Conventional Representation

To draw every part of a workpiece in full detail would be very time-consuming and would also be likely to make the drawing less readable. This particularly applies to parts that are often repeated, such as screw threads. Over the years, therefore, draughtsmen have developed many conventions or symbols to replace the detail representation of the feature these have gradually been standardised on a more or less international basis. I have already dealt with those referring to the sectioning of different materials on page 40. Figs 56 and 57 illustrate those referring to machine parts. In almost every case the convention is a stylised version of the appearance of the finished part.

In Fig. 56 are those for screw threads: (a), (b) and (c) show a semi-pictorial type for external, internal in section, and thread in hidden detail. Neither the pitch nor the core diameter are strictly to scale but should be in good proportion. Note that the extra penetration of the tapping drill is shown. This is a simple method, very clear even to a layman, and about the only way in which a male thread may be sectioned to show internal detail as at (d). It is very easy to draw, though can be ragged if hurried.

The type shown at (a), (f) and (g) was the British Standard preferred for many years. The thin lines represent the tops of the threads and the thick ones the root. Again, neither pitch nor core diameter need be accurately to scale. Draughtsmen would reverse their tee-squares on the board and use the tapered backs to get the slight angle to the lines. Note that (a) shows a right-hand thread and so does (b), although it appears to be reversed; this is because you are seeing the backot the thread in section. In more recent years the lines were drawn without slope, as at (h). There is no disadvantage, as the type of thread must be specified anyway.

The current British Standard convention is shown in (j), (k) and (I). Note the line showing the depth of full thread, and the slight taper beyond it to indicate the imperfect threads at the ends. It is clearly much quicker to draw than either of the other conventions.

All three methods are acceptable, and you can choose that which suits you best. However, for amateur use I would avoid the third type (j) to (I). It does not look like a

Fig.56 Conventional representation of screw threads.

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  • Amethyst
    What is the conventional representation of external screw thread?
    7 years ago

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