If not, they had better not let me patent my flogging-machine. Luckily it is not invented yet.
The cutting edges of drills come under the same rules as other cutting edges. You might, for instance, hold a large drill flat on the rest, and use either edge as a turning-tool. You will see at once that these edges will not cut if made in the usual way, but only scrape. The bevel wants to be ground only to 3°, as before explained, to give the proper clearance, and the cutting edge requires to be then made by grinding back the upper surface, which is just the same in effect as is produced by twisting the metal or cutting a spiral groove, which hollows out this upper surface and gives it cutting power. It is no use grinding a sharper-looking bevel, or making more of a point—you only weaken the edge; m or n is quite pointed enough, though the first is a right angle and the second greater; and, for cast-iron, a rounded point, showing no angle at all, will do just as well, or better, when once it has begun to penetrate. Do not be deceived, therefore, by making drills look pointed and keen, for, I repeat, they are scraping tools only, unless you file an edge by bevelling back the upper face of each side of the point. If you were to make a very thick, strong drill, you might begin by grinding back the two sides to 3°, to form the accidental front line of the point or section angle, and then grind back, at 45° from this line, the upper face, by which you would do just what you did to give the graver cutting edges of 60°—only a drill thus formed must have a point of 90°. It would cut in two directions, like one for a drill stock and bow.
I hope my bigger boys will not pass over the remarks on cutting edges interspersed in this book, for, once understood, they will be found to be most valuable. Indeed, they cannot work intelligently until they understand exactly the nature and principles of the tools which they have to use. In drilling iron, use water or oil, or soap and water, or soda-water—either will do; but the holes are drilled in the ships’ armour-plating with soap and water to cool the drill; and very well it answers, for these plates are several inches thick, but the holes are soon made. When working in brass and gun-metal, use no water, but work the drill quite dry. The same rules, in short, apply to drilling as to turning or planing metal; and if you could see the action of a well-made American twist-drill, you would recognise this similarity, for you would see the metal come forth in long, bright curls, as pretty and shining as those of your favourite young lady or loving sister—one of which you have, I daresay.
To give you some idea of what a straight course a drill will take, if rightly made and skilfully used, I may tell you that a twist-drill has been run through a lucifer-match from end to end without splitting it; and as to the fineness possible, I have seen a human hair with an eye drilled through it, by which, needle-like, it was threaded with the other end of itself.
I told you how to bore a cylinder, which is but drilling on a larger scale, and in Fig. 65 I sketched the method of doing this in the lathe with a rosebit. But I did not explain another tool used just in the same way, but which will bore holes in solid iron wonderfully. Fig. 65, L, H, K, is one of these. This is an engineer’s boring-bit, and is made of all sizes, from that required to bore the stem of a tobacco-pipe—(don’t smoke, boys, it will dry up your brains)—to that which would bore a cannon. A rod of steel is forged with a boss or larger part at one end. This is centred in the lathe, and the centre-marks are well drilled, and not merely punched, especially that at the small end. The boss is then turned quite cylindrical, after which it is filed[4] away exactly to the diametrical line, as you will see by inspection of L. The end is then ground off a little slanting, to give, as before, about 3° of clearance. The cutting edge thus obtained, and the end in which the centre hole still remains, are carefully hardened. You thus have a tool which will bore splendidly, but you must give it entrance by turning a recess first of all in the work, or drilling, with a drill of equal size, a little way into the material. Used like the rosebit, this tool will run beautifully straight, so that you can bore very deep, long holes with it, and cylinders can be most beautifully bored with it. I think you would be able to make these tools with a little care; but, when you harden them, only heat and dip the extremities, or it is ten to one your steel rod will bend and warp in cooling, and you will not be able to rectify it. If the ends are quite hard, it is as well that the rest should be soft, as the tool will not then be so liable to get broken.
There are many other tools used for boring iron and steel, but you need not trouble yourself at present to learn anything of them—they are no use to you now.
I have headed this chapter “Hardening and Tempering” tools, but as yet I have only partially explained the process, which is a very curious one; and though the result is highly necessary in many cases, it is by no means well understood what really takes place in the process, or why this effect should occur in steel, but not in iron, or brass, or other metals.
