A universal drill is one which is suitable for making holes into iron, steel, brass, gun-metal, a number of other metals, and a large number of woods. The tool consists of a cylindrical piece of steel in which are cut two thread-grooves, these grooves being channels along which the shavings travel after being detached from the work with the cutting end of the drill. Fig. 425 denotes a tool of this character, which is adapted to originate holes in places where none exist, and also to enlarge holes that are made with other tools. Such a drill may be used in crank-braces, swing-braces, lathes, and vertical drilling-machines; but the tool is specially adapted for lathes, because the cuttings from the work easily travel along the thread-grooves while the drill is in a horizontal position. Drills of this form never need any forging to repair them, all the mending which is necessary through the proper wear of the tool being effected by ordinary grinding; and because the drill is parallel, all the holes made with it are of the same diameter, although the cutting part may have been shortened an inch or two inches with repeated breaking and grinding.

Slot Drills

A slot drill is one with a concave extremity, instead of a conical point. Slot drills are represented by Fig. 426, and are used in slot-drilling machines, the concave form of the cutting part being necessary to permit the traversing motion which produces the desired slot in the work. The straight part of the tool being of a cylindrical shape, it is very strong and adapted to its special uses.