catory motion, ground on both sides, so as to scrape equally well in both directions, shown at a in Fig. 13; and those worked with a continuous circular motion, ground on one

Fig. 13.

side only, shown at b, so as to cut in one direction; drills of the former description are usually fixed in a shaft, the tail-end of which is conical, and rests in a counter-sink formed in a breast-plate worn by the operator, through which the requisite pressure is imparted. Upon the shaft is fixed a sheave or pulley, around which the string of a steel bow passes; by imparting an alternate rectilinear motion to the bow, the wheel, shaft, and drill are caused to revolve alternately in opposite directions, thereby penetrating the material which is being operated upon.

The second class of drill is usually employed either in a brace, consisting of a crank, as shown at a Fig. 14; but when the hole is

E

Fig. 14.

to be drilled in a position which does not allow sufficient room for the brace, another kind of stock, called a ratchet brace, b, is made use of. This consists of a stout shaft, furnished at one end with a socket to receive the tang, or tail-end of the drill, and at the other with a screw, on the head of which is a hard conical point, and by means of which the requisite pressure is imparted to the drill; in the centre of the shaft is a ratchet-wheel, firmly fixed, and embraced by the forked end of an arm or lever, furnished with a pall, acted upon by a spring, which causes it to fall into the teeth of the ratchet-wheel; thus, when the arm is moved in the direction in which the drill is made to cut, the pall catches in the teeth of the ratchet-wheel, and drives the wheel forward, but on reversing the motion, the pall slides over the teeth, leaving the drill stationary. A great variety of minor drill stocks have been devised, but

they are calculated rather to entertain the amateur than to render efficient service to the practical engineer; we shall, therefore, not encumber our space with a description of them.

There are a class of scraping tools, known as broaches, or rhymers, employed for cleaning out circular holes; they consist of taper, triangular, square, hexagonal, or octagonal tools, of which the thickness is inconsiderable, when compared with the length.

Square holes are cleared out by means of steel drifts, consisting of taper steel bars, in which notches are filed at regular intervals, in order to give rise to cutting edges. A drift is forced through the aperture to be cleared by striking it with a hammer, and as these tools are made very hard, breakage frequently occurs.

We must now proceed to describe the methods by means of which screws are produced by manual labor. For the smallest screws a plate of dies, called a screw-plate, is employed; it consists of a plate of steel, in which threads have been cut, which are at certain parts filed away, in order that cutting edges may be formed, and also to afford a means of egress to the metal removed from the screw which is being cut. By means of this apparatus, the threads on a screw are partly cut, and partly squeezed up, being, therefore, not so perfect as those produced by the action of point tools, which will presently be described.

For the production of larger screws, such as the threads of bolts, dies made n two or more parts are used, the cutting edges appearing on the edges of the dies. These dies are used by means of stocks of the form shown, Fig. 15. In the centre of the stock is a

Fig. 15.

rectangular opening containing V-shaped ridges, which, fitting grooves in the dies, retain them in the stock. At one end of the rectangular opening, the ridges are cut away, in order to allow of the introduction of dies as required, which dies are adjusted by means of a set screw, in some cases formed on the end of one handle, of which construction we do not, however, approve, deem

ing it preferable to have the handles firmly fixed, the dies being set up by separate screws.

Having concluded our remarks upon the means used for producing small solid screws without employing machinery, it is necessary to give an account of the method employed in making small hollow screws, or nuts. These are produced, first by drilling, and then by cutting threads by means of a hard steel screw, which we will now proceed to describe. Upon a piece of the best round steel, accurately turned, a screw is cut with great care, so as to be truly formed throughout; a portion of this thread is then removed by filing three or four grooves along the sides of the top, cutting edges being thereby formed, and a means of egress afforded the particles of metal cut away; the thread is also reduced towards the point of the top, thereby imparting to it a taper form, in order that it may gradually cut the thread, so that it may not overstrain either the tool or the material being wrought. Usually, for tapping a nut, two taper taps, and one plug, or parallel tap are used, being formed with a square head, which fits a rectangular opening in the centre of a stock, or tap-wrench, which is handed round in the same manner as die-stock. Oil is used for lubricating these tools, in order to prevent their becoming heated.

A pair of shears for shearing metals is also used; they are precisely similar in their action to ordinary scissors, their edges being ground to an angle of about 85°. The blades are very short, and broad in proportion to the length of the shears. The ends of the handles are curved round, so as to meet and prevent the shears from closing too far.

There are other tools used by fitters, but as they are not, properly speaking, cutting tools, we shall defer the description of them to the chapter wherein we propose to treat of the manipulations included under the general head of fitting.

We will now proceed to describe the forms of the various tools used for operating upon metal by means of machinery. The machines themselves will be described in the following chapter.

The first tool of which we shall speak is known as the point tool; it is forged from square steel, as in fact are most of the machine tools. Two views are shown of it in the accompanying Fig. 16. This tool produces a surface consisting of very narrow grooves, the metal being generally removed by the action of the point and one

side of the cutting edge; it is used in the lathe, in the shaping machine, and in the planing machine.

Another tool has been derived from that which we have just described, which is most frequently used in the lathe, for taking large cuts, whereby the greater bulk of the superfluous metal is removed. This tool may be said to consist of one side of the point tool, the whole of its cutting edge being inclined to the axis of the work.

The next tool which we have to mention is the spring tool; it is formed with a spring or bend, as shown in Fig. 17, which enables it to yield to any hard particles which may occur in the metal being wrought, instead of tearing them out after the manner of the point tool. The spring tool is also much broader, somewhat rounded on the end, in order to prevent the danger of its corners from cutting too deep into the work. Hence this tool produces fewer ridges than the point tool, and such as do occur are more gradual in their ascent; the point tool is, however, capable of executing the work more accurately.

We may next speak of the side tool used for boring small cylinders, and also for cutting internal screws. A plan of this tool is shown in Fig. 18; when intended for boring, it is usually formed with a cylindrical part drawn out behind the cutting edge, in order to allow it to pass freely to the bottom of the cavity. When the tool is intended for cutting internal screws, the cutting edge should be made to protrude farther from the central axis than when the tool is employed for boring purposes.

Fig. 19 represents a parting-tool. It is used for cutting through or dividing work, and is made widest at the cutting edge, in order that the metal behind may not come in contact with the sides of

the cut. A tool very similar in form to this is used for cutting the threads of screws, its form being square, V, or otherwise according to the form of the thread required to be cut.

Fig. 19.

A number of hand tools shown in Fig. 20, are used with the lathe: a being termed a gravér, b a flat tool capable of springing slightly, c a screw tool, d an internal-screw tool. Following the points of these screw tools, are threads which determine the pitch of the screw being cut.

We have yet to notice the slotting tool, which is used in the vertical-motion slotting machine; it is of the form shown, Fig. 21, and is chiefly used for slotting out wheels to receive the keys or wedges by which they are fixed to their shafts.

Fig. 21.

The machine tools already mentioned, are used with the turning lathe, planing machine, and slotting machine; but there are also