Modern Machine-Shop Practice Part 123

Fig. 1765 represents the auger bit, which is provided with a conical screw S which pulls it forward into the wood. Its two wings W have cutting edges at D, D, which, being in advance of the cutting edges A, B, sever the fibre of the wood, which is afterwards cut off in layers whose thickness is equal to the pitch of the thread upon its cone S. The sides of the wings W obviously steady the auger in the hole, as do also the tops T of the twist. This tool is more suitable for boring across the grain than lengthways of it, because when boring lengthways the wings W obviously wedge themselves between the fibres of the wood.

[Ill.u.s.tration: Fig. 1766.]

This is obviated in Cook's auger bit, shown in Fig. 1766, in which the cutting edge is curved, so that whether used either across or with the grain the cutting edge produces a dished seat and cuts the fibre endways while removing the material in a spiral layer. The curve of the cutting edge is such that near the corners it lies more nearly parallel to the stem of the auger than at any other part, which tends to smooth the walls of the hole. This tool while very serviceable for cross grain is especially advantageous for the end grain of the wood.

[Ill.u.s.tration: Fig. 1767.]

[Ill.u.s.tration: Fig. 1768.]

In the smaller sizes of auger bits the twist of the spiral is made coa.r.s.er, as in Fig. 1767, which is necessary to provide sufficient strength to the tool. For the larger sizes the width of the top of the flute (T, Fig. 1765), or the land, as it is termed, is made narrow, as in Fig. 1768, for holes not requiring to be very exact in their straightness, while for holes requiring to be straight and smooth they are made wider, as at D, in Fig. 1769, and the wings A, B in the figure extend farther up the flutes so as to steady the tool in the walls of the hole and make them smoother. It is obvious that the conical screw requires to force or wedge itself into the wood, which in thin work is apt to split the wood, especially when it is provided with a double thread as it usually is (the top of one thread meeting the cutting edge A in Fig. 1765, while the top of the other thread meets cutting edge B).

[Ill.u.s.tration: Fig. 1769.]

[Ill.u.s.tration: Fig. 1770.]

In boring end-grain wood, or in other words lengthways of the grain of the wood, the thread is very apt to strip or pull out of the wood and clog the screw of the auger; especially is this the case in hard woods.

This may be to a great extent avoided by cutting a spiral flute or groove along the thread, as in Fig. 1770, which enables the screw to cut its way into the wood on first starting, acts to obviate the stripping and affords an easy means of cleaning. The groove also enables the screw to cut its way through knots and enables the auger to bore straight.

In boring holes that are parallel with the grain or fibre of the wood, much more pressure is required to keep the auger up to its cut and to prevent the thread cut by the auger point from pulling or stripping out of the wood, in which case it clogs the thread of the auger point and is very difficult to clean it out, especially in the case of hard woods.

[Ill.u.s.tration: Fig. 1771.]

[Ill.u.s.tration: Fig. 1772.]

Furthermore, after the thread has once stripped it is quite difficult to force the auger to start its cut again. To obviate these difficulties, the screw is fluted as shown. It is obvious also that this flute by imparting a certain amount of cutting action, and thereby lessening the wedging action of the screw, enables it to bore, without splitting it, thinner work than the ordinary auger. But it will split very thin work nevertheless; hence for such work as well as for holes in any kind of wood, when the hole does not require to be more than about twice as deep as that diameter, the centre bit shown in Figs. 1771 and 1772 is employed, being an excellent tool either for boring with the grain or across it. The centre B is triangular and therefore cuts its way into the work, and the spur or wing A extends lower than the cutting edge C, which on account of its angle cuts very keenly.

[Ill.u.s.tration: Fig. 1773.]

Fig. 1773 represents the twist drill which is used by the wood-worker for drilling iron, its end being squared to fit the carpenter's brace.

[Ill.u.s.tration: Fig. 1774.]

[Ill.u.s.tration: Fig. 1775.]

Fig. 1774 represents an extension bit, being adjustable for diameter by reason of having its cutting edge upon a piece that can be moved endways in the holder or stem. This piece is ruled with lines on its face so that it may be set to the required size. Its upper edge is serrated with notches into which a dish screw or worm meshes, so that by revolving the worm the bit piece is moved farther out on the spur or wing side or end, it being obvious that the spur must meet the walls of the hole. A better form of extension bit for the end grain of wood is shown in Fig. 1775, the cutting edge being a curve to adapt it to severing the fibre in end-grained wood, as was explained with reference to Good's auger bit.

[Ill.u.s.tration: Fig. 1776.]

Fig. 1776 represents a drill for stone work, whose edge is made curved to steady it. This tool is caused to cut by hammer blows, being slightly revolved upon its axis after each blow, hence the curved shape of its cutting edge causes it to sink a dish-shaped recess in the work which holds that end steady. The end of the tool is spread because the corners are subject to rapid wear, especially when used upon hard stone, and the sides of the drill would bend or jam in the walls of the hole in the absence of the clearance caused by the spread. To prevent undue abrasion water is used.

In soft stones the hammer blows must be delivered lightly or the cutting edge will produce corrugations in the seat or bottom of the hole, and falling into the same recesses when revolved after each blow the chipping action is impaired and finally ceases. To prevent this the cutting edge is sometimes curved in its length so that the indentations cross each other as the drill is revolved, which greatly increases the capacity of the drill, but is harder to forge and to grind.

[Ill.u.s.tration: Fig. 1777.]

