Modern Machine-Shop Practice Part 72

" 5 " " 2-1/32 " 2-1/2 " "

These sockets are manufactured ready to receive the drills, but are left unturned at the shank end so that they may be fitted to the particular lathe or machine in which they are to be used, no standard size or degree of taper having as yet been adopted.

A twist drill possesses three cutting edges marked A, B, C respectively in Fig. 1043, and of these C is the least effective, because it cannot be made as keen as is desirable for rapid and clean cutting, and therefore necessitates that the drill be given an unusually fine rate of feed as compared with other cutting tools.

The _land_ of the drill--or, in other words, the circ.u.mference between the flutes--is backed off to give clearance, as is shown in Fig. 1044, a true circle being marked with a dotted line, and the drill being of full diameter from A to B only. The object of this clearance is to prevent the drill from seizing or grinding against the walls of the hole, as it would otherwise be apt to do when the outer corner wore off, as is likely to be the case.

[Ill.u.s.tration: Fig. 1043.]

[Ill.u.s.tration: Fig. 1044.]

Twist drills having three and more flutes have been devised and made, but the increased cost and the weakness induced by the extra flutes have been found to more than counterbalance the gain due to an increase in the number of cutting edges, Further, the increase in the number of flutes renders the grinding of the drill a more delicate and complicated operation.

The keenness and durability of the cutting edge of a twist drill are governed by the amount of clearance given by the grinding to the cutting edge, by the angle of one cutting edge to the other, and by the degree of twist of the flute. Beginning with the angle of the front face, we shall find that it varies at every point in the diameter of the drill, being greatest at the outer corner and least at the centre of the drill, whatever degree of spirality the groove or flute may possess. In Fig.

1045, for example, we may consider the angle at the corner C and at the point F in the length of the cutting edge. The angle or front rake of the corner C is obviously that of the outer edge of the spiral C D, while that of the point F is denoted by the line F _f_, more nearly parallel to the drill axis, and it is seen that the front rake increases in proportion as the corner C is approached, and diminishes as the drill centre or point is approached.

[Ill.u.s.tration: Fig. 1045.]

[Ill.u.s.tration: Fig. 1046.]

[Ill.u.s.tration: Fig. 1047.]

It follows, then, that if the angle of the bottom face of the drill be the same from the centre to the corner of the drill, and we consider the cutting edge simply as a wedge and independent of its angle presentation to the work, we find that it has a varying degree of acuteness at every point in its length. This may be seen from Fig. 1046, in which the end face is ground at a constant angle from end to end to the centre line of the drill, and it is seen that the angle A represents the wedge at point C and the angle B the wedge at the point F in the length of the cutting edge, and it follows that the wedge becomes less acute as the centre of the drill is approached from the point C. If, then, we give to the end face a degree of clearance best suited for the corner C, it will be an improper one for the cutting edge near the drill point; or if we adopt an angle suitable for the point, it will be an improper one for the corner C.

This corner performs the most cutting duty, because its path of revolution is the longest, or rather of the greatest circ.u.mference, and it operates at the highest rate of cutting speed for the same reason, hence it naturally wears and gets dull the quickest.

As this wear proceeds the circ.u.mferential surface near this corner grinds against the walls of the hole, causing the drill to heat and finally to cease cutting altogether.

For these reasons it is desirable that the angle of the end face, or the angle of clearance, be made that most suitable to obtain endurance at this corner. It may be pointed out, however, that the angle of one cutting edge to the other, or, what is the same thing, its angle to the centre line of the drill, influences the keenness of this corner. In Fig. 1045, for example, each edge is at an angle of 60 to the drill axis, this being the angle given to drills by the manufacturers as most suitable for general use. In Fig. 1047, the angle is 45, and it will be clearly seen that the corner C is much less acute; an angle of 45 is suitable for bra.s.s work or for any work in which the holes have been cored out and the drill is to be used to enlarge them.

[Ill.u.s.tration: Fig. 1048.]

Referring again to the angle of clearance of the end faces, it can be shown that in the usual manner of grinding twist drills the conditions compel the amount of clearance to be made suitable for the point of the drill, and therefore unsuitable for the corner C, giving to it too much clearance in order to obtain sufficient clearance for the remainder of the cutting edge. Suppose, for example, that we have in Fig. 1048 a spiral representing the path of corner C during one revolution, the rate of feed being shown magnified by the distance P, and the spiral will represent the inclination of that part of the bottom of the hole that is cut by corner C, and the angle of the end face of the drill to the drill axis will be angle R. The actual clearance will be represented by the angle between the end face S of the drill and the spiral beneath it, as denoted by T. But if we take the path of the point F, Fig. 1045, during the same revolution, which is represented by the spiral in Fig. 1049, we find that, in order to clear the end of the hole, it must have more angle to the centre line of the drill, as is clearly shown, in order to have the clearance necessary to enable the point F to cut, because of the increased spiral. It follows that, if the same degree of clearance is given throughout the full length of the cutting edge, it must be made suitable for the point of the drill, and will therefore be excessive for the corner C.

