In all the drills previously described, except fig. 474, the size of the point is lessened each time of sharpening; but to avoid this loss of size, a small part is often made parallel, as shown in fig. 479. In fig. 480, this mode is extended by making the drill with a cylindrical lump, so as to fill the hole: this is called the re-centering drill. It is used for commencing a small hole in a flat-bottomed cylindrical cavity; or else, in rotation with the common piercing drill, and the half-round bit, in drilling small and very deep holes in the lathe: see sect. iv. p. 567. Fig. 480 may be also considered to resemble the stop-drill, upon which a solid lump or shoulder is formed, or a collar is temporarily attached by a side screw, for limiting the depth to which the tool can penetrate the work.

Fig. 481, the cone countersink, may be viewed as a multiplication of the common single-cutting drill. Sometimes, however, the tool is filed with four equi-distant radial furrows, directly upon the axis, and with several intermediate parallel furrows sweeping at an angle round the cone. This makes a more even distribution of the teeth, than when all are radial as in the figure, and it is always used in the spherical cutters, or countersinks, known as cherries, which are used in making bullet-moulds.

On comparison, it may be said the single-chamfered drill, fig. 476, cuts more quickly than the double-chamfered, fig. 470, but that the former is also more disposed of the two, to swerve or run from its intended position. In using the double-cutting drills, it is also necessary to drill the holes at once to their full sizes, as otherwise the thin edges of these tools stick abruptly into the metal, and are liable to produce jagged or groovy surfaces, which destroy the circularity of the holes; the necessity for drilling the entire hole at once, joined to the feebleness of the drill-bow, limits the size of these drills.

In using the single chamfered drills, it is customary, and on several accounts desirable, to make large holes by a series of two or more drills; first the run of the drill is in a measure proportioned to its diameter, therefore the small tool departs less from its intended path, and a central hole once obtained, it is followed with little after-risk by the single-cutting drill, which is less penetrative. This mode likewise throws out of action the less favourable part of the drill near the point, and which in large drills is necessarily thick and obtuse; the subdivision of the work enables a comparatively small power to be used for drilling large holes, and also presents the choice of the velocity best suited to each progressive diameter operated upon. But where sufficient power can be obtained, it is generally more judicious to enlarge the holes previously made with the pointed drills, by some of the group of pin drills, figs 482 to 485, in which the guide principle is very perfectly employed: they present a close analogy to the plug center-bit, and the expanding center-bit, used in carpentry.

The ordinary pin-drill, fig. 482, is employed for making countersinks for the heads of screw-bolts inlaid flush with the surface, and also for enlarging holes commenced with pointed drills, by a cut parallel with the surface; the pin-drill is also particularly suited to thin materials, as the point of the ordinary drill would soon pierce through, and leave the guidance less certain. When this tool is used for iron, it is fluted as usual, and a, represents the form of one edge separately.

Figs. 482.

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Fig. 483 is a pin-drill principally used for cutting out large holes in cast-iron and other plates. In this case the narrow cutter removes a ring of metal, which is of course a less laborious process than cutting the whole into shavings. When this drill is applied from both sides, it may be used for plates half an inch and upwards in thickness; as should not the tool penetrate the whole of the way through, the piece may be broken out, and the rough edges cleaned with a file or a broach.

Fig. 484 is a tool commonly used for drilling the tube-plates for receiving the tubes of locomotive boilers; the material is about 3/4 inch thick, and the holes 1$ diameter. The loose cutter a, is fitted in a transverse mortise, and secured by a wedge; it admits of being several times ground, before the notch which guides the blade for centrality is obliterated. Fig. 485 is somewhat similar to the last two, but is principally intended for sinking grooves; and when the tool is figured as shown by the dotted line, it may be used for cutting bosses and mouldings on parts of work not otherwise accessible.

Many ingenious contrivances have been made to ensure the dimensions and angles of tools being exactly retained. In this class may be placed Mr. Roberts's pin-drill, figs. 486 and 487; in action it resembles the fluted pin-drill, fig. 482, but the iron stock is much heavier, and is attached to the drilling-machine by the square tang; the stock has two grooves at an angle of about 10 degrees with the axis, and rather deeper behind than in front. Two steel cutters, or nearly parallel blades represented black, are laid in the grooves; they are fixed by the ring and two set screws, s s, and are advanced as they become worn away, by two adjusting screws, a a, (one only seen,) placed at the angle of 10° through the second ring; which, for the convenience of construction is screwed up the drill-shaft just beyond the square tang whereby it is attached to the drilling-machine. The cutters are ground at the extreme ends, but they also require an occasional touch on the oilstone, to restore the keenness of the outer angles, which become somewhat rounded by the friction. The diminution from the trifling exterior sharpening, is allowed for by the slightly taper form of the blades.

Drills For Metal Used By Hand Continued 20068

The process of drilling, generally gives rise to more friction than that of turning, and the same methods of lubrication are used, but rather more commonly and plentifully; thus oil is used for the generality of metals, or from economy, soap and water; milk is the most proper for copper, gold, and silver; and cast iron and brass are usually drilled without lubrication, as described at page 538. For all the above-named metals, and for alloys of similar degrees of hardness, the common pointed steel drills are generally used; but for lead and very soft alloys, the carpenters' spoon bits and nose bits are usually employed, with water. For hardened steel and hard crystalline substances, copper or soft iron drills, such as fig. 67 or 71, page 178, Vol. I., supplied with emery powder and oil are needed; or the diamond drill-points 66, 68, and 70 are used for hardened steel, with oil alone.*

Having considered the most general forms of the cutting parts

* The boring tools used for the mineral substances, are partly adverted to in the ninth chapter of Vol. I; beginning with the bits used for the softest materials, those for boring through earth, sand, and clay, in order to obtain water, are enlarged copies of the shell, nose, and spiral bits used in carpentry, attached to long vertical rods which are screwed together like jointed gun rods, and are worked by a cross at the earth's surface. The rods are drawn up by a windlass, and joint after joint is unscrewed, until the bit, with its contained earth, is brought to the surface. Various attempts have been made to avoid the tedious necessity for raising the rods, by the employment of a hollow cylinder or magazine resting on the bit, to receive the borings, and to be drawn up occasionally to be emptied.

In boring large holes the earth is generally excavated by the process of "miser-ing up." The rods terminate in the "miter," which is a cylindrical iron case sometimes two to three feet diameter, with a slightly conical bottom, in which there is a slit much like the mouth of a plane, and covered with a leather flap to prevent the escape of the earth that has been collected.

In sinking the Artesian wells, lined with cast-iron tubes attached end to end by internal flanges or screws, a spring tool is used, which expands when it is thrust beneath the lower end of the series of pipes. See the account of sinking the Artesian well at Messrs. Truman, Hanbury, and Co.'s Brewery, Minutes of Conversation, Inst. of Civil Eng., 1842, p. 192.

The common pointed drill, is used for mineral substances not exceeding in hardness those enumerated under the terms, 1, 2, 3, of the Table of hardness, p. 158, Vol. I., of drills, we will proceed to explain the modes in which they are put in action by hand-power, beginning with those for the smallest diameters, and proceeding gradually to the largest.