broken in much less time than the counter-borer could, thus making it cheaper to use, both in point of execution, and cost of repair when injured.

The tool in fig. 15 is, as may be seen, adjustable, and will cut holes of varying diameters, according to the sizes it is constructed for. The arrangement is simple, and consists of a central head forged solid on the shank. This head is planed out for the reception of the tools of cutters, and has, further, a wrought-iron ring shrunk over it. This ring is tapped out to receive the set screws which hold the tools fast. Behind the tools are wedges, which, when driven or slacked off, advance or retract the cutters with great nicety; the taper is planed in the shank for the wedges, so that the cutters always stand vertically. The wedges should all be planed at once, so that there will be no variation in them, and several sizes should be provided, so that holes of any diameter can be made. The cutters need not all travel in the same track, but each may set a little inside of the one that forms the size; in this way they cut freer and are less liable to break. This tool is useful, not only in the boiler shop, but also in the finishing department, for by changing the character of the cutters, work of almost any kind can be done.

Here is another plan (fig. 16) for a boring tool or tube sheet-borer more properly, for this is the object it was designed for. It is not so good a tool as the first one for some purposes, but as all

persons may not have the same opinion we give it place. It is not adjustable, except limitedly, it

costs more to make at first, but it will work faster and do equally as good, if not better work than the ordinary adjustable cutter shown in fig. 14 The bar, A, is merely forged with a larger portion on the end, and is grooved on four sides to admit the cutters; these are simply square nosed, offset on one side, and the cutting part, of course, curved to suit the circle it works in; a wrought-iron ring, B, is then slipped over the cutters to hold them firmly in place and adjust them so that all the points may work at once; this ring has set screws for each cutter, and one of the cutters may be made to countersink the sheet at the same time, if it is preferred to do it on the side drilled from. The burrs or ragged edges left on the under side of the sheet by this tool will be very slight indeed, if it is properly made, and can be rubbed off with an old file.

Still another drill for boring tube sheets is giver in fig. 17, herewith. It is one commonly used,

Fig. 17.

and is a very efficient tool when well made. It is costly to construct, however, and requires to be turned in the lathe by an experienced workman, and afterward filed up so as to cut properly. The spaces between the pin and the cutters are very troublesome to cut out in the lathe with an ordinary tool, as the work in revolving strikes square and suddenly on the lathe tool, and soon dulls it, or else breaks off the end and throws the drill out of the centres. A useful cutter for making these drills is shown in fig. 18. It is simply a steel bar turned up and bored out the size of the "tit" or pin on the drill, and has teeth cut all round the circumference, as shown below. The

portions so difficult to remove in the lathe. The drill may revolve in the steady rest, or the barrel cutter may be so used and the work screwed up to it by inserting the centre in the dead centre of the lathe; by employing this tool much time may be saved and better work done.

A work might be specially devoted to the drill; it is one of the most indispensable of the minor instruments employed by mechanics, and it is only reasonable to add, that the tool most in use, simple though it be, deserves all the attention that can profitably be given to it.

Although the rose bit is not in any sense a drill it is of the same class, and is indispensable to good work in the drilling machine; for if a man does not know how to grind a mill or make one, and the holes he makes are neither round, square, nor oval, then he has only to use the rose bit and he will have a perfectly round, straight hole. Fig. 19 is the bit. It may be made wholly of steel, or the

Fig. 19.

shank may be iron, and the cutting end only of steel. The end is composed of a series of fine cutters arranged regularly all around, and the body is a shade smaller at its upper end than at the lower. When the hole is drilled in the work

to nearly the right size, the drill is taken out and replaced with this bit, which cuts regularly and steadily all around, and corrects any untruth in the first hole. There should be but very little metal left for it to work on, and the job must be well oiled during the process. If these conditions are observed the hole will be a true cylinder.

CHAPTER III.

THE DRILL AND ITS OFFICE-CONTINUED.

THERE is still another kind of drill for peculiar work, which is employed by some machinists, though for our own part we see no special virtue in it, for it is troublesome to use and to make, and very liable to break. It is called the tit or centre drill, and fig. 20 is an engraving of it. The centre marked out by the punch is of course the point where the tit is inserted on the work. This tit is the cause of all the trouble with the tool; it must be filed up in the vice, it tries the tool-dresser's patience to harden it, for the small quantity of metal

Fig. 20.