Fig. 29 Illustrating the rules governing the use of dotted lines. The cases are discussed in the text.

parts. Solis black has been used for the packing; cross-hatching is preferred in this situation.

(3) If three or more dotted lines meet at a point the meeting should be on dashes in each case, Fig. 29e.

(4) When drawing arcs in dotted lines the end of the curve should coincide with the end of a dash, Fig. 29k and (I).

Fig. 29f shows the treatment of screw threads in hidden detail, but this method of representation is now permitted rather than preferred. We shall be dealing with this matter later. Shown at (g) and (h) is the treatment of gear teeth in cases where the profile is shown complete, (g) is for small teeth, and (h) for large. For very large teeth there might be more dashes on the flanks, but there should always be a bent dash at the corners.

Drawing these dotted (really they are dashed) lines is not easy - there is a temptation to rush the job. The most common fault is excessive length of the dash with ragged ends. Try to keep a ratio of dash to space of between 2:1 and 3:1. As already noticed, current practice is to draw these lines thin, but I try to make them just a little more emphatic than centrelines, though not as bold as outlines.

Sectioned views

The sectioned view is by far the most effective way of showing the internal shape and dimension of an object - whether it be a complete engine or a small part. It is clear, and provided that the conventions are observed seldom prone to misinterpretation. The view imagines that the part has been cut or broken and shows the cut face, which is indicated by section lines or hatching, drawn (as a rule) at an angle of 45 degrees to the main axis of the drawing - Fig. 30. You will see that where two parts meet the section lines run in opposite directions. To get over the difficulty when three parts meet (as for the packing in the sketch) the draughtsman has used full black, but it would be more usual to use cross hatching here. Note that the section lines are thinner than the outline and regularly spaced - a most important matter.

Many find difficulty in achieving this, but it is quite easy. All you have to do is to scribe a line along each edge of your 45-degree set-square, 1,5mm from the edge on one, 2.5mm on another, and 3.5mm from the edge on the third. When you have drawn one section line, advance the square until the line lies below the scribed line on the square and draw another, and so on. As you may, occasionally, need section lines at 30 or 60 degrees rather than 45, do the same on your 60-degree square as well.

Section m 'EF'

Fig. 31 Cross-section of an air compressor designed by the author. Because the wall thicknesses are so small, section lines have not been used.

Section m 'EF'

Fig. 31 Cross-section of an air compressor designed by the author. Because the wall thicknesses are so small, section lines have not been used.

Before going on to the conventions and rules governing sectional views, a word about one special case or, rather, practice. On many drawings you may find that there are no section lines at all. This is, in part, due to sheer laziness, but there is a problem when drawings have to be reduced considerably in size for reproduction in a magazine or book. The section lines may then come so close together that they appear as a blur, and to avoid this the draughtsman has left them off, Fig. 31. This is a pity, as it can cause confusion. The problem can be avoided either by drawing the section lines slightly farther apart or, better still, by asking the printer to lay on a tint, as I have done in Fig. 32. It does add slightly to the cost, but a drawing is such an important part of any constructional article that this is warranted.

Fig. 32 "Shading" using a tint, can be helpful on small parts.



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