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Drills and Drilling
Drilling may be effected in two ways, by rotating the drill and holding the work stationary, or vice versa. The most satisfactory results, however, are obtained by revolving the work and gradually bringing the drill into contact with it. Although it is not always possible to do this, owing to the shape of the article to be drilled. A drill of the shape shown in Fig. 109 is preferable for drilling hard steel, while the shape shown in Fig. 110 is best suited for drilling soft steel, brass, etc. Oil or glycerine diluted with alcohol is the best lubricant for the softer metals, but when drilling hard steel turpentine should be used. Drills of the form shown in Fig. 111, are used for drilling flat bottomed holes, for countersinking screw heads, etc. See also Countersinks. The twist drill shown in Fig. 112 is desirable when drilling deeply, as this form of drill heats slowly and the particles are carried to the surface of the work. Pivot drills, like those shown in Fig. 113, can be purchased from material dealers, mounted on cards and ready for use, at such small cost that it will scarcely pay the watch-maker to make them.
Drills of a form indicated by Fig. 114 are recommended highly by Saunier and are known as semi-cylindrical drills. They are made from cylindrical steel rods, rounded at their ends and filed down to a trifle less than half their thickness. The length of the point should be greater or less according to the nature or the metal to be operated upon, but under no circumstances must the point itself be sharp. This form of drill should be sharpened on the round side and not on the flat surface. It possesses, says Saunier, the advantages that when placed in a drill-chuck it can be turned exactly round, of the required diameter and finish; so that whenever replaced in the chuck, one can be certain beforehand that the hole drilled will be of a definite diameter. With such a drill the hole is smoothed immediately after it is made by one or the other cutting edges.