. Machinery for metalliferous mines : a practical treatise for mining engineers, metallurgists and managers of mines. ingle Screw Column for Small Drifts and Shafts. For this reason the supports upon which the rock drills are fixedmust, as a general rule in mining, be strong, but light and portable, inorder that they may be readily removed previous to each blast. The support in ordinary use is a light steel column, as shown infig. 125, fitted with a lengthening screw, by which it is tightened upeither horizontally or vertically across the tunnel, a block of wood beingalways placed between the
. Machinery for metalliferous mines : a practical treatise for mining engineers, metallurgists and managers of mines. ingle Screw Column for Small Drifts and Shafts. For this reason the supports upon which the rock drills are fixedmust, as a general rule in mining, be strong, but light and portable, inorder that they may be readily removed previous to each blast. The support in ordinary use is a light steel column, as shown infig. 125, fitted with a lengthening screw, by which it is tightened upeither horizontally or vertically across the tunnel, a block of wood beingalways placed between the ends and the rough, uneven surface of the rock. In tunnels of large sectional area the double-screw column (fig. 126)is used with the drill mounted on the arm. In small tunnels and shaftsthe single-screw column, however, is preferred, the drill being mounteddirectly on the column by means of the clamp (fig. 127), or the patent i84 MACHINERY FOR INIETALLIFEROUS MINES. Ingersoll-Scrgeant clamp (fig. 128), in which the swinging jaw is l)roