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Fig. 61 Machining marks (a) on older drawings (b) immediate post-WWII standard, (c) 1953 standard, not recommended, (d) Current "preferred" symbol, (e) Machining mark with surface roughness number stated in Microns or micro-inches.

Then, following US practice, the tick shown in Fig. 61(c) became the standard. It did p. not last long, as it was not conspicuous enough. The current symbol is that shown in Fig. 61(d)-a triangle with a tail. For what it is worth, BS 308 specifies that the triangle be equilateral, and the tail at 60°. If an indication of the fineness of the finish is deemed to be necessary this is to be stated by including the roughness figure, in either micro-inches on imperial drawings or microns on metric ones, as shown at Fig. 61 (e) (that shown is in microns, about 125 micro-inches).

As in the case of tolerances, to specify surface finish requires considerable experience. Further, it cannot be checked without a very specialised piece of equipment. It is best to avoid any such indications on drawings for model engineers or amateur machinists. Quite apart from anything else, precision of dimension does not always walk hand-in-hand with an apparently high surface finish. A turned finish, for example, ranges from 250 m-in, when roughing to 16 when finishing and even 4 m-in. under really good conditions. Grinding can range from 64 to 4 m-in. reaching down to 1 m-in, with special care. The best advice I can give to the model engineer is simply to indicate which surfaces on a casting or forging need a machined surface, and leave it to the machinist to use his or her judgement. It is, however, often prudent to indicate to the patternmaker the amount of machining allowance required. The cylinder head castings for one of our engines needed no more than 0.015 inch, the castings being so accurate that they were simply machined on a surface grinder. Amateur foundrymen may need a rather larger allowance.

Section 8

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