Modern Machine-Shop Practice Part 122

[Ill.u.s.tration: Fig. 1744.]

In addition to its value as an adjustable boring tool this device may be used to cut out sweeps and curves, and is especially adapted to cutting those of double eyes. This operation is shown in Fig. 1744, in which D is the double eye, A is the tool stock, F is the adjusting screw, and C is the cutter. The circular ends of connecting rod strips and other similar work also fall within the province of this tool, and in the case of such work upon rods too long to be revolved this is an important item, as such work has now to be relegated to that slowest and most unhandy of all machine tools, the slotting machine.

It is obvious that any of the ordinary forms of cutter may be used in this stock.

[Ill.u.s.tration: Fig. 1745.]

For enlarging a hole for a certain distance the counterbore is employed.

Fig. 1745 represents a counterbore or pin drill, in which the pin is cut like a reamer, so as to ream the hole and insure that the pin shall fit accurately. The sides are left with but little clearance and with a dull edge, so that they will not cut, the cutting edges being at _e_, _c_ and the clearance on the end faces.

[Ill.u.s.tration: Fig. 1746.]

For counterboring small holes or for facing the metal around their ends, the form of counterbore shown in Fig. 1746 is employed. The pin must be an accurate fit to the hole, and to capacitate one tool for various sizes of holes the bit is made interchangeable. The stock has a flat place on it to receive the pressure of the screw that secures the counterbore, and the end of the stock is reduced in diameter, so that the counterbore comes against a shoulder and cannot push up the stock from the pressure of the feed; the end of the counterbore is bored to receive the t.i.t pin, thus making it permissible to exchange the pin, and use various sizes in the same counterbore.

[Ill.u.s.tration: Fig. 1747.]

[Ill.u.s.tration: Fig. 1748.]

Twist drills for use in wood work are given a conical point, as was shown with reference to lathe drills, and when the holes are to be countersunk, an attachment, such as shown in Fig. 1747, may be used. It is a split and threaded taper, so that by operating the nut in one direction it may be locked to the drill, while by operating it in the other it will be loosened, and may be adjusted to any required distance from the point of the drill, as shown in Fig. 1748.

[Ill.u.s.tration: Fig. 1749.]

For larger sizes of holes a stock and cutter, such as shown in Fig.

1749, may be employed, receiving a facing of counterboring cutter such as A, or a countersink bit such as B, and the bit may be made to suit various sizes of holes by making its diameter suitable for the smallest size of hole the tool is intended for, and putting ferrules to bring it up to size for larger diameters.

The cutters are fastened into the stock by a small key or wedge, as shown. By having the cutter a separate piece from the stock, the cutting edges may be ground with greater facility, while one stock may serve for various sizes of cutters. The slot in the stock should be made to have an amount of taper equal to that given to the key, so that all the cutters may be made parallel in their widths or depths, and thus be more easily made, while at the same time the upper edge will serve as a guide to grind the cutting edges parallel to, and thus insure that they shall stand at a right angle to the axis of the stock, and that both will therefore take an equal share of the cutting duty.

When cutters of this kind are used to enlarge holes of large diameter it is necessary that the pin be long enough to pa.s.s down into a bus.h.i.+ng provided in the table of the machine, and thus steady the bar or stock at that end.

For coning the mouths of holes the countersink is employed, being provided with a pin, as shown in Fig. 1750; and it is obvious that the pin may be provided with bus.h.i.+ngs or ferrules. The smaller sizes of countersinks are sometimes made as in Fig. 1751, the coned end being filed away slightly below the axis so as to give clearance to the cutting edge.

[Ill.u.s.tration: Fig. 1750.]

[Ill.u.s.tration: Fig. 1751.]

[Ill.u.s.tration: Fig. 1752.]