This fault is inseparable from the method of grinding drills in ordinary drill-grinding machines, which is shown in Fig. 1050, the line A A representing the axis of the motion given to the drill in these machines. It is obvious that the line A A being parallel to the face of the emery-wheel, the angle of clearance is made equal throughout the whole length of the cutting edge. This is, perhaps, made more clear in Fig. 1051, in which we have supposed the drill to take a full revolution upon the axis A A, and as a result it would be ground to the cylinder represented by the dotted lines. We may, however, place the axis on which the drill is moved to grind it at an angle to the emery-wheel face, as at B, Fig. 1052, and by this means we shall obtain two important results: (1) The angle of B may be made such that the clearance will be the same to the actual surface it cuts at every point in the length of the cutting edge, making every point in that length equally keen and equally strong, the clearance being such as it is determined is the most desirable. (2) The clearance may be made to increase as the heels of each end face are approached from the cutting edge. This is an advantage, inasmuch as it affords freer access to the oil or other lubricating or cooling material. If we were to prolong the point of the drill sufficiently, and give it a complete revolution on the axis B, we should grind it to a cone, as shown by the dotted lines in Fig. 1052.

[Ill.u.s.tration: Fig. 1049.]

[Ill.u.s.tration: Fig. 1050.]

[Ill.u.s.tration: Fig. 1051.]

[Ill.u.s.tration: Fig. 1052.]

[Ill.u.s.tration: Fig. 1053. Top View.]

[Ill.u.s.tration: Fig. 1054. Sectional View.]

In Fig. 1053 we have a top, and in Fig. 1054 a sectional, view of a conical recess cut by a drill, with a cylinder R lying in the same. P represents in both views the outer arc or circle which would be described by the outer corner, Fig. 1045, of the drill, and Q the path or arc described or moved through by the point at F, Fig. 1045, of the drill. At V and W are sectional views of the cylinder R, showing that the clearance is greater at V than at W. The cylinder obviously represents the end of a drill as usually ground. In Figs. 1055 and 1056 we have two views of a cone lying in a recess cut by a drill, the arcs and circles P and Q corresponding to those shown in Fig. 1055, and it is seen that in this case the amount of clearance between V and P and between W and Q are equal, V representing a cross-section of the cone at its largest end, and W a cross-section at the point where the cone meets the circle Q. It follows, therefore, that drills ground upon this principle may be given an equal degree of clearance throughout the full length of each cutting edge, or may have the clearance increase or diminished towards the point at will, according to the angle of the line B in Fig. 1052.

In order that the greatest possible amount of duty may be obtained from a twist drill, it is essential that it be ground perfectly true, so that the point of the drill shall be central to the drill and in line with the axis on which it revolves. The cutting edges must be of exactly equal length and at an equal degree of angle from the drill axis. To obtain truth in these respects it is necessary to grind the drill in a grinding machine, as the eye will not form a sufficiently accurate guide if a maximum of duty is to be obtained. The cutting speeds and rates of feed recommended by the Morse Twist Drill and Machine Company are given in the following table.

[Ill.u.s.tration: Fig. 1055. Top View.]

[Ill.u.s.tration: Fig. 1056. Sectional View.]

The following table shows the revolutions per minute for drills from 1/16 in. to 2 in. diameter, as usually applied:--

+----------+------+------+------++----------+------+------+------+ | Diameter |Speed |Speed |Speed || Diameter |Speed | Speed|Speed | |of Drills.| for | for | for ||of Drills.| for | for | for | | |Steel.|Iron. |Bra.s.s.|| |Steel.| Iron.|Bra.s.s.| +----------+------+------+------++----------+------+------+------+ | inch. | | | || inch. | | | | | 1/16 | 940 | 1280 | 1560 || 1-1/16 | 54 | 75 | 95 | | 1/8 | 460 | 660 | 785 || 1-1/8 | 52 | 70 | 90 | | 3/16 | 310 | 420 | 540 || 1-3/16 | 49 | 66 | 85 | | 1/4 | 230 | 320 | 400 || 1-1/4 | 46 | 62 | 80 | | 5/16 | 190 | 260 | 320 || 1-5/16 | 44 | 60 | 75 | | 3/8 | 150 | 220 | 260 || 1-3/8 | 42 | 58 | 72 | | 7/16 | 130 | 185 | 230 || 1-7/16 | 40 | 56 | 69 | | 1/2 | 115 | 160 | 200 || 1-1/2 | 39 | 54 | 66 | | 9/16 | 100 | 140 | 180 || 1-9/16 | 37 | 51 | 63 | | 5/8 | 95 | 130 | 160 || 1-5/8 | 36 | 49 | 60 | | 11/16 | 85 | 115 | 145 || 1-11/16 | 34 | 47 | 58 | | 3/4 | 75 | 105 | 130 || 1-3/4 | 33 | 45 | 56 | | 13/16 | 70 | 100 | 120 || 1-13/16 | 32 | 43 | 54 | | 7/8 | 65 | 90 | 115 || 1-7/8 | 31 | 41 | 52 | | 15/16 | 62 | 85 | 110 || 1-15/16 | 30 | 40 | 51 | | 1 | 58 | 80 | 100 || 2 | 29 | 39 | 49 | +----------+------+------+------++----------+------+------+------+

To drill one inch in soft cast iron will usually require: For 1/4 in.

drill, 125 revolutions; for 1/2 in. drill, 120 revolutions; for 3/4 in.

drill, 100 revolutions; for 1 in. drill, 95 revolutions.