If you heat a piece of bright steel over a clear gas jet or fire which will not smoke it, you will see several colours arise as the metal gets hotter and hotter, until finally it becomes red. These are due to oxidation, which is so long a word that I am not sure I can stop to explain it thoroughly. Let us see, however, what we can make of it. The air we breathe contains two gases, oxygen and nitrogen, with a small proportion of a third called carbonic acid. Neither of these alone will support life, or keep the fire burning, or enable vegetables to live and grow, but it is the first which is in this the chief support. The second is only used by Nature as we use water to brandy, viz., to dilute it and render it less strong. If we breathed oxygen alone, we should live too fast, and wear out our bodies in a few hours. If we breathed nitrogen only, we should die, and so of carbonic acid. Now this oxygen seizes upon everything in a wonderful and sometimes provoking manner. If you leave a bright tool out of doors to get damp, down comes our friend oxygen and rusts it. It combines with the iron and makes oxide of iron, which is what we call rust. I suppose, however, this oxygen comes more from the water than the air, because water is made also of two gases, hydrogen and this same oxygen. It is certain that oxygen in this case always finds any bright tools that we leave about in the wet, and coats them with a red jacket very speedily. Then if you look at a blacksmith at work, you will see scales fall from the hot iron as he hammers it. These are black, but our old friend has been at work, and united with the red-hot metal and formed another oxide of iron, called black oxide. We can understand this. If a man eats a good deal, or drinks a good deal, he gets red in the face; if he eats till he chokes himself, he gets black in the face, and I suppose it is much the same when oxygen eats too much iron. Well, when we begin to heat the steel, down comes oxygen and begins his work; and first he looks very pale; then he gets more bilious and yellower; then he gets hotter and shows a tinge of red with the yellow forming orange; then he begins to get purple, then blue, then deeper blue; and finally black before he gets absolutely red and white hot.
Now to temper steel, we first heat it red-hot, not minding these colours, and then we cool it suddenly in cold water. This renders it very hard indeed. No file will cut it, or drill penetrate it; but if we strike it, behold it breaks like glass! This is too hard for general work, for the edge will break and chip if it meets with any hard spot in the metal, or chances to bite in too deep. Its teeth are too brittle, and so get broken off. For this reason we have to “let down,” or temper, the tool, and we proceed as follows: The part to be tempered is ground quite bright. It is then laid upon a bar of iron heated red-hot, or if small, it is held over a gas jet or in a candle; heated, in short, in any way most suitable and convenient. And now, first, our friend oxygen puts on a pale yellow face as before. This will do for turning steel and iron, but is still too hard for general work. Then comes the orange, and this presently tends slightly to blue; at which point, if the tool is instantly cooled in water, it will be found to bear a good edge, hard, but sufficiently tough for work. Most tools for metal and drills are let down to something between the yellow and blue, and we know that the more they approach blue, the softer they will be. Thus we can easily manage our tools;—some to bear hard blows, like axes, which are tempered to a blue colour; some like files, which a blow will break, but which are famous for their own special work—these are let down only to a pale yellow; others, like springs and saws, are let down to a more thorough blue, because they are required to be elastic and tough, but are not needed to be so particularly hard. Then tools like turnscrews, and bradawls, and gimblets are left even softer, sometimes not tempered or hardened at all, but just forged and ground to the required shape.
Now, I fancy some of my sharp boys will say that the first description I gave of the mode of hardening and tempering was not exactly like this; nor was it, yet in principle it is the same. For instance, if you give a drill to a smith to make, he will do as I then said. He will heat the extreme point red-hot, then dip the point in water, give a rub on the stone or bricks of the forge, and watch the colours. This can be done when the tool is of sufficient substance to retain heat enough after the edge has been dipped to re-heat that edge sufficiently. In this case there is no need to chill the whole tool and then heat it again. But in the case of small drills and tools, pen-knife-blades, and other articles of this nature, there will not be sufficient heat retained, after dipping, to bring up to the surface the desired colours; for oxygen likes a hot dinner as well as you do, and if the iron is not hot enough he will have nothing to do with it.