The auxiliary implements which are required for use with drilling-machines consist of drills, rosebits, guide-drills, boring-bars and their distinct cutters, el-chucks, spindle-chucks, vices, and holdfast plates with their screw-bolts and nuts. Some of these, such as el-chucks, spindle-chucks, plates, and bolts, are the same in shape as those described in pages 216, 217, and 218, for planing-machines; consequently, it is only necessary to here mention the implements that are specially intended for drillers.

Drills

A machine-drill is a straight piece of steel, one end of which is conical for being held tight in the boss of the machine's drill-spindle, and the other end of which is thinned and pointed for cutting. The pointed extremity of a drill is a pivot on which the drill rotates while passing through the metal, and the two straight sides that bound the end are provided with two edges, which are the cutting edges, each drill having only two.

Figs. 838, 839, and 840 represent ordinary drills having conical points, which differ in shape from each other to render them suitable for various work. A drill with an end which is not very taper is shown by Fig. 838, and is fit for making holes that are to be comparatively shallow, but which require to be smooth. Fig. 839 denotes one with a sort of medium end for all sorts of holes in various metals. Fig. 840 shows one which cuts freely without being very steady; this one is provided with a couple of grooves in the cutting part, to form keen edges, one of which grooves is represented by the line across the end. Drills of this sort are termed roughers; if made in this form they are much keener than those denoted by Fig. 838 or 839, the keenness being the result of thinning the cutting edges, and therefore making them of comparative small angle. In order to properly shape these grooves it is necessary to form them with a true edge of an ordinary grindstone. This edge is what is termed sharp, when speaking of a grindstone, and will allow a drill to be held thereon without damaging or wasting the point.

Drills which have these grooved points are imitations of the joiners' centre-bits employed for wood; the pin-drills shown by Fig. 842 are also of similar character. While the grooved points remain entire they are very effectual for easy cutting; but when they are become worn and broken they lose their keen character by the grinding which is necessary, their shapes being thus made different to those they originally possessed. After being re-ground, all, or nearly all, of their previous keenness is lost, unless new grooves are formed ; and if this is done the drill-end is very much thinned, and therefore weakened. The re-grinding also reduces the extreme diameter of the drill-point, which is termed the drill's size; and one or two grindings and groovings will reduce it sufficient to require it to be re-forged. By this it may be easily perceived that these drills are specially adapted for roughly boring holes where none exist, being more suited to gun-metal and iron than to hard steel. If a large amount of metal is to be removed, previous to finishing a hole with a smoothing tool, a keenly cutting drill is suitable to commence with, although it may make a rough hole; and if the drill-end is reduced in diameter with re-grinding, the difference of size is not important. For such work the drill-point may be shaped several times by grinding, without making it too small for its intended use.