This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
It will therefore be readily understood that if one lip of a drill stands before the other to the extent of 1/100 in. only, the prominent lip, or portion of a lip, will have to remove the whole thickness of the metal from the hole at each turn. The lip of the drill will not stand such treatment; and it is therefore obvious that if this were attempted the prominent lip would either break or become too rapidly blunted. To get over these difficulties, the driller would no doubt reduce his feed by one-half, or to 1/200 in. per turn, which would mean about half the number of holes drilled in a given time.
This nice accuracy, although absolutely required, cannot be produced by hand grinding; neither can a common drill, having a rough black stem more or less eccentric, be ground accurately, even by aid of a grinding machine with mechanism for holding it. To grind any drill accurately, it must be concentric and perfectly true throughout with the shank, as that part has to be held by the drill-grinding machine. If the drilling is to be done in the most rapid manner, in other words, at the smallest cost, and if the best class of work is also desired, it seems certain that a twist-drill, with all the accuracy which can possibly be imparted to it in its manufacture, and the greatest care employed in the resharpening, is the only instrument which can be employed.
About a quarter of a century ago both Sir Joseph Whitworth and Greenwood, of Leeds, made some twist-drills; but it is to be presumed that a large amount of success was not achieved with them, and for some reasons the system was not persevered with. After that period the Manhattan Firearms Company in America produced some beautifully finished twist-drills. Though the workmanship in these was of a superior description, the drills would not endure hardship. It was found that the 2 lips were too keen in their cutting angles, and that they were too apt to drag themselves into the metal they were cutting, finally to dig in and to jam fast, and to twist themselves into fragments. Morse then took the matter up, and by diminishing by about 50 per cent. the keenness of the cutting lips of the twist-drills, made a great success of them. He used the grinding line A B, Fig. 1252, and an increasing twist. In such a drill, of the standard length, and before it is worn shorter by grinding, the twist is so rapid towards the lips that the angle they present, or what has been already referred to as the angle of the cutting surface, is very nearly the same as that which W. F. Smith had previously established for cutters cutting metals, as in Fig. 1254.
If, however, the angle of twist is made to increase towards the lips, it will of course decrease towards the shank, as in Fig. 1251. The shorter the drill is worn, the more obtuse the cutting angle becomes, and the less freedom will it have; supposing, of course, that the angle, when the drill was new, was the most efficient. Suppose this decrease of twist were carried still further by lengthening the drill, a cutting angle of 90° would eventually be arrived at. The old common style of drill usually has a cutting edge which is so obtuse as not to cut the metal sweetly, but on the contrary to have more of a tearing action, and thus put so much torsional strain on the drill that fracture is certain to take place, even if what the writer would now consider a moderate feed was put on by the drilling machine.
It is therefore obviously advantageous to adopt from the first the best cutting angle for all twist-drills, and to preserve this same angle through the whole length of the twisted part, so that, however short the drill may be worn, it always presents the same angle, and that the most efficient which can be obtained. This cutting angle is easy to fix, and becomes an unalterable standard which will give the best attainable results. This has been adopted at the Gresley Works, Manchester, and of course applies to both lips.
A common drill may " run," as it is usually termed, and produce a hole which is anything but straight. This means that the point of the drill will run away from the denser parts of the metal it is cutting, and penetrate into the opposite side which is soft and spongy. This is especially the case in castings; where, for instance, a boss may be quite sound on the one side, while on the other a mass of metal may be full of blow-holes, or so drawn away by contraction in cooling as to be very soft and porous. In such cases, it is perfectly impossible to prevent a common drill from running into the soft side. This sort of imperfect hole is most trying to the fitter or erector, and if it has to be tapped, to receive a screwed belt or stud, is most destructive to steel taps. The taps are very liable to be broken, and an immense loss of time may also take place in attempting to tap the hole square with the planed face. A twist-drill, on the other hand, from its construction is bound to penetrate truly, and produce holes which are as perfect as it is possible to make them.
The next important step in twist-drills has been to fix a standard shape and angle of clearance for both lips, which should also give the best attainable result. This angle might be tampered with if the regrinding were done by hand, and too much or too little clearance might easily be imparted to the drill from want of sufficient knowledge on the part of the workman. If too little clearance, Fig. 1248, or in some cases none at all, is given to the drill, the cutting lips then cannot reach the metal, consequently they cannot cut. The self-acting feed of the drilling machine keeps crowding on the feed until either the machine or the drill gives way. Usually it will be the latter.
Again, if too much clearance is given, Fig. 1249, the keen edges of the lips dig into the metal and imbed themselves there, and of course break off.