The tubular drill-rod passes through the serew-shaft and is held firmly by a chuck, the motion of the screw-shaft being thus com municated to the drill-rods and bit.
In order to run the serew-shaft back after it has been fed forward its full length, it is only neeeAsary to release the chuck and to loosen the nut on the frictional gear, thus allowing the gear to run loose; then the screw-shaft will run up with the same motion which carried it down, but with a velocity sixty times greater—that is, the speed with which the screw-shaft feeds up is to the speed with which it fed the drill down as sixty to one—the revolving veloci ty in both eases being the same. The chock and nut are then tightened, giving the screw shaft a grip un the drill-rod in a new place, and the drill is ready for another run. The drill rods may be extended to any desired length by simply adding fresh pieces of tubing, the suc cessive lengths being quickly coupled together by an inside shoulder-nipple coupling, and having a hole bored through the center to admit of the passage of the water. In order to withdraw the drill-rods, they are uncoupled below the chuck ; the swivel-head, which is hinged, is unbolted and swung beck—thereby moving the screw-shaft to one side, and afford ing a clearance for the rods to be raised by the hoisting-gear on the machine, without moving the latter from its place.
2. The hydraulic feed is illustrated in the form of diamond drill shown in Fig. 21. This is an improved method which is substituted for the gear or differential feed, described above. The feed-motion here is accomplished. as its name indicates. by hydraulic pressure. through the medium of two small cylinders and pistons, the piston-rods being connected by a suitable cross-head to the plain hollow spindle, which takes the place of the serew-shaft of the differ ential feed, and carries the drill-rod. Both ends of the hydraulic cylinders are connected by
a system of pipes and hose to the plumps that supply the water necessary in drilling with the diamond bit. The quantity of water admitted to the cylinders is controlled by a four-way cock. which also admits water to either end of the cylinders, as the operator may require. Thus, it will be readily understood, the amount of pressure on the bit is directly under the control of the operator, and only limited by the water-pressure from the supply-pumps; the range being, in ordinary cases, from nothing op to 4,000 lbs. The changes through the whole range of pressure, and also the reversing the motion of the feed, are accomplished by simply moving a small lever while the machine is running at full speed. A pressure-gauge is placed on the pipe leading to the hydraulic cylinders, so that the operator can at all times see just how much pressure there is on the bit. With any constant pressure this feed gives an auto matic adjustment of the speed with which the drill is fed forward, the rate of progression depending upon the hardness of the material, being from frequently less than 1 in. per minute in very hard rock to over 2 ft.. per minute in a soft substance like coal. The operator, after some experience, can, by comparing the pressure shown by the gauge with the rate of pene tration of the drill, tell about what kind of material the bit is boring through, and can make use of the knowledge thus obtained either for speed or for safety. The method of coupling the drill-rods and of withdrawing them is similar to that already described.