In broach channeling, a line of holes is driven, leaving a dividing wall of about three quarters of an inch between the holes. When these holes are completed to the depth and extent required, the rotation pawls are released. and the drill is made to break down the dividing walls by means of a broach, and without rotating the piston.
The Diamond Chonneling 3fachike is represented in Fig. 9. This is the only machine used for stone channeling other than the percussive machines herein before described. In some cases no boiler is placed on the machine, thus enabling the drill spindle to be tipped backward. Diamond channeling machines have been largely used in the Vermont Marble Quarries, where progress has been made to a depth of 400 ft., fol lowing a vein under the hill. Their extreme adaptability to any angle, and the fact that the carriage does not move on the track while the machine is working, gives it a special value in angular quarries and places where the floor is not level or regular. The track upon which the machine is mounted is made of a special rail, on one side of which is a rade; the ear moves in this rack through a pinion. and by means of a hand crank the machine is moved a definite distance after each hole. Holes are drilled on the line of the proposed channel in the same manner as diamond drills are operated. .11 stationary
engine revolves a spindle on the end of which is a diamond bit. This bit differs from that used for pros pecting in that it is solid instead of cored out, so that it bores the hole and discharges the cuttings to the full diameter of the hole. The bits are usually about in diameter, and the holes are drilled close together, leaving, a slight space between, which is afterwards bored out by the same bit through a guide piece which follows in an adjacent hole.
Fig. 10 illustrates a special tripod carrying a drill for putting in lewis-holes. This tripod is of the regular pattern, except the center bar, which car ries the drill, is extended in length, and is perforated with a slot 7 in. long, which allows the drill clamp to move 0 in., to cover the centers of three parallel holes, 3 in. each. center to center, When the three holes are finished, a broach is inserted in place of the drill without moving the tripod, and the lewis-hole finished by broaching down the partitions. This obviates the clinically of breaking down the partitions in the old plan of diverging holes, as shown in the right side of Fig. 11.