Modern Machine-Shop Practice Part 73

Referring to a 1/2 inch twist drill, it is said:

"The time occupied from the starting of each hole in a hammered sc.r.a.p-iron bar till the drill pierced through it varied from 1 minute 20 seconds to 1-1/2 minutes. The holes drilled were perfectly straight. The speed at which the drill was cutting was nearly 20 feet per minute in its periphery, and the feed was 100 revolutions per inch of depth drilled. The drill was lubricated with soap and water, and went clean through the 2-3/4 inches without being withdrawn, and after it had drilled each hole it felt quite cool to the hand, its temperature being about 75. It is found that 120 to 130 such holes can be drilled before it is advisable to resharpen the twist drill. This ought to be done immediately the drill exhibits the slightest sign of distress. If carefully examined after this number of holes has been drilled, the prominent cutting parts of the lips which have removed the metal will be found very slightly blunted or rounded to the extent of about 1/100th inch, and on this length being carefully ground by the machine off the end of the twist drill, the lips are brought up to perfectly sharp cutting edges again.

"The same sized holes, 1/2 inch diameter and 2-3/4 inches deep, have been drilled through the same hammered sc.r.a.p-iron at the extraordinary speed of 2-3/4 inches deep in 1 minute and 5 seconds, the number of revolutions per inch being 75. An average number of 70 holes can be drilled in this case before the drill requires resharpening. The writer considers this test to be rather too severe, and prefers the former speed.

"In London, upward of 3000 holes were drilled 5/8 inch diameter and 3/8 inch deep through steel bars by one drill without regrinding it. The cutting speed was in this instance too great for cutting steel, being from 18 to 20 feet per minute, and the result is extraordinary. Many thousands of holes were drilled 1/8 inch diameter, through cast iron 7/16ths inch deep with straight-shank twist drills gripped by an eccentric chuck in the end of the spindle of a quick-speed drilling machine. The time occupied for each hole was from 9 to 10 seconds only.

Again, 1/4-inch holes have been drilled through wrought copper 1-3/8 inches thick at the speed of one hole in 10 seconds. With special twist drills, made for piercing hard Bessemer steel, rail holes, 13/16ths inch deep and 29/32nds inch diameter, have been drilled at the rate of one hole in 1 minute and 20 seconds in an ordinary drilling machine. Had the machine been stiffer and more powerful, better results could have been obtained. A similar twist drill, 29/32nds inch in diameter, drilled a hard steel rail 13/16ths inch deep in 1 minute, and another in 1 minute 10 seconds. Another drill, 5/8 inch diameter, drilled 3/4 inch deep in 38 seconds, the cutting speed being 22 feet per minute. This speed of cutting rather distressed the drill; a speed of 16 feet per minute would have been better. The steel rail was specially selected as being one of the hardest of the lot."

[Ill.u.s.tration: Fig. 1060.]

Drills ground by hand may be tested for angle by a protractor, as in Fig. 1061, and for equal length of cutting edge by resting them upon a flat surface, as B in Fig. 1062, and applying a scale as at S in the figure. In the case of very small drills, it is difficult to apply either the protractor or the scale, as well as to determine the amount of clearance on the end face. This latter, however, may be known from the appearance of the cutting edge at the point A in Fig. 1063, for if the line A is at a right angle to E, there is no clearance, and as clearance is given this line inclines as shown at B in the figure, the inclination increasing with increased clearance, as is shown at C. When this part of the edge inclines in the opposite direction, as at D in the figure, the curved edges _e_ _f_ stand the highest, and the drill cannot cut. The circ.u.mferential surface of a drill should never be ground, nor should the front face or straight side of the flute be ground unless under unusual conditions, such as when it is essential, as in drilling very thin sheet metal, to somewhat flatten the corner (C in Fig. 1062), in order to reduce its tendency to run forward, in which case care must be taken not to grind the front face sufficiently to reduce the full diameter. In Fig. 1064, for example, that part of the circ.u.mference lying between A and B being left of full circle, the faces of the flutes might be ground away as denoted by the dotted lines C D without affecting the drill diameter.

[Ill.u.s.tration: Fig. 1061.]

[Ill.u.s.tration: Fig. 1062.]

[Ill.u.s.tration: Fig. 1063.]

[Ill.u.s.tration: Fig. 1064.]

[Ill.u.s.tration: Fig. 1065.]

[Ill.u.s.tration: Fig. 1066.]

Fig. 1065 represents the Farmer lathe drill, in which the flutes are straight and not spiral, by which means the tendency to run forward when emerging through the work is obviated.

[Ill.u.s.tration: Fig. 1067.]