Fig. 1752 refers to a device for drilling square holes. The chuck for driving the drill is so constructed as to permit to the drill a certain amount of lateral motion, which is rendered necessary by the peculiar movement of the cutting edges of the drill which does not rotate on a fixed central point, but diverges laterally to a degree proportional to the size of the hole. For the chuck the upper part of the cavity of a metal cylinder is bored out so as to fit on the driving spindle. Below this bore a square recess is made, and below this latter and coming well within the diameter of the square recess, is a circular hole pa.s.sing through the end of the chuck. The drill holder or socket is in a separate piece, the bottom portion of which is provided with a square or round recess for holding the drill shanks, and is held firmly in its socket by means of a set-screw. The upper part of the socket consists first of a screw (Fig. 1752) at S; secondly, of a squared shoulder B; thirdly, of a cylindrical shoulder D, and the circular part E, the drill shank being inserted at H. N is a nut holding the drill socket in the chuck. The socket being inserted in the chuck, the loose square collar C, which has an oblong rectangular slot in it, is put in, pa.s.sing over the squared part of the socket. The nut N is then screwed up, bringing the face of E up to the face of the chuck, but not binding C, because C is thinner than the recess in which it lies. When this is done the socket will readily move in a horizontal plane to such a distance as the play between the two sides of the loose collar C and two of the sides of the recess will permit, while in the other direction it will move in a horizontal plane such distance as the play between the two sides of the square shoulder of the socket and the ends of the rectangular slot in the loose collar C will permit. The amount of this horizontal motion is varied to suit the size of the square hole to be drilled. Near to the lower end or cutting edges of the drill, there is fixed above the work a metal guide plate F having a square hole of the size requiring to be drilled. The drill is made three-sided, as shown, the dimensions of the three sides being such that the distance from the base to the apex of the triangle is the same as the length of the sides of the hole to be drilled. The drill may then be rotated through F as a guide, when it will drill a square hole.

The method of operation is as follows: The three-sided drill being fixed in the self-adjusting chuck, the guide bar with the square guide hole therein rigidly fixed above the point in the work where it is required to drill, the drilling spindle carrying the chuck drill is made to revolve, and is screwed or pressed downwards, upon which the drill works downwards through the square guide hole, and drills holes similar in size and form to that in the guide. The triangular drill for drilling dead square holes may also be used without the self-adjusting drill chuck in any ordinary chuck, when the substance operated upon is not very heavy nor stationary; then, instead of the lateral movement of the drill, such lateral movement will be communicated by the drill to the substance operated upon.

In making oblong dead square-cornered holes, either the substance to be operated upon must be allowed to move in one direction more than another, or the hole in the guide plate must be made to the shape required, and the drill chuck made to give the drill greater play in one direction.

[Ill.u.s.tration: Fig. 1753.]

The boring bars and cutters employed in drilling and boring machines are usually solid bars having fixed cutters, the bars feeding to the cut.

[Ill.u.s.tration: Fig. 1754.]

[Ill.u.s.tration: Fig. 1755.]

[Ill.u.s.tration: Fig. 1756.]

Figs. 1753, 1754, 1755, and 1756, however, represent a bar having a device for boring tapers in a drilling or boring machine. It consists of a sleeve A fixed to the bar S, and having a slideway at an angle to the bar axis. In this slideway is a slide carrying the cutting tool and having at its upper end a feed screw with a star feed. Fig. 1753 shows the device without, and Fig. 1754 with, the boring bar. A is a sleeve having ribs B to provide the slideway C for the slide D carrying the cutting-tool T. The feed screw F is furnished with the star G between two lugs H K. A stationary pin bolted upon the work catches one arm of the star at each revolution of the bar, and thus puts on the feed. To take up the wear of the tool-carrying slide, a gib M and set-screws P are provided, and to clamp the device to the boring-bar it is split at Q and furnished with screws R. The boring-bar S, furthermore, has a collar at the top and a nut N at the bottom. The tool, it will be observed, can be closely held and guided, the degree of taper of the hole bored being governed by the angle of the slideway C to the axis of the sleeve.

CHAPTER XX.--HAND DRILLING AND BORING TOOLS AND DEVICES.

