The frequent necessity, in metal works, for the operation of drilling holes, which are required of all sizes and various degrees of accuracy, has led to so very great a variety of modes of performing the process, that it is difficult to arrange with much order the more important of these methods and apparatus.

The ordinary piercing drills for metal do not present quite so . much variety as the wood drills. The drills for metal are mostly pointed; they consequently make conical holes, which cause the point of the drill to pursue the original line, and eventually to produce the cylindrical hole. The comparative feebleness of the drill-bow limits the size of the drills employed with it to about one-quarter of an inch in diameter; but as some of the tools used with the bow agree in kind with those of much larger dimensions, it will be convenient to consider as one group the forms of the edges of those drills which cut when moved in either direction.

Figs. 370, 371, and 372, represent, of their largest sizes, the usual forms of drills proper for the reciprocating motion of the drill bow, because, their cutting edges being situated on the line of the axis, and chamfered on each side, they cut, or rather scrape, with equal facility in both directions of motion.

Fig. 370 is the ordinary double-cutting drill, the two facets forming each edge meet at an angle of about 50 to 70 degrees, and the two edges forming the point meet at about 80 to 100; but the watch-makers, who constantly employ this kind of drill, sometimes make the end as obtuse as an angle of about 120 degrees; the point does not then protrude through their thin works long before the completion of the hole. Fig. 371, with two circular chamfers, bores cast-iron more rapidly than any other reciprocating drill, but it requires an entry to be first made with a pointed drill. By some this kind is also preferred for wrought-iron and steel. The flat-ended drill, Fig. 372, is used for flattening the bottoms of holes. Fig. 373 is a duplex expanding drill, used by the cutlers for inlaying the little plates of metal in knife handles; the ends are drawn full size.

Fig. 374 is also a double-cutting drill; the cylindrical wire is filed to the diametrical line, and the end is formed with two facets. This tool has the advantage of retaining the same diameter when it is sharpened. It is sometimes called the Swiss drill, and was employed by M. Le Riviere, for making the numerous small holes in the delicate punching machinery for manufacturing perforated sheets of metal and pasteboard. These drills are sometimes made either semi-circular or flat at the extremity.

The square countersink, Fig. 375, is also used with the drill-bow; it is made cylindrical, and pierced for the reception of a small central pin - after which it is sharpened to a chisel-edge, as shown. This countersink is in some measure a diminutive of the pin drills, Figs. 382 to 385; and occasionally circular collars are fitted on the pin for its temporary enlargement, or around the larger part to serve as a stop and limit the depth to which the countersink is allowed to penetrate, for inlaying the heads of screws. The pin is removed when the instrument is sharpened.

By way of comparison with the double-cutting drills, the ordinary forms of those which only cut in one direction are shown in Figs. 376, 377, and 378. Fig. 376 is the common single cutting drill for the drill-bow, brace, and lathe. The point, as usual, is nearly a rectangle, but is formed by only two facets, which meet the sides at about 80° to 85°; and therefore lie very nearly in contact with the extremity of the hole operated upon, thus strictly agreeing with the form of the turning tools for brass. Fig. 377 is a similar drill, particularly suitable for horn, tortoise-shell, and substances liable to agglutinate and clog the drill. The chamfers are rather more acute, and are continued around the edge behind its largest diameter, so that, if needful, the drill may also cut its way out of the hole.

Fig. 378, although never used with the drill-bow, nor of so small a size as in the wood-cut, is added to show how completely the drill proper for iron, follows the character of the turning tools for that metal; the flute or hollow filed behind the edge, gives the hook-formed acute edge required in this tool, which is in other respects like Fig. 376; the form proper for the cutting edge is shown more distinctly in the diagram a, Fig. 382.

Care should always be taken to have a proportional degree of strength in the Shafts of the drills, otherwise they tremble and chatter when at work or they occasionally twist off in the neck; the point should be also ground exactly central, so that both edges may cut. As a guide for the proportional thickness of the point, it may measure at b, Fig. 379, the base of the cone, about one-fifth the diameter of the hole, and at p, the point, about one-eighth, for easier penetration; but the fluted drills are made nearly of the same thickness at the point and base.

In all the drills previously described, except Fig. 374, 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. 379. In Fig. 380 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. Fig. 330 may be also considered to resemble the slop-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.