The grinding line A B, Fig. 1252, was introduced in the States to assist the operator in keeping both lips of the drill identically the same. To arrive at this, however, is more than can be accomplished by hand grinding, as not less than 3 points have to be carefully watched, viz..

(1) That both lips are exactly the same length.

(2) that both make the same clearance angles.

(3) that both make the same angle with the centre line on the body of the drill. If these are not attended to, the drill lips may for instance be both ground so as to converge exactly to the grinding lines at the point of centre of the drill, and may still be of such different lengths and angles as to produce very bad results in drilling.

Much ingenuity has been expended on machnes for the grinding of the 2 lips with mechanical accuracy. The one which has been the most successful in the United States has 3 motions, ingeniously combined with each other. So many motions, however, entail complication; and this, added to a system of holding the drill which was not sufficiently reliable, failed to produce the extreme accuracy it is requisite to impart to the 2 angles.

The grinding line, too, is found to be more or less a source of weakness. It is therefore advisable to dispense with it if possible; and where a good twist-drill grinding machine is used, the grinding line is seldom or ever looked at, and in that case is useless. If it is still desirable to have grinding lines (as in some cases where hand grinding has to be relied upon), they should be made as faint as possible, and not cut deeply in to the thin central part of the drill so as to weaken it.

Fig. 1247 is drawn exaggerated, in order to show the ill effect of grinding one lip of a drill longer than the other.

A simple and efficient twist-drill grinding machine was so much needed, that within the last 3 years the writer has designed one. The twist-drill in this machine has only one motion imparted to it to produce the 2 lips of each drill as perfect fac-similes of each other and with the desired amount of clearance. Many of these machines are now at work. That the drills ground by them are accurate is proved by the holes drilled being so nearly the size of the twist-drill itself that in many cases the drill will not afterwards drop vertically through the drilled hole by its own gravity; in other words, the hole is no larger than the drill which has drilled it.

It is not generally known that this is the most severe test that can be made of the accuracy of regrinding, and of the uniformity of all parts of the twist-drill.

The whole of the drilling in many establishments is now done entirely by twist-drills. Since their introduction it is found that the self-acting feed can be increased about 00 per cent.; and in several engineering works the feeds in some machines have been increased by fully 200 per cent., and consequently 3 holes are now being drilled in the same time that one was originally drilled with the old style of drill and with old machines.

It may be interesting to give a few results out of numerous tests and experiments made with twist-drills. Many thousands of holes 1/2 in. in diameter and 2 3/4 in. deep have-been drilled, by improved 1/2-in. twist-drills, at so high a rate of feed that the spindle of the drilling machine could be seen visibly descending and driving the drill before it. The time occupied from the starting of each hole, in a hammered scrap-iron bar, till the drill pierced through it, varied from 1 min. 20 sec. to 1 1/2 min.

The holes drilled were perfectly straight. The speed at which the drill was cutting was nearly 20 ft. per minute in its periphery, and the feed was 100 revolutions per in. of depth drilled.

The drill was lubricated with soap and water, and went clean through the 2 3/4 in. without being withdrawn; and after it had drilled each hole, it felt quite cool to the hand, its temperature being about 75° F. 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 1/100 in.; 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 in. in diameter and 2 3/4 in deep, have been drilled through the same hammered scrap iron at the extraordinary speed of 2 3/4 in. deep in 1 minute and 5 seconds, the number of revolutions per in. 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. The drills in both cases were driven by a true-running, drilling machine spindle, having a round taper hole, which also was perfectly true; and the taper shank and body, or twisted part of the drills, also ran perfectly concentric when placed in the spindle, or in a reducer, or socket having a taper end to fit the spindle. When the drills run without any eccentricity, there is no pressure, and next to no friction on the sides of the flutes; the whole of the pressure and work being taken on the ends of the drills. Consequently they are not found to wear smaller in diameter at the lip end, and they retain their sizes, with careful usage, in a wonderful manner.

The drills used were carefully sharpened in one of the twist-drill grinders mentioned above.

In London upwards of 3000 holes were drilled 5/8 in. in diameter and 3/8 in. 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 ft. to 20 ft. per minute; and the result is extraordinary.

Many thousands of holes were drilled 1/8 in. in diameter, through cast iron 7/16 in. 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-in. holes have been drilled through wrought copper, 1 3/8 in. thick, at the speed of one hole in 10 seconds.