H

A SECTION AA

A SECTION AA

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

Fig. 33 Some of the rules and conventions governing the use of sections. Note the example at (a) - a very bad mistake often made by the amateur, (b) is the correct representation as explained in the text.

Fig. 33 shows a few of the conventions but starts at (a) with an all too common mistake. The section has been drawn, but the lines indicating the edges visible behind the cutting plane have been left off, so that the spindle appears to be in two pieces. The correct interpretation of such a section is seen at (b). Don't forget this point: all edges visible behind the cutting plane must be shown.

When large areas are to be sectioned, it is in order to draw section lines, hatching, around the edges only, as at (c). However, if there is any detail within the mass, say a hole, it must be hatched round, as indicated. If the hole or feature is small the lines may be closer together around it.

Clarity is all-important, and situations may arise where hatching at 45 degrees may cause confusion. Two cases are shown at (d), where the section lines are set to avoid running parallel to an edge.

We have already noted in passing that there can be a problem where two parts meet a third in section. The convention is shown at (e). Two parts are hatched at the same angle, but the lines are staggered where they meet - this is regarded as preferable to hatching one of them at a different angle. There are also difficulties with very thin sections. These are dealt with by using a thick black line instead of hatching, (f); the thickness need not be exactly to scale. It is, however, important that such a line be solid black, as otherwise it may be mistaken for the conventional representation of insulating material.

Indicating the position of the section plane is, of course, very important, especially on a complicated object. This is shown at (g), where a thin line runs through the boss, terminated by two thick arrows

Fig. 34 Shafts, keys, webs and similar parts are never sectioned as there is no hidden detail to reveal.

Fig. 34 Shafts, keys, webs and similar parts are never sectioned as there is no hidden detail to reveal.

pointing towards the sectional view and lettered for identification. Note that the convention of projection — first or third angle - used on the rest of the drawing should be followed where possible, though the arrows will prevent any confusion. However, if the section is taken on a normal centreline, as at (h), then there is no need for any arrows provided the section is properly projected. Finally on this sheet I show at (i) how dimensions must be treated. The figures are set in a space in the hatching, but the dimension line runs over them.

Parts that are never sectioned

There is no point in sectioning a part that contains no hidden detail. For this reason shafts, keys, bolts, rivets, wheel spokes, and webs are never sectioned on their longways centreline. This is illustrated on Fig. 34. You will see that the parts I have mentioned are all unsectioned. In the case of the web, it is sometimes prudent to write the word WEB across it, especially when dealing with webs inside castings. The sectioning of these parts is the second most common error in amateur drawings. To do so is not only a waste of time and pencil jzJLa

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Fig. 35 There is no need for the dotted lines at (a), as the apparently "missing" detail is evident from the outline, (b) is correct.

lead, but also makes the drawing much more difficult to interpret.

Fig. 35 illustrates yet another error. There is absolutely no need to show the hidden detail as on the left-hand view. The detail is not, in fact, hidden, as it can be deduced very simply from the outline of the wheel, as seen in the right-hand view. Dotted lines may be necessary on a sectioned view, of course, but they should only be used when necessary. The guiding rule is to include sufficient information for the part to be made, or the construction followed, yet to avoid cluttering the drawing with so much detail that it becomes confusing. Better to show a few more views than to smother one with hatching and dotted lines.

Other forms of sectional views

At Fig. 36a we have the half section. This is extremely useful for symmetrical objects, one half an outside view and the other sectioned. Note that there is no full line at the junction. At (b) is the partial section, which is self-explanatory; there is no point in sectioning the complete object in a case like this. Fig. 36c is a section on two planes. The cutting plane is indicated by the thin lines thickened at the change of direction and at the ends. In this case the alternative method of indicating direction is shown - the arrows point to the line, instead of forming part of it as on Fig. 33g and on Fig. 36f. Note that in this example one of the branches is in full section and the other in half section, and that a limited amount of hidden detail is shown to avoid the need to draw a further detail of the coring.

Revolved sections are shown at (d) and (e). For a relatively short piece the