The rates of feed for twist drills are thus given by the same Company:--

Diameter of Revolutions per inch drill. depth of hole.

1/16 inch 125 1/4 " "

3/8 " 120 to 140 1/2 " " "

3/4 " 1 inch feed per minute 1 " " " " "

1-1/2 " " " " "

Taking an inch drill as an example, we find from this table that the rate of feed is for iron 1/100th inch per drill revolution, and as the drill has two cutting edges it is obvious that the rate of feed for each edge is 1/200th inch per revolution. But it can be shown that this will only be the case when the drill is ground perfectly true; or, in other words, when the drill is so ground that each edge will take a separate cut, or so that one edge only will cut, and that in either case the rate of feed will be diminished one-half.

In Fig. 1057, for example, is shown a twist drill in which one cutting edge (_e_) is ground longer than the other, and the effect this would produce is as follows. First, suppose the drill to be fed automatically, the rate of feed being 1/100th inch, and the whole of this feed would fall on cutting edge _e_, and, being double what it should be, would in the first place cause the corner _c_ to dull very rapidly, and in the second place be liable to cause the drill to break when _c_ became dull.

[Ill.u.s.tration: Fig. 1057.]

[Ill.u.s.tration: Fig. 1058.]

In the second place the drill would make a hole of larger diameter than itself, because the point of the drill will naturally be forced by the feed to be the axis or centre of cutting edge revolution, which would therefore be on the line _b_ _b_. This would cause the diameter of hole drilled to be determined by the radius of the cutting edge _e_ rather than by the diameter of the drill. Again, the side of the drill in line with corner _c_ would bind against the side of the hole, tending to grind away the clearance at the corner _c_, which, it has been shown, it is of the utmost importance to keep sharp. But a.s.suming 1/200th inch to be the proper feed for each cutting edge, and the most it can carry without involving excessive grinding, then the duty of the drill can only be one-half what it would be were both cutting edges in action.

In Fig. 1058 is shown a twist drill in which one cutting edge is ground longer than the other, and the two cutting edges are not at the same angle to the axis _a_ _a_ of the drill.

Here we find that the axis of drill rotation will be on the line _b_ from the point of the drill as before, but both cutting edges will perform some duty. Thus edge _e_ will drill a hole which the outer end of _f_ will enlarge as shown. Thus the diameter of hole drilled will be determined by the radius of corner _c_, from the axis of drill revolution, and will still be larger than the drill. A drill thus ground would drill a more true and round hole than one ground as in Fig. 1057, because as both cutting edges perform duty the drill would be steadied.

[Ill.u.s.tration: Fig. 1059.]

The rate of feed, however, would require to be governed by that length of cutting edge on _f_ that acts to enlarge the hole made by _e_, and therefore would be but one-half what would be practicable if the drill were ground true. Furthermore, the corner _c_ would rapidly dull because of its performing an undue amount of duty, or in other words, because it performs double duty, since it is not a.s.sisted by the other corner as it should be. In both these examples the drill if rigidly held would be sprung or bent to the amount denoted by the distance between the line _a_ _a_, representing the true axis of the drill, and line _b_ _b_, representing the line on which the drill point being ground and one-sided compels the drill to revolve; hence one side of the drill would continuously rub against the walls of the hole the drill produced, acting, as before observed, to grind away the clearance that was shown in figure and also to dull corner _c_.

Fig. 1059 shows a case in which the point of the drill is central to the drill axis _d_ _d_, but the two cutting edges are not at the same angle.

As a result all the duty falls on one cutting edge, and the hole drilled will still be larger in diameter than the drill is, because there is a tendency for the cutting edge _e_ to push or crowd the drill over to the opposite side of the hole.

It will be obvious from these considerations that the more correctly the drill is ground, the longer it will last without regrinding, the greater its amount of feed may be to take an equal depth of cut, and the nearer the diameter of the hole drilled to that of the drill--the most correct results being obtained when the drill will closely fit into the hole it has drilled and will not fall through of its own gravity, a result it is somewhat difficult to attain.

Professor John E. Sweet advocates grinding twist drills as in Fig. 1060 (which is from _The American Machinist_), the object being to have a keener cutting edge at the extreme point of the drill.

In a paper on cutting tools read before the British Inst.i.tution of Mechanical Engineers the following examples of the efficiency of the twist drill are given--