with short, stiff, steel wire teeth, must be used to clean the file thoroughly after every few strokes, and the file rechalked after each cleaning.
Use the file in both hands as illustrated in Figure 120, and run every stroke the full length of the barrel, maintaining a constant firm pressure. It is not necessary to bear down hard enough to spring the barrel—just a firm steady pressure both ways, so that the file takes a cut "coming and going." A little practice will enable you to feel the right pressure, so that the file will just take hold. Excessive pressure will result in too deep a cut and make "flats" a sixteenth of an inch or wider along the barrel, and these are hard to work out.
Too little pressure, however, is as bad as not enough. A file's purpose is to cut—not to scratch. Each stroke should remove an appreciable amount of metal. Some men never acquire the knack of fine surface filing—the fear of cutting too deep causes them to merely scour and scratch the surface. It is surprising how many full pressure strokes are required to reduce a piece of stock a thousandth of an inch.
When you learn to strike a barrel with the proper pressure, neither too light nor too heavy, the cuts taken will have scarcely any appreciable width—or in other words, there will be no flats visible on the surface.
Nevertheless, the flats are there, and the next step is to polish them out. Emery cloth is the usual polishing medium, but I prefer carborundum or- alundum cloth when it is obtainable, due to its longer cutting life, and faster work. This material is sold by ma-
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