Modern Machine-Shop Practice Part 121

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To drive all drills by placing them directly in the socket of the drilling machine spindle would necessitate that all the drills should have their shanks to fit the drilling machine socket. This would involve a great deal of extra labor in making the drills, because the socket in the machine spindle must be large enough to fit the size of shank that will be strong enough to drive the largest drill used in the machine, hence the small drills would require to be forged down from steel equal to the full diameter of the shank of the largest drill. To obviate this difficulty the sockets already described with reference to drilling in the lathe are used.

The employment of these sockets preserves the truth of the bore of the drilling machine spindle by greatly diminishing the necessity to insert and remove the shank from the drill spindle, because each socket carrying several sizes of drills (as given with reference to lathe work) the sockets require less frequent changing.

[Ill.u.s.tration: Fig. 1729.]

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 circ.u.mference on the side of the flat place, leaving it to fit on the other half circ.u.mference 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 1-1/4 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.

[Ill.u.s.tration: Fig. 1730.]

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 pa.s.sing 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 bra.s.s or hard composition bra.s.s, 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.

[Ill.u.s.tration: Fig. 1731.]

[Ill.u.s.tration: Fig. 1732.]

The thread shown on the end of the drill spindle in figure is to receive chucks for holding and driving drills.

[Ill.u.s.tration: Fig. 1733.]

The various forms of small drill chucks ill.u.s.trated 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.

[Ill.u.s.tration: Fig. 1734.]

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 sh.e.l.l reamer shown in Fig. 1734, which fits on a taper mandrel through which pa.s.ses a square key fitting into the square slot shown in the sh.e.l.l reamer.

[Ill.u.s.tration: Fig. 1735.]

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.

[Ill.u.s.tration: Fig. 1736.]

[Ill.u.s.tration: Fig. 1737.]

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.

[Ill.u.s.tration: Fig. 1738.]

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.

[Ill.u.s.tration: Fig. 1739.]

[Ill.u.s.tration: Fig. 1740.]

[Ill.u.s.tration: Fig. 1741.]

It is obvious that the pin E in the figure must be long enough to pa.s.s 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.

[Ill.u.s.tration: Fig. 1742.]

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 cla.s.s 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 pa.s.ses further up the cutter stock. Thus the point of D serves instead of a hole and pin guide.

[Ill.u.s.tration: Fig. 1743.]

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 quant.i.ty 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.