When a twist drill is to be used for wood and is driven by a machine it is termed a bit, and is provided with a conical point to steady it, and two wings or spurs, as in Fig. 1066, which sever the fibres of the wood in advance of their meeting the main cutting edges and thus produce a smooth hole. The sharp conical point is used in place of the conical screw of the ordinary wood auger to avoid the necessity of revolving the drill or bit backwards to release the screw in cases in which the hole is not bored entirely through the work.

[Ill.u.s.tration: Fig. 1068.]

When the drill revolves and the work is to be held in the hands a rest or table whereon to rest the work and hold it fair is shown in Fig.

1067, the taper shank fitting in the dead centre hole and the tailstock spindle being fed up by hand to feed the drill to its cut. The face A A of the chuck is at a right angle to the shank, and a coned recess is provided at the centre, as denoted by the dotted lines, to permit the drill point to pa.s.s through the work without cutting the chuck.

[Ill.u.s.tration: Fig. 1069.]

For larger work a table, such as shown in Fig. 1068, is used, the cavity C permitting the drilling tool to pa.s.s through the work, there being a hole H provided for that purpose. The stem S fits in place of the dead centre. For cylindrical work the rest or chuck shown in Figs. 1069 and 1070 may be employed. It consists of a piece fitted to the tail spindle in place of the dead centre, its end being provided with [V]-grooves.

These grooves are made true with the line of centres of the lathe, so that when the work is laid in them it will be held true. It is obvious that one groove would be sufficient, but two are more convenient--one for large work and one for small work--so that the side of the shaft to be drilled shall not pa.s.s within the fork, but will protrude, so that the progress of the work can be clearly seen. In Fig. 1070 an end view of this chuck is shown. It may be observed, however, that when starting the drill care must be taken to have it start true, or the drill may bend, and thus throw the work out of the true. For this reason the drills should be as short as possible when their diameters are small.

For square work this cla.s.s of work table or chuck may be formed so as to envelop the work and prevent its revolving, thus relieving the fingers of that duty, and it may be so formed as to carry the work back or off the drill when the latter is retired after the drilling is performed.

[Ill.u.s.tration: Fig. 1070.]

Another and quite convenient method of holding work to be drilled by a revolving drill in the lathe is shown in Fig. 1071. It consists of simply a bracket, _a_ _b_, fitted to the tool-box of the slide rest, carrying a spindle with one end screwed to receive any face plates or chucks that fit the lathe live spindle. The bracket is kept in position by two pins in the under side of it, fitting into holes in the bottom piece of tool-box. If it be required to drill a straight row of holes, the spindle is fixed by the set-screws in its bracket, and the work is bolted to the face plate at the proper level, and traversed across opposite the drill in the lathe mandrel, by the cross screw of the slide rest, while it is fed up to the drill by the upper screw or the rack and pinion.

For circular rows of holes the centre line of the spindle is adjusted parallel with and at a proper distance from that of the mandrel. For holes in the edge of the work, the whole top of slide rest is turned round till the spindle is at right angles with the mandrel.

[Ill.u.s.tration: Fig. 1071.]

Work merely requiring to be held fast for drilling is bolted on one side of the face plate, and can then be adjusted exactly to the drill by the combined motions of the cross screw and the face plate on its centre.

Small round work, while drilled in the end, can be held in a scroll chuck screwed on the spindle the same as a face plate.

The convenience of this device consists in this, that the work turned on the chuck may be drilled without moving it from the chuck, which may be so set as to cause the drilled holes to be at any required angle to the work surface, which is quite difficult of accomplishment by other ordinary means.

On account of the readiness with which a flat drill may be made to suit an odd size or employed to recess work with a flat or other required shape of recess, flat drills are not uncommonly used upon lathe work, and in this case they may be driven in the drill chucks already shown. A very convenient form of drill chuck for small drills is shown in Fig.

1072. It consists of a cylindrical chuck fitting from A to B into the coned hole in the live spindle so as to be driven thereby. At the protruding end C there is drilled a hole of the diameter of the wire forming the drill. At the end of this hole there is filed a slot D extending to the centre of the chuck. The end of the drill is filed half round and slightly taper, as shown in Fig. 1073 at D, so that the half-round end of the drill will pa.s.s into the slot of the chuck, therefore forming a driving piece which effectually prevents the drill from slipping, as is apt to occur with cylindrical stem or shank drills.

If one size of wire be used for all drills, and the drill size be determined by the forging, the drill will run true, being held quite firmly, and may be very readily inserted in or removed from the chuck.

[Ill.u.s.tration: Fig. 1072.]

