By Courtesy of the Cleveland Twist Drill Company.

Next to a drill being properly made and tempered, it is of the utmost importance that its cutting edges be properly ground to get the maximum results in drilling. This means that both cutting edges must have the same inclination to the axis of the drill, and be of exactly the same length; this will, of course, bring the center of the eutting edges in the true center of the drill and will produce a round and smooth hole. To get maximum results all these requirements must be carefully observed. It is not sufficient to have one condition correct, but all of them. If the point be central but the angle of the cutting edges different, the drill will bind on the side of the hole opposite to that side of the point which is cutting, will drill too large a hole, and all the work will fall on the one cutting edge. Fig. 8 illustrates this, while Fig. 9 shows a point ground with equal angles but of different lengths, which will result in the hole being too large.

When both angle and length of cutting edges are wrong the drill will be laboring under the severe conditions shown in Fig. 10, and the support spoken of n paragraph on "diametric support " entirely lost.

Another very important feature of grinding a dri 1 point is the lip clearance or proper backing off of the cutting edge. To do this correctly, even on a machine, is a difficult problem. Our idea of the correct form to which the lip of a drill should be ground is that of a segment of a cone whose axis is on line a b Fig. 11, and at an angle b d c to the axis of the drill. There is, however, a difference of opinion among engineers as to just what shape this end of the drill should be, some favoring that shape which corresponds to a segment of a cylinder, some an inverted cone, and still others a cone of irregular contour.

The machines which grind on the last named system nearly all come under the head of form or cam machines, that is, the shape is produced by copying a template or the motion of a cam.

In Fig. 11 the dotted lines show the complete frus-trum of the cone; by comparison that of Fig. 12 will be seen to be too near the center, and the curvature at that point correspondingly more, which we found to consume about 20 per cent more power than that of

Fig. 11. Fig. 13 illustrates the point whose surface is a segment of cylinder, and Fig. 14 represents the inverted cone with an axis on line a b, dotted lines show the frustrum complete. In both these forms of point. (Fig. 13 and 14) the curvature is too small at the outside periphery compared with that of the inside or center when the clearance angles are correct. Increased durability is claimed for this form of point, due to the support the cutting edge has from the small curvature at the periphery where the most severe work is done.

This is probably true, due is in turn onset by the fact that when the curvature at the periphery is correct for good work, that of the center is excessive, and under heavy feed pressure the edge chips out and breaks, evidence of which we are constantly brought in contact with.

There are various shapes of flute and angles of spiral on the drills made by different manufacturers, the shape of flute varying only by a small amount, while the angle of spiral ranges from 18° to 35°.

Theoretically the finer pitch of the spiral grooves or the greater angle of the spiral with the axis, the easier it should sever and bend or curl the chip; but practical considerations arise which counteract the mere saving of chips, and it becomes advisable to make this angle somewhat more acute than would otherwise be the case. Among the practical objections to a very fine pitch of spiral may be mentioned the weakness of the cutting edge and its inability to carry off the heat generated. It also packs up with chips more readily.

From a large number of tests made we have found that the practical limit to this angle of spiral for the regular commercial article is between 30° and 25°, assuming that the average drill is to drill a hole from one to three diameters deep. For deeper holes than this a smaller angle might be advisable, and for shorter holes a greater one. The difference in torsional stress on the drill does not vary any considerable amount when the angle of spiral ranges between 30o and 25o with the axis. We therefore use an angle of 27 1/2° for reasons which facilitate the operation of milling the grooves and to simplify the curves on the cutter, to produce a straight lip of the form shown in Fig. 5. This angle of 27 1/2° with the axis makes the spiral groove of all drills start at the point with a pitch equal to six diameters of the drill blank. This with a uniform web increase retains a strict uniformity in the pitch of grooves, and curves of cutters for the entire system of regular drills and is the form which our experience has shown to be the most effective for the average work a drill is called upon to do.

The subject of the speed at which a drill should run and the feed per revolution is one on which engineers differ very radically, and the extremes of heavy feed with slow speed and light feed with fast speed are both supported by indisputable data. No rule can be given to cover all cases, and the ordinary tables published should be considered as guides only; the correct speeds should determined by good, sound judgment for each particular case. One thing is certain, if the drill chips out at the edge there is either too much lip clearance or too much feed, and a drill split up the web is sure evidence of improper grinding or excessive feed pressure, and no drill manufacturer ought to be expected to replace a split drill unless there is a "flaw" apparent in the break. Fig. 15 illustrates the angle of lip clearance; 12° is the best for the average rate of feed; for heavier feeds this angle may be increased to 15°. Observing the end view of this figure, the center of the drill will be found to be at an angle with the cutting edges, and should be approximately as shown. The remedy for drills that are properly ground, chipping at the cutting edges is to decrease the feed and increase the speed, which, if a little care is taken to arrange properly, will produce as much work as before, with a longer life to the drill. When the extreme outer corners deteriorate too rapidly, it is evidence of too much speed, so that the best performance of a drill will be found where the effect of the work on the tool is somewhere between these two conditions. '

If no table is at hand or operator is in doubt as to correct speed for the drill, start with a periphery speed of 30 feet per minute for soft tool and machinery steel,

45 ft. per minute for cast iron, 60 ft. for brass, and a feed of from .005 to .007 of an inch per revolution, and then attain maximum results by noting conditions of the drill and following instructions in preceding paragraph.

We have seen 50 point carbon steel drilled with one of our 2 in. drills at a periphery speed of 60 ft. per minute and a feed of .005 inch per revolution, but we do not think it is good practice, as we have found in our own work that the majority of cases are better suited to high speed and light feed carried to the point at which the outside corners commence to wear away. For automatic machines where holes do not exceed two diameters of the drill in depth, and under a flood of lard oil, high speeds and light feeds are especially recommended. For holes deeper than this it becomes a matter of getting rid of the chips, and a form like Fig. 5 is efficient with slower speeds and heavier feeds, as the bottom of the hole is approached. Always endeavor in automatic drilling to get a small compact roll to the chip, and if possible keep it intact the entire depth of the hole.

A heavier feed should be used in drilling brass, especially in automatic machines, to insure chips working out, and if lubricated at all it should be flooded.

High speeds in cast iron tend to wear away the small portion of the drill that represents the diameter - see Fig. 2 - and we think that 35 ft. per minute should not be exceeded. Feed may be from .007 in. to .015 in. per revolution, according to the kind of metal drilled.

The drilling of hard material is facilitated by reducing angle of spiral with the axis, as shown in Fig. 16, so as to permit of heavier feed pressure without chipping the edge, and using turpentine as a lubricant, but extreme care and judgment is needed to do this without unfitting the drill for urther use. This form of drill will be found efficient in drilling soft material where the regular form has a tendency to "hog in ". Drills are made to feed to their work easier by thinning the extreme point. This is a delicate operation and requires some skill on the operator's part, but is a decided improvement in hand feed drilling. To thin this point properly a round face emery wheel is necessary, and the drill should look like Fig. 17 when finished, care being taken to preserve the true center of the drill and not weaken it by extending the ground portion too far back. Concluded from October Number.