Conventional Representation Of Straight Knurling

screw thread, and those not accustomed to it may be confused especially when shown in hidden detail. I have the further objection to it that to the amateur drawer-reader it can be confused with a shaft bearing two flats — see sketch (m). For all

my own drawings I use the first method, (a, b, c). It is very clear, particularly in hidden detail, and I don't find it difficult to make a presentable job of the outline. I do use method (h) occasionally, but never in hidden detail, as the drawing becomes a mass of dotted lines. A male thread in section is impossible with any method other than (d). Square and Acme threads cannot be shown by similar conventions. Fig. 56 (n) is the pictorial convention - it is not a true projection, as the lines are all straight instead of being curves. This is used on presentation drawings i.e. those where an impression must be created. But on working drawings these threads are shown as at (o) and (p) for male and sectioned female threads respectively. (Hidden detail would be shown as at (o) but dotted lines.) The same convention is used for Acme threads unless they are large enough to draw the proper Acme profile, which is seldom. Both square and Acme threads must be defined by notes anyway, so there is no need to distinguish the conventions.

Springs are indicated in a number of ways, but those shown in Fig. 57 (a) and (b) are preferred: (a) is a compression spring, and you will see that the semi-pictorial representation enables the type of end to be indicated clearly although it should also be defined in a note.

Similarly, with the tension spring (b) the type of end can be indicated. It is good practice for these ends to be detailed in a scrap view alongside the spring so that they

Fig. 57

Conventional representation of machine parts, (h) is an example of "Draughtsmans Privilege", explained in the text.

Fig. 57

Conventional representation of machine parts, (h) is an example of "Draughtsmans Privilege", explained in the text.

can be properly dimensioned. Splines and gears carry teeth, the drawing of which „ could be a labour and an unnecessary labour at that. The convention is that one or two teeth only are drawn and then indicated as in Fig. 57 (c) and (d). The spline and tooth form are, of course, defined in a note. In the case of gears the pitch circle should be drawn. In the sectional view on the right you can see two methods of indicating the teeth. At the top a chain-dotted line is drawn on the pitch circle; where the two wheels meet I have shown a couple of diagonal lines, either can be used. There is no need to detail ball or roller bearings, the convention at (e) will suffice. The same symbol is used for both, the type being defined in a note. Alongside at (f), are the symbols for knurling Some draughtsmen like to hatch the complete area but this is unnecessary for a workshop drawing although desirable on assembly and general arrangements. Flats and squares are usually shown on end views or sections, but can be indicated on elevations as at (g). To end this section, a case of draughtsman's privilege. In any NVQ examination the projection of a hexagon nut in the way seen at (h) would lose marks, but it is common in commercial drawings -1 always draw them this way on design drawings. The reason is that we see the nut taking up the maximum amount of room in both views, and so can check that the nut can, in fact, be turned. The one situation where we do show two flats of the hexagon would be where we wish to have one flat of the bolt-head hard up against a web so that it is prevented from turning when fitting the nut.

There are a number of other conventions, of course, but they are of less interest to the amateur. Full details can be obtained by reference to British Standard No. 308 (at your local public library).

Section 6

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