Drill shanks are sometimes made parallel, with a flat place as at a in Fig. 1729, to receive the pressure of the set-screw by which it is driven. To enable the shank to run true it must be a close fit to the socket and should be about five diameters long. The objection to this form is that the pressure of the set-screw tends to force the drill out of true, as does also the wear of the socket bore.
These objections will obviously be diminished in proportion as the drill shank is made a tight fit to the socket, and to effect this and still enable the drill to be easily inserted and removed from the socket, the drill shank may be first made a tight fit to the socket bore, and then eased away on the half circumference on the side of the flat place, leaving it to fit on the other half circumference which is shown below the dotted line b in the end view in the figure. The set-screw is also objectionable, since it requires the use of a wrench, and is in the way and liable to catch the operator’s clothing.
There is, however, one advantage in employing a set-screw for twist drills, inasmuch as that, on account of the front rake on a twist drill, there is a strong tendency for the drill, as soon as the point emerges through the work, to run forward into the work and by ripping in become locked. This is very apt to be the case if there is any end play in the driving spindle, because the pressure of the cut forces the spindle back from the cut; but so soon as the drill point emerges and the pressure is reduced, the weight of the spindle acting in concert with the front rake on the drill causes the spindle to drop, taking up the lost motion in the opposite direction. In addition to this the work will from the same cause lift and run up the drill, often causing an increase in the duty sufficient to break the drill.
If the spindle has no lost motion and the work is bolted or fastened to the table or in a chuck, the drill if it has a taper shank only will sometimes run forward and slip loose in the driving socket. This, however, may be obviated by feeding the drill very slowly after its point emerges through the work.
Yet another form in which the cylindrical shanks of drills have been driven is shown in Fig. 1730. The shank is provided with a longitudinal groove turning at a right angle; at its termination the socket is provided with a screw whose point projects and fits into the shank groove. The drill is inserted and turned to the right, the end of the screw driving the drill and preventing it from coming out or running forward.
Flat drills are usually provided with a square taper shank such as shown in Fig. 1730, an average amount of taper being 11⁄4 inches per foot.
There are several disadvantages in the use of a square shank.
1st. It is difficult to forge the drill true and straight with the shank.
2nd. It is difficult to make the square socket true with the axial line of the machine spindle, and concentric with the same from end to end.
3rd. It is difficult to fit the shank of the drill to the socket and have its square sides true with the axial line of the drill.
4th. It is an expensive form of shank to fit. It is a necessity, however, when the cutting duty is very heavy, as in the case of stocks carrying cutters for holes of large diameter.
In order to properly fit a square shank to a socket it should be pressed into the socket by hand only, and pressed laterally in the direction of each side of the square. If there is no lateral movement the shank is a fit, and the spindle may be revolved to see if the drill runs true, as it should do if the body of the drill is true with the shank (and this must always be the case to obtain correct results). The drill must be tried for running true at each end of the cylindrical body of the drill, which, being true with the square shank, may be taken as the standard of truth in grinding the drill, so that supposing the hole in the driving spindle to be true and the drill shank to be properly fitted, the drill will run true whichever way inserted. If the body of the drill runs out of true it will cause a great deal of friction by rubbing and forcing the cuttings against the sides of holes, especially if the clearance be small or the hole a deep one.
In fitting the shank, the fitting or bearing marks will show most correctly when the shank is driven very lightly home, for if driven in too firmly the bearing marks will extend too far in consequence of the elasticity of the metal. If the hole in the spindle is not true with the axial line of the spindle, or if the sides of the hole are not a true square or are not equidistant from the axial line of the spindle, the drill must be fitted with one side of its square shank always placed to the same side of the square in the socket, and these two sides must therefore be marked so as to denote how to insert the drill without having to try it in the socket. Usually a centre-punch mark, as at e, Fig. 1731, is made on the drill and another on the collar as at f.
To enable the extraction of the drill from the socket the latter is provided with a slot, shown in figure at c, the slot passing through the spindle and the end of the drill protruding into the slot, so that a key driven into the slot will force the drill from the socket. The key employed for this purpose should be of some soft metal, as brass or hard composition brass, so that the key shall not condense or press the metal of the keyway, and after the key is inserted it should be lightly tapped with a hammer, travelling in the direction of the line of the spindle and not driven through the keyway.
The drill should not be given a blow or tap to loose it in the spindle, as this is sure in time to make its socket hole out of true.
The thread shown on the end of the drill spindle in figure is to receive chucks for holding and driving drills.
The various forms of small drill chucks illustrated in connection with the subject of lathe chucks are equally suitable for driving drills in the drilling machine.
Fig. 1732, however, represents an excellent three-jawed chuck for driving drills, the bite being very narrow and holding the drill with great firmness.
Fig. 1733 represents a two-jawed drill chuck in which the screws operate a pair of dies for gripping parallel shank drills, the screws being operated independently.
In other forms of similar chucks the bite is a V recess parallel to the chuck axis, the only difference between a drill chuck for a drilling machine and one for a lathe being that for the former the jaws do not require outside bites nor to be reversible.
Holes that are to be made parallel, straight, cylindrically true in the drilling machine, are finished by the reamer as already described with reference to lathe work, and it is found as in lathe work that in order that a reamer may finish holes to the same diameter, it is necessary that it take the same depth of finishing cut in each case, an end that is best obtained by the use of three reamers, the first taking out the irregularities of the drilled hole, and the second preparing it for the light finishing cut to be taken by the third.
All the remarks made upon the reamer when considered with reference to lathe work apply equally to its use in the drilling machine.
Another tool for taking a very light cut to smooth out a hole and cut it to exact size is the shell reamer shown in Fig. 1734, which fits on a taper mandrel through which passes a square key fitting into the square slot shown in the shell reamer.
