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Fig. 67 A typical "assembly" drawing, part in section to show detail.

Heathrow Airport Diagram

and the drawing the cotter. The sheet was A4 size.

Almost as important is the ASSEMBLY DRAWING, as this is needed with few exceptions to guide the erector. This also is drawn to scale with some precision, and usually carries no dimensions other than those necessary either to give the eventual customer some idea of the size, or to indicate the normal length of some adjustable part. Considerable use will be made of sectional views, but it is not considered necessary to give sections of standard parts - valves, bearings, etc. -unless there is something special about them. Nor are sections necessary when a sub-assembly drawing carries both detail and arrangement views.

Accuracy and clarity are again essential, and some care is often needed in the choice of scale of the drawing, even on a large engine there may be some very small details. Fig. 67 is an example that has served both as an assembly and as a display drawing. In this case detail assemblies took care of matters like the governor drive and the arrangement of the parallel motion linkwork. This drawing was on A3 size.

DESIGN DRAWINGS are seldom, if ever, seen outside the drawing office. These are the medium through which the designer converts his ideas into reality. They are, in part, ephemeral, as the india-rubber is as much an essential tool as the pencil. On the other hand, they are usually 'on the board' for far longer than any other type of drawing. This fact, and the need for meticulous accuracy demand special paper and also very hard, and hard pencils.

Unfortunately this makes it very difficult to reproduce them, Fig. 68 shows the original design drawing for the engine shown in Fig. 67. You will see that the lines were (in the original drawing) very thin. Also, that some parts, designed separately, are merely sketched in freehand. There are notes, mere scribbles, all over the place. On the other hand, a singular paucity of dimensions; this is the exception to the rule, as the drawing must be sufficiently accurate to scale from if need be. (In practice, of course, there is an intermediate state between the design and the workshop drawing - that of detailing, which is a further design process in which individual parts are refined in design.) You will appreciate that Fig. 68 shows only part of the drawing -the original was nearly 4ft long. I should add, too, that though Figs 67 and 68 look very much alike, the former was not made from the latter. Assembly drawings must always be made using the details from the workshop drawings.

The tools for the job

We had better start with pencils. If all you need is something to make rough sketches then the ordinary HB pencil (perhaps with a rubber at the end) will serve, but for anything more than this you should obtain some of proper draughtsman 's quality. These are made by Royal Sovereign, Venus, Faber-Castell, Staedtler and others. The 'lead' (really graphite) is compounded to give a very uniform and black line. They are available in grades from about 6B to 9H. Tastes differ, but for what it is worth I use grade B for sketching, H for hand sketches, 2H and 3H for detail drawings, and 4H and 6H for design work -the harder grade for such things as centrelines and the softer for outlines. For dimensions and lettering H or2H are used.

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