A Treatise On The Principles And Practice Of Dock Engineering

86 DOCK ENGINEERING. Drilling tools are divisible into two classes—hand drills and machine drills. Hand drills are round bars of iron or steel, witli a steel cutting edge, either cruciform or chisel shaped, and are of two sizes. The short hand drill can be manipulated by one man. He holds the drill in the left hand and strikes it with a hammer in his right. Sometimes two men are engaged—one as a holder and the other as a striker. The drill should be slowly rotated. Long hand drills, or jumpers, necessitate the attendance of several men. If driven vertically, the drill is lifted by their combined effort and allowed to fall, being caught at its rebound and at the same time turned through a slight angle. If the cutting be horizontal, the drill is projected backwards and forwards by a swaying movement of the holders. Hand drilled holes are from | inch to 2 inches in diameter, and the depth varies, of course, according to circumstances. For biasting purposes from 2 to 4 feet will suffice. The rate of drilling depends upon the nature of the material, but raay be taken between the limits of 5 to 10 feet per 10-hour day. The cutting edge will require re-sharpening, at intervals represented by from 6 to 18 inches of excavation in depth. Machine drills are niuch more rapid in action than hand drills, and they aiso work more economically, but their installation is expensive and only justifiable in the case of extensive operations. Machine drills are of two kinds—percussive and rotary. The former are identical in principle with hand drills, the distinction lying simply in the nature of the motive power applied, which may be steam, compressed air, or electricity. Instead of using a single cutting edge, however, several chisels may be worked in combination, especially where large holes are required. For vertical boring thedrill is often surged bya wire ropeleading over sheaves to a winch. The chisels vary in width up to 24 inches, but the vibration due to such a heavy chisel as this last is apt to cause frequent breakages in the rods. Rotary drills are tubular, with extremities fitted with hardened steel teeth or diamonds, the latter being more general. The drill consists of two parts the boring bit and the core lifter. In the course of action the former makes an annular cutting, leaving an internal core upstanding, which, when the operation is finished, is gripped by a loose toothed ring contained within, and caught in its turn by, the coned inner surface of the drill. The core, being thus jamrned in the drill, is broken away at the root by a few additional revolutions. In ordinary rock, machine drills can bore holes, 2 to 3 inches in diameter, at rates varying from 1 to 10 feet per hour. Blasting Agents.—The agents most commonly used are :— Gunpowder ; Nitro-ylycerine and its compounds, such as dynamite ; and Gun cotton and its compounds, such as tonite.