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Machining is the process by which metal is formed into a desired shape and size with great accuracy. Power-driven machines are used to cut, shave, grind, or drill the metal into prescribed dimensions, so that separate pieces may be easily assembled into the complete product. Before the metal can be fed into the machines, however, it must be marked with machining directions so that a machinist or a machine operator can perform the proper operations. This important preliminary work is the task of the layout worker.
To perform this job, layout workers use their knowledge of standard machine tools; the sequence of machining operations; the nature of shop practice; and the working properties of steel, cast iron, aluminum, and other metals and metal alloys. After studying blueprints or other specifications, layout workers use a variety of different instruments to mark the metal, indicating machining points. The scriber, which marks lines on the surface of the metal, the center punch, which marks the centers of the ends of pieces to be drilled or machined, and the key seat or box rule, which draws lines and lays off distances on curved surfaces, are commonly used instruments.
Determining exactly where the marks must be placed is, of course, extremely important. To ensure precision, workers use dividers for transferring and comparing distances, L or T squares for determining right angles, and calipers and micrometers for accurate measurement. High degrees of accuracy in measurement are required for this work, since some specifications call for tolerances as close as one thousandth of an inch.