The expanding center-bit, shown on a much smaller scale in fig. 460, is a very useful instrument; it has a central stem with a conical point, and across the end of the stem is fitted a transverse bar, adjustable for radius. Where the latter carries only a lancet-shaped cutter it is used for making the margins of circular recesses, and also for cutting out disks of wood and thin materials generally; when, as in Mr. James Stone's modification, the expanding center-bit has two shearing points or nickers, and one chisel-formed cutter, it serves for making grooves for inlaying rings of metal or wood in cabinet-work, and other purposes.* - See Appendix, note A W., p. 1001.

The above tools being generally used for woods of the softer kinds, and the plankway of the grain, the shearing point and oblique chisel of the center-bit, fig. 457, are constantly retained, but the corresponding tools used for the hard woods assume the characters of the hard wood tools generally. For instance, a, fig. 458, has a square point, also two cutting edges, which are nearly diametrical, and sharpened with a single chamfer at about 60 degrees; this is the ordinary drill used for boring the finger-holes in flutes and clarionets, which are afterwards chamfered on the inner side with a stout knife, the edge of which measures about 50 degrees. The key-holes, are first scored with the cup-key tool, b, and then drilled, the tools a, and b, being represented of corresponding sizes, and forming between them the annular ridge which indents the leather of the valve or key.

When a, fig. 458, is made exactly parallel, and sharpened up the sides, it cuts hard mahogany very cleanly in all directions of the grain, and is used for drilling the various holes in the small machinery of piano-fortes; this drill (and also the last two), is put in motion in the lathe; and in fig, 459, the lathe-drill for hard woods, called by the French langue de carpe, the center-point and the two sides melt into an easy curve, which is sharpened. all the way round, and a little beyond its largest part. Various tools for boring wood have been made with spiral stems, in order that the shavings may be enabled to ascend the hollow worm, and thereby save the trouble of so frequently withdrawing the bit. For example, the shaft of fig. 461. the singlelip auger, is forged as a halt round bar, nearly as in the see: above; it is then coiled into an open spiral with the flat side outwards, to constitute the cylindrical surface, and the end is formed almost the same as that of the shell auger, fig.155. The twisted-gimlet, fig.462, is made with a conical shaft, around which is filed a half-round groove, the one edge of which becomes thereby sharpened, so as gradually to enlarge the hole after the first penetration of the worm, which, from being smaller than in the common gimlet, acts with less risk of splitting.

Boring Tools Section I Boring Bits For Wood Contin 20059

Fig. 460.

* See Trans. Soc. of Arts, vol. xxxi. p. 250.

Boring Tools Section I Boring Bits For Wood Contin 20060

The ordinary screw auger, fig. 463, is forged as a parallel blade of steel, (seen in the section, fig. 464, which also refers to 463 and 465,) it is twisted red-hot, the end terminates in a worm by which the auger is gradually drawn into the work, as in the gimlet, and the two angles or lips are sharpened to cut at the extreme ends, and a little up the sides also.

The same kind of shaft is sometimes made as in fig. 464, with a plain conical point, with two scoring cutters and two chisel edges, which receive their obliquity from the slope of the worm: it is as it were a double center-bit, or one with two lips grafted on a spiral shaft. The same shaft has been also made, as in fig.465, with a common drill point, and proposed for metal, but this seems scarcely called for; but it is in this form very effective in Hunter's patent stone-boring machine, intended for stones not harder than sandstones; the drill is worked by a cross, guided by a tube, and forced in by a screw cut upon the shaft carrying the drill; so that the stone is not ground to powder, but cast off in flakes with very little injury to the drill.