HAND DRILLING AND BORING TOOLS.--The tools used for piercing holes in wood are generally termed boring tools, while those for metal are termed drilling tools when they cut the hole from the solid metal, and boring tools when they are used to enlarge an existing hole. Wood-boring tools must have their cutting edges so shaped that they sever the fibre of the wood before dislodging it, or otherwise the cutting edges wedge themselves in the fibre. This is accomplished, in cutting across the grain of the wood, in two ways: first, by severing the fibre around the walls of the hole and in a line parallel to the axial line of the boring tool, and removing it afterwards with a second cutting edge at a right angle to the axis of the boring tool; or else by employing a cutting edge that is curved in its length so as to begin to cut at the centre and operate on the walls of the hole, gradually enlarging it, as in the case of Good's auger bit (to be hereafter described), the action being to cut off successive layers from the end of the grain or fibre of the wood. Tools for very small holes or holes not above one-quarter inch in diameter usually operate on this second principle, as do also some of the larger tools, such as the nail bit or spoon bit and the German bit.

[Ill.u.s.tration: Fig. 1757.]

[Ill.u.s.tration: Fig. 1758.]

The simplest form of wood-piercing tool is the awl or bradawl, shown in Figs. 1757 and 1758, its cutting end being tapered to a wedge shape whose width is sometimes made parallel with the stem and at others spread, as at C D in figure. It is obvious that when the end is spread the stem affords less a.s.sistance as a guide to pierce the hole straight.

It is obvious that the action of an awl is that of wedging and tearing rather than of cutting, especially when it is operating endways of the grain.

Thus in Fig. 1758 is shown an awl operating, on the right, across the grain, and, on the left, endwise of the same. In the former position it breaks the grain endwise, while in the latter it wedges it apart. Awls are used for holes up to about three-sixteenths of an inch in diameter.

[Ill.u.s.tration: Fig. 1759.]

Fig. 1759 represents the gimlet bit having a spiral flute at F and a spiral projection at S S, which, acting on the principle of a screw, pulls the bit forward and into its cut. These bits are used in sizes from 1/16 inch to 1/2 inch. The edge of the spiral flute or groove here does the cutting, producing a conical hole and cutting off successive layers of the fibre until the full diameter of hole is produced. The upper part of the fluted end is reduced in diameter so as to avoid its rubbing against the walls of the hole and producing friction, which would make the tool hard to drive.

[Ill.u.s.tration: Fig. 1760.]

[Ill.u.s.tration: Fig. 1761.]

Figs. 1760 and 1761 represent the German bit, which is used for holes from 1/16 inch to 3/8 inch in diameter. This, as well as all other bits or augers, have a tapered square by which they are driven with a brace, the notch shown at N being to receive the spring catch of the brace that holds them in place. The cutting edges at A and B are produced by cutting away the metal behind them.

[Ill.u.s.tration: Fig. 1762.]

[Ill.u.s.tration: Fig. 1763.]

Fig. 1762 represents the nail bit, which is used for boring across the grain of the wood. Its cutting edge severs the fibre around the walls of the hole, leaving a centre core uncut, which therefore remains in the hole unless the hole is pierced entirely through the material. If used to bore endways or parallel with the direction of the fibre or grain of the wood it wedges itself therein.

The groove of the nail bit extends to the point, as shown by the dotted line in the figure. Nail bits are used in sizes from 1/16 to 3/8 inch.

Fig. 1763 represents the spoon bit whose groove extends close to the point, as shown by the dotted line C.

[Ill.u.s.tration: Fig. 1764.]

Fig. 1764 represents the pod or nose bit, whose cutting edge extends half way across its end and therefore cuts off successive layers of the fibres, which peculiarly adapts it for boring endways of the grain, making a straight and smooth hole. It is made in sizes up to as large as four inches, and is largely used for the bores of wooden pipes and pumps, producing holes of great length, sometimes pa.s.sing entirely through the length of the log.

[Ill.u.s.tration: Fig. 1765.]