Reamers may be driven by drill chucks, but when very true and parallel work is required, and the holes are made true before using the reamer, it is preferable to drive them by a socket that permits of their moving laterally. Especially is this the case with rose-bits. Fig. 1735, which is taken from The American Machinist, represents a socket of this kind, being pivoted at its driving or shank end, and supported at the other by two small spiral springs. The effect is that if the socket does not run quite true the reamer is permitted to adjust itself straight and true in the hole being reamed, instead of rubbing and binding against its walls, which would tend to enlarge its mouth and therefore impair its parallelism.
Cotter drills, slotting drills, or keyway drills, three names designating the same tool, are employed to cut out keyways, mortises, or slots.
Fig. 1736 represents a common form of cotter or keyway drill, the cutting edges being at a, a, and clearance being given by grinding the curve as denoted by the line c. In some cases a stock s and two detachable bits or cutters c, c, are used as in Fig. 1737, the bits being simple tools secured in slots in the stock by set-screws, and thus being adjustable for width so that they may be used to cut keyways of different widths.
The feed of keyway drills should be light, and especial care must be taken where two spindles are used, to keep them in line, or otherwise the keyway will not come fair, as is shown in Fig. 1738, where the half drilled from side a and that drilled from side b are shown not to come fair at their point of junction c. This is more apt to occur when a deep keyway is drilled one half from each side. Hence in such a case great care must be exercised in setting the work true, because the labor in filing out such a keyway is both tedious and expensive.
In producing holes of above or about two inches in diameter, cutters such as shown in Fig. 1739 may be employed. a is a stock carrying a cutter b secured in place by a key c. Holes are first drilled to receive the pin d, which serves as a guide to steady the stock. The amount of cutting duty is obviously confined to the production of the holes to receive the pin and the metal removed from the groove cut by the cutters, so that at completion of the cutter duty there comes from the work a ferrule or annular ring that has been cut out of the work.
For use on wrought iron or steel the front faces of the cutters may be given rake as denoted by the dotted line at e, and smooth and more rapid duty may be obtained if the cutter be set back, as in Fig. 1740, the cutting edge being about in a line with line a, in which case the front face may be hollowed out as at b, and take a good cut without the digging in and jumping that is apt to occur in large holes if the cutter is not thus set back. The larger the diameter of the work the greater the necessity of setting the cutting edge back, thus in Fig. 1741 the cutter is to be used to cut a large circle out of a plate p, as, say, a man-hole in a boiler sheet. The cutter c is carried in a bar b secured in the stock a by a screw, and unless the cutter is set well back it is liable to dip into the work and break.
It is obvious that the pin e in the figure must be long enough to pass into the hole in the plate before the cutter meets the plate surface and begins to cut, so that the pin shall act as a guide to steady the cutter, and also that in all cutters or cutter driving stocks the shank must be either of large diameter or else made square, in order to be able to drive the cut at the increased leverage over that in drilling.
In these forms of tube plate cutters it is necessary to drill a hole to receive the pin d. But this necessity may be removed by means of a cutter, such as shown in Fig. 1742, which is given simply as a representative of a class of such cutters. a is a cutter stock having the two cutters b b fitted in slots and bolted to it. c is a spiral spring inserted in a hole in a and pressing upon the pin d, which has a conical point. The work is provided with a deep centre-punch mark denoting the centre of the hole to be cut. The point of d projects slightly beyond the cutting edges of the cutters, and as it enters the centre-punch mark in the work it forms a guide point to steady the cutters as they rotate. As the cutters are fed to their cut, the pin d simply compresses the spiral spring c and passes further up the cutter stock. Thus the point of d serves instead of a hole and pin guide.
A simple form of adjustable cutter is shown in Figs. 1743 and 1744. It consists of a stock a a with the shank b, made tapering to fit the socket of a boring or drilling machine. Through the body of the stock is a keyway or slot, in which is placed the cutter c, provided in the centre of the upper edge with a notch or recess. Into this slot fits the end of the piece d, which is pivoted upon the pin e. The radial edge of d has female worm teeth upon it. f is a worm screw in gear with the radial edge of d. Upon the outer end of f is a square projection to receive a handle, and it is obvious that by revolving the screw f, the cutter c will be moved through the slot in the stock, and hence the size of the circle which the cutter will describe in a revolution of the stock a may be determined by operating the screw f. Thus the tool is adjustable for different sizes of work, while it is rigidly held to any size without any tendency whatever either to slip or alter its form. The pin g is not an absolutely necessary part of the tool, but it is a valuable addition, as it steadies the tool. This is necessary when the spindle of the machine in which it is used has play in the bearings, which is very often the case with boring and drilling machines. The use of g is to act as a guide fixed in the table upon which the work is held, to prevent the tool from springing away from the cut, and hence enabling it to do much smoother work. It is usual to make the width of the cutter c to suit some piece of work of which there is a large quantity to do, because when the cutter is in the centre of the stock both edges may perform cutting duty; in which case the tool can be fed to the cut twice as fast as when the cutter is used for an increased diameter, and one cutting edge only is operative. The tool may be put between the lathe centres and revolved, the work being fastened to the lathe saddle. In this way it is exceedingly useful in cutting out plain cores in half-core boxes.
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.
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.
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 tit pin, thus making it permissible to exchange the pin, and use various sizes in the same counterbore.
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.
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 pass down into a bushing 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 bushings 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.
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 passing 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, passing 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.
The boring bars and cutters employed in drilling and boring machines are usually solid bars having fixed cutters, the bars feeding to the cut.
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.
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.
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 assistance 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.
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.
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.
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.
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 passing entirely through the length of the log.
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.
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.
In the smaller sizes of auger bits the twist of the spiral is made coarser, 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).
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.
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.