Another screw auger, which is perhaps the most general after the double-lipped screw auger, fig. 463, is known as the American screw auger, and is shown in fig. 466; this has a cylindrical shaft, around which is brazed a single fin or rib; the end is filed into a worm as usual, and immediately behind the worm a small diametrical mortise is formed for the reception of a detached cutter, which exactly resembles the nicking point and chisel edge of the center-bit; it may be called a center-bit for deep holes. The parts are shown detached in fig. 467. The loose cutter is kept central by its square notch, embracing the central shaft of the auger: it is fixed by a wedge driven in behind, and the chisel edge rests against the spiral worm. Spare cutters are added in case of accident, and should the screw be broken off, a new screw and mortise may be made by depriving the instrument of so much of its length. The instrument will be found on trial extremely effective; and on account of the great space allowed for the shavings, they are delivered perfectly, until the worm is buried a small distance beneath the surface of the hole.

The Americans have also invented an auger, said to be thoroughly applicable to producing square holes, and those of other forms: the tool consists of a steel tube, of the width of the hole, the end of the tube is sharpened from within, with the corners in advance or with four hollowed edges. In the center of the square tube works a screw auger, the thread of which projects a little beyond the end of the tube, so as first to penetrate the wood, and then to drag after it the sheath, and thus complete the hole at one process; the removed shavings making their escape up the worm and through the tube. For boring long mortises, two or more square augers are to be placed side by side, but they must necessarily be worked one at a time.*

Fig. 468, the last of this group of spiral drills, is used in

* This is described in Gill's Technical repository, vol. xi. page 317. The author has never seen one; it seems far too complex an instrument for general purposes, and its success appears to be overrated. The tools, figs. 461 to 466, are also ascribed to America; whether truly or not it is impossible to say. Fig. 461 is in partial use. The twisted gimlet is a good tool, but as it is somewhat more expensive than the common kind, it is less used. These several instruments are probably derived from the common screw auger, fig. 463, which is, I believe, English.

Germany, and two of the instruments were brought from that country and deposited in the Museum of the Society of Arts, by Mr. Bryan Donkin.* The tool acts as a hollow taper bit or rimer, and the screw-form point and shaft, assist in drawing it into the wood; but the instrument must pass entirely through for making cylindrical holes.†

The most usual of the modes of giving motion to the various kinds of boring bits, is by the ordinary carpenter's brace with a crank-formed shaft. The instrument is made in wood or metal, and at the one extremity has a metal socket, called the pad, with a taper square hole, and a spring-catch used for retaining the drills in the brace when they are withdrawn from the work, and at the other, it has a swivelled head or shield, which is pressed forward horizontally by the chest of the workman; or when used vertically, by the left hand, which is then commonly placed against the forehead. ‡

The ordinary carpenter's brace is too familiarly known to require further description, but it sometimes happens, that in corners and other places there is not room to swing round the handle, the angle-brace, fig. 469, is then convenient. It is made entirely of metal, with a pair of bevil pinions, and a winch handle that is placed on the axis of one of these, at various distances from the center, accord to the power or velocity required. Sometimes the bevil wheel attached to the winch haudle, is three or four times the diameter of the pinion on the drill; this gives greater speed but less power. §

The augers, which from their increased size require more power, are moved by transverse handles; some augers are made with shanks, and are rivetted into the handles just like the gimlet; occasionally the handle has a socket or pad, for receiving several augers, but the most common mode, is to form the end of the shaft into a ring or eye, through which the transverse handle is tightly driven. The brad-awls, and occasionally the other tools requiring but slight force, are fitted in straight handles; many of the smaller tools are attached to the lathe mandrel by means of chucks, and the work is pressed against them, either by the hand, or by a screw, a slide, or other contrivance; figs. 458 and 459, are always thus applied.

Fig. 469.

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• See Trans., vol. xliv., p. 75.

† The cooper's bit Lb sometimes made with a gimlet worm, a semi-conical shell, and a conical plug to stop the hole until the tap is inserted.

‡ The carpenter's brace is somotimes fixed vertically, with the power of revolving and of being depressed by a lever, in some respects like the smith's press drill, fig. 494, page 558. See also Manuel du Tourneur, 1816, Plate IX., vol ii.

§ Fig. 460 is reduced from Plate IX. of the Manuel du Tourneur.