But the flat drill possesses several disadvantages: thus, referring to figure, it must be enough smaller at A than at B to permit the cuttings to find egress, and this taper causes the diameter of the drill to be reduced at each drill grinding. The end B may, it is true, be made parallel for a short distance, but in this case the cuttings will be apt to clog in the hole unless the drill be frequently removed from deep holes to clear the cuttings. For these reasons the fluted drill or the twist drill is preferable, especially as their diameters are maintained without forging. For deep holes, as, say, those having a depth equal to more than twice the diameter, the flat drill, if of small diameter, as, say, an inch or less, is unsuitable because of the frequency with which it must be removed from the hole to clear it of cuttings.

[Ill.u.s.tration: Fig. 1073.]

For fluted or twist drills the lathe may run quicker than for a flat drill, which is again an advantage. It sometimes becomes convenient in the exigencies which occur in the work of a general machine shop to hold a drill in a dog or clamp and feed it into the work with the lathe dead centre. In this case the drill should be held very firmly against the dead centre, or otherwise the drill may, when emerging through the back of the hole, feed itself forward, slipping off the dead centre, and causing the drill to catch and break, or moving the work in the chuck, to avoid which the drill should have a deep and well countersunk centre.

[Ill.u.s.tration: Fig. 1074.]

A very effective drill for holes that are above two inches in diameter and require enlarging is shown in Fig. 1074. It consists of a piece of flat steel A, with the pieces of wood B fastened on the flat faces, the wood serving to steady the drill and prevent it from running to one side in the work. This drill is sometimes used to finish holes to standard size, in which case the hole to be bored or drilled should be trued out a close fit to the drill for a distance equal to about the diameter of the drill, and the face at the entrance of the hole should be true up.

This is necessary to enable the drill to start true, which is indispensable to the proper operation of the drill.

This drill is made by being turned up in the lathe, and should have at the stock end a deep and somewhat large centre, so that when in use it may not be liable to slip off the dead centre of the lathe. The drill is held at the stock end by being placed in the lathe dead centre and is steadied, close to the entrance of the hole in the work, by means of a hook which at one end embraces the drill, as shown in Fig. 1075, in which A represents the hook and B the drill.

[Ill.u.s.tration: Fig. 1075.]

This drill will bore a parallel hole, but if the same be a long or a deep one it is apt to bore gradually out of true unless the bore of the hole is first trued from end to end with a boring tool before using the drill. It is often employed to enlarge a hole so as to admit a stout boring tool, and to remove the hard surface skin from which the boring tool is apt to spring away.

[Ill.u.s.tration: Fig. 1076.]

[Ill.u.s.tration: Fig. 1077.]

HALF-ROUND BIT OR POD AUGER.--For drilling or enlarging holes of great depth (in which case it is difficult to drill straight holes with ordinary drills), the half-round bit--Figs. 1076 and 1077--is an excellent tool. Its diameter D is made that of the required hole, the cutting being done at the end only from A to B, from B to C being ground at a slight angle to permit the edge from A to B to enter the cut. When a half-round bit is to be used on iron or steel, and not upon bra.s.s, it may be made to cut more freely by giving the front face rake as at E F, Fig. 1078.

[Ill.u.s.tration: Fig. 1078.]

[Ill.u.s.tration: Fig. 1079.]

To enable a bit of this kind to be adjusted to take up the wear, it may be formed as in Fig. 1079, in which a quarter of the circ.u.mference is cut away at _a_, and a cutter _c_ is bolted in position projecting into a recess at _b_ to secure the cutter in addition to the bolts. Pieces of paper may be inserted at _b_ to set out the cutter.

An excellent form of boring bar and cutter is shown in Figs. 1080 and 1081.

Fig. 1082 shows a side view of the cutter removed from the bar; Fig.

1081 an end, and Fig. 1080 a side view of the bar and cutter. The cutter is turned at A and B to fit the bore of the bar. The cutting edge C extends to the centre of the bar, while that at D does not quite reach the centre. These edges are in a line as shown in the end view. On account of the thickness of the cutter not equaling the diameter of the bore through the bar there is room for a stream of water to be forced through the bar, thus keeping it cool and forcing out the cuttings which pa.s.s through the pa.s.sages G and H in the bar. The cutter drives lightly into the bar. By reason of one cutting edge not extending clear to the centre of the cutter there is formed a slight projection at the centre of the hole bored which serves as a guide to keep the cutter true, causing it to bore the hole very true.

[Ill.u.s.tration: Fig. 1080.]

For finishing the walls of holes more true, smooth, and straight, and of more uniform diameter than it is found possible to produce them with a drill, the reamer, or rymer, is employed. It consists of a hardened piece of steel having flutes, at the top of which are the cutting edges, the general form of solid reamer for lathe work being shown in Fig.