The turret-head can be adjusted in position upon the machine and firmly clamped by the bolt a (Jig. 4). As the turret slide b MONTS to the left, the turret c remains firmly locked in position, the forward motion of the turret slide being arrested by an adjustable stop-screw (a').
moving the side to the right a locking-pin is withdrawn from a hole in the bottom of the turret by a lever upon which a heavy spring presses and holds the pin in the hole. As the slide moves to the right, the end of the lever rides under a steel piece, which is free to swing in the opposite direc tion and withdraws the pin from the turret. As the movement of the slide continues, a ratchet-pawl engages with a ratchet-wheel, and turns the tur ret, the pawl being brought to its neutral position by a coil-spring. Mean while, a projection upon the lever has passed from under the steel piece, and the spring is acting, so that as soon as the hole in the turret collies under the pin, the pin is forced in. At this instant the relative motion of the slide and slide-rest is arrested by the contact of two lugs, so that in rapid working there is no danger that the locking-pin will miss the hole.
The principal tool-holders used in the turret-head are shown in Figure 5. A is the "box-tool," which, containing two cutters—one for rough ing and one for finishing—can easily be taken out and ground. The shape of the box-tool allows easy access for measuring the work. 13 is a die-holder, the die being held by three screws, which allow the die to be so adjusted as to swing concentrically with the spindle. The part b' fits freely upon B and is clamped in the turret. C is a tap-holder and works the same as the (lie-holder; e is for holding hollow mills and other tools, and a' is the stock, with which the rapidity of the wire-feed is gauged. Figure 6 (pi. 23) shows the cross-feed.
Drills and bore a cylindrical hole in wood or metal it suffices to place upon the material a pointed steel tool provided with scraping- or cutting-edges and to rotate it with a constant pressure in the direction of its axis. A tool of this kind is called a "drill." The forms of the cutting-edges vary greatly, according to the different materials to be drilled.
.fetal the drills for metals illustrated on Plate 24, Figures and 2 represent straightway drills; Figure 3 a drill for boring brass and soft metals; Figure 4 a countersink; and Figure 5 a countersink and drill combined. Figure 6 shows the ordinary single-cutting drill, in which the point is nearly a rectangle formed by only two facets symmetrical to the axis. Drills of this kind, being liable to run out of centre, have been improved by increasing the length of the parallel portion next to the edges and by maintaining it at a width equal to the diameter of the bore (jig. 7). In the drill shown in Figure 8, which is intended for lathe use, there is a single edge on the end of a semi-cylinder, the central point, as in Figure 9, being wanting. The twist-drill (jig. to) may be considered the most complete form for metals, continuous removal of shavings being effected through two helical channels.
manipulating drills the crank-brace (fig. 12) is mostly used. The " feed " of the drill is effected by a powerful screw, with a box in a horizontal arm of a frame connected to the material to be drilled. While the right hand moves the handle of the brace in a cir cle, the left effects gradual advance of the drill by a bar pushed through the head of the feed-screw. A suitable combination of crank-brace and ratchet brace is shown in Figure r r. The brace does not receive a continuous rota tion, but a pendulum-like oscillation, through which the advance of the drill by means of pawl, ratchet-wheel, and screw is automatically effected. Figures 14 and 15 represent ratchet-drills employed by engineers, machin ists, bridge-builders, ship-builders, boiler-makers, etc. Figure 3 CAL 26) exhibits a hand-drill which can be fastened to a work-bench or to the flange of a casting by means of a clamp and a stud, or can be permanently bolted to a bench or any flat surface. The post and arm are both round, and are held in split bearings, which allow the drills to be placed in any position and at any angle. The crank-handle, to suit the size of the hole being drilled, is adjustable, and it can be used either at the end of the arm (B) or at the end of the spindle (A).