A drilling-bar or drilling-rod is a cylindrical rod of steel, of any suitable length and thickness, in which is formed a slot for containing a distinct steel cutter and fixing-key. Boring-bars or drilling-bars are used in a variety of machines and in several different ways; and when one is used in a drilling-machine, it is held at its upper end in the boss of the drill-spindle, and rotated in the same manner as drills and rosebits ; but no portion of the rod is capable of cutting, all the cutting tools used with the bar being distinct pieces. The boring-rod is, therefore, a holder for containing and properly guiding the various cutters while in use.

There are two principal modes in which the rod may be used, one of which consists in causing the rod's lower end to rotate in a hole already drilled in the object being bored, and the other mode consisting in causing the lower end to rotate in a suitable hole formed in the table of the machine. When the rod is so used that its end fits a hole in the piece of work beneath, the end in the hole resembles the pivot end of a pin-drill, because the guide-hole first made contains the rod's end and guides it during its passage downwards at the time of cutting, the cutter which is keyed in the rod performing in a manner similar to the cutting part of a pin-drill. The rod's end is also guided if situated in a hole in the machine-table, in which case, the hole is so bored, that its axis is exactly in line with the drill's axis of rotation. By referring to Fig. 837, a drilling-rod in the situation here referred to, may be seen fixed.

A boring-rod may have two or three slots for cutters, and for some purposes, all may be used at one time, when two or three cutters are required to cut at one time. Fig. 847 represents a rod having two cutters keyed in their places, and are fixed the one at right-angles to the other, in order to prevent the strain of both cutters acting in the same direction across the rod. The boring-rod shown by Fig. 848 is a comparative strong one, in which is keyed a cutter for facing; an operation which is to be detailed. It may be here mentioned that through all boring-rods being comparatively slender, the thickest rod which can be used for a stated work, is always selected, to avoid vibration while in contact with the object being bored.

To avoid the vibration and bending of a drilling-rod while cutting, is an important desideratum, when a long hole of comparative small diameter is to be bored, because the trembling of the rod always makes the hole rough, and sometimes entirely prevents the cutting of the cutter or cutters in the hole. An object with a small hole will not admit a rod of great thickness, consequently, a slender one must be used, and vibration prevented, as far as convenient. The mode of treatment resorted to for avoiding the trembling of a long-rod, consists in steadying it. This consists in adopting means for counteracting the strain of the cutter; and this is effected by fixing a piece termed, a steady, in the rod, so that one end of the steady shall be keyed tight in a slot, and the other end project from the rod a sufficient distance to bear tight against the side of the hole being bored. The steady should be of steel, and that end intended to bear in contact with the side of the hole, should be curved and smoothed, that an easy movement may be secured, and friction to some extent avoided. When only one steady is employed for the purpose, its friction surface is made to bear upon that side of the hole which is opposite to the side at which the cutter is cutting; but when two steadies are used in one bar, they are keyed at an angular position of about a hundred and twenty degrees to each other, and also to the cutter. This arrangement is named triangular, or tri-radial; the other mode, in which only one steady is used, being termed diametrical.