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Ebook Engineering fundamentals: Part 2

(BQ) Part 2 book "Engineering fundamentals" has contents: Marking out, basic bench fitting, drilling techniques and drilling machines, centre lathe and turning techniques, milling machines and milling techniques, grinding machines and processes. | 7 Marking out When you have read this chapter you should be able to understand how to: • Identify and select marking-out tools for making lines. • Identify and select marking-out equipment for providing guidance. • Identify and select marking-out equipment for providing support. • Identify and select different types of datum. • Identify and use different co-ordinate systems. • Mark out workpieces having square, rectangular, circular and irregular-shaped sections. 7.1 Marking-out equipment (tools for making lines) Marking out is, essentially, drawing on metal so as to provide guide lines for a fitter or a machinist to work to. A pencil line would not be suitable. The hard metal surface would soon make a pencil blunt and the line would become thick and inaccurate; also a pencil line is too easily wiped off a metal surface. Usually the line is scribed using a sharp pointed metal tool, such as a scriber, that cuts into the surface of the metal and leaves a fine, permanent line. 7.1.1 Scriber This is the basic marking-out tool. It consists of a handle with a sharp point. The pointed end is made from hardened steel so that it will stay sharp in use. Engineers’ scribers usually have one straight end and one hooked end, as shown in Fig. 7.1. It is essential that the scribing point is kept sharp. Scribing points should not be sharpened on a grinding machine. The heat generated by this process tends to soften the point of the scriber so that it soon becomes blunt. The scribing point should be kept needle sharp by the use of an oil stone (see Fig. 7.27). 7.1.2 Figure 7.1 Scriber Centre and dot punches Typical centre and dot punches are shown in Fig. 7.2. They are used for making indentations in the surface of the metal. There are two types of punch. Figure 7.2(a) shows a dot punch. This has a relatively fine point of about 60◦ or less and is used for locating the legs of such instruments as dividers and trammels. Figure 7.2(b) shows a centre punch. This is heavier than