The simplest hand-drilling device employed for metal is the fiddle bow drill shown in Fig. 1777. It consists of an elastic bow B, having a cord C, which pa.s.ses around the reel R, at one end of which is the drill D, and at the other a stem having a conical or centre point fitting into a conical recess in a curved breast-plate. The operator presses against this plate to force the drill to cut, and by moving the bow back and forth the cord revolves the drill.

[Ill.u.s.tration: Fig. 1778.]

As the direction of drill revolution is reversed at each pa.s.sage of the bow, its cutting edges must be formed so as to cut when revolved in either direction, the shape necessary to accomplish this being shown in the enlarged side and edge views at the foot of the engraving. It is obvious that a device of this kind is suitable for small holes only, as, say, those having a diameter of one-eighth inch or less. But for these sizes it is an excellent tool, since it is light and very sensitive to the drill pressure, and the operator can regulate the amount of pressure to suit the resistance offered to the drill, and therefore prevent the drill from breakage by reason of excessive feed. In place of the breast-plate the bow drill may be used with a frame, such as in Fig.

1778. the frame being gripped in a vice and having a pin or screw A. If a pin be used, its weight may give the feed, or it may be pressed down by the fingers, while if a screw is used it must be revolved occasionally to put on the feed.

[Ill.u.s.tration: Fig. 1779.]

Fig. 1779 represents a hand-drilling device in which the cord pa.s.ses around a drum containing a coiled spring which winds up the cord, the latter pa.s.sing around the drill spindle, so that pulling the cord revolves the spindle and the drill, the drum and spiral spring revolving the drill backwards.

[Ill.u.s.tration: Fig. 1780.]

Fig. 1780 represents a drilling device in which the drill is carried in a chuck on the end of the spindle which has right and left spiral grooves in it, and is provided with a barrel-shaped nut, which when operated up and down the spindle causes it to revolve back and forth.

The nut or slide carries at one hand a right-hand, and at the other a left-hand nut fitting into the spindle grooves, and cut like a ratchet on their faces. Between these is a sleeve, also ratchet cut, but sufficiently short that when one nut engages, the other is released, with the result that the drill is revolved in one continuous direction instead of back and forth, and can therefore be shaped as an ordinary flat drill instead of as was shown in Fig. 1777. The drill is fed to its cut by hand pressure on the handle or k.n.o.b at the top.

[Ill.u.s.tration: Fig. 1781.]

Fig. 1781 represents Backus' brace for driving bits, augers, &c., the construction of the chuck being shown in Fig. 1782. The two tongues are held at their inner ends by springs and are coned at their outer ends, there being a corresponding cone in the threaded sleeve, so that s.c.r.e.w.i.n.g up this sleeve firmly grips the tool shank and thus holds it true, independent of the squared end which fits into the inner tongue that drives it.

In another form this brace is supplied with a ratchet between the chuck and the cranked handle, as shown in Fig. 1783, the construction of the ratchet being shown in Fig. 1784. The ring is provided on its inner edge with three notches, so that by pulling it back and setting it in the required notch the ratchet will operate the chuck in either direction or lock it for use as an ordinary brace. The ratchet enables the tool to be used in a corner in which there would be no room to turn the crank a full revolution. This end may, however, be better accomplished by means of the Backus' patent angular wrench shown in Fig. 1785, which consists of a frame carrying a ball-and-socket joint between it and the chuck, as shown.

[Ill.u.s.tration: Fig. 1782.]

[Ill.u.s.tration: Fig. 1783.]

[Ill.u.s.tration: Fig. 1784.]

[Ill.u.s.tration: Fig. 1785.]

[Ill.u.s.tration: Fig. 1786.]

[Ill.u.s.tration: Fig. 1787.]

Figs. 1786 and 1787 represent the brace arranged to have a gear-wheel connected or disconnected at will, the object of this addition being to enable a quick speed to the chuck when the same is advantageous.

[Ill.u.s.tration: Fig. 1788.]

For drilling small holes in metal, the breast drill shown in Fig. 1788 is employed. It consists of a spindle having journal bearing in a breast-plate at the head, and in a frame carrying a bevel gear-wheel engaging with two gear-pinions that are fast upon the spindle, this frame and the bevel gear-wheel being steadied by the handle shown on the right. At the lower end of the spindle is a chuck for holding and driving the drill, which is obviously operated by revolving the handled crank which is fast upon the large bevel gear. The feed is put on by pressing the body against the breast-plate.

It is obvious that but one bevel pinion would serve, but it is found that if one only is used the spindle is apt to wear so as to run out of true, and the bore of the gear-wheel rapidly enlarges from the strain falling on one side only. To avoid this the spindle is driven by two pinions, one on each side of the driving gear as in figure.

Breast-drills do not possess enough driving power to capacitate them for drills of above about quarter inch in diameter, for which various forms of drill cranks are employed.

[Ill.u.s.tration: Fig. 1789.]

Fig. 1789 represents a drill crank which receives the drill at A, and is threaded at B to receive a feed screw C, which is pointed at D; at E is a loose tube or sleeve that prevents the crank from rubbing in the operator's hands when it is revolved.

[Ill.u.s.tration: Fig. 1790.]

To use such a drill crank a frame A, Fig. 1790, is employed, being held in a vice and having at T a table whereon the work W may be rested. The feed is put on by uns.c.r.e.w.i.n.g the screw S in this figure against the upper jaws of A; holes of about half inch and less in diameter may be drilled with this device.

[Ill.u.s.tration: Fig. 1791.]