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Fig. 74 An exercise in reading drawings. Each sketch has one or two missing lines. Copy the sketches and see if you can find them. Warning; in all but the top row there is one sketch in third angle projection; all the rest are in first angle.

symbol accordingly. However, to reduce your anxiety I will reveal that all the sketches in the first row are in first angle and that there is only one in third angle in each of the other rows. Note that some of the missing lines may be hidden detail - dotted lines. If you run into difficulties remember that if there is a line on the drawing there will be line joining two points on the object, and that there must be a corresponding line on every view of the object. However, the line may appear as hidden detail, and if the line on the object would lie vertically on the paper, then it will appear only as a point. If you find yourself in real trouble over any of the sketches, make a little model out of Plasticine or balsa wood. This is quite a regular procedure; most engineers keep a stock of each in their offices.

In the case of published drawings, if you have any difficulty reading them, by far the best procedure is to make a new drawing. Even if it is no more than a straight copy of the original the very act of copying will help to clarify matters. But if there is much hidden detail in the form of dotted lines, then make sectional drawings of those parts. If you have no drawing tackle then make sketches on squared paper; it helps even if not exactly to scale. Note that this is not a confession of weakness. Such procedures are very common, especially in design offices, and unless things have changed a lot over recent years, patternmakers always copy complex drawings full size onto "scribing board". Imagine what the fully dimensioned drawing of the crankcase of the engine in Fig. 75 would look like when it had eight cylinders - drawn at less than full size at that. I commend this procedure of redrawing to you: I use it a lot. If nothing else it may reveal the missing or erroneous dimensions, to say nothing of the odd missing line.

Fig. 75 An assembly drawing of a "National" diesef engine, 16-1/2" bore x21" stroke (From "Diesel Engine Design", T.D.Walshaw - Geo. Newnes, 1949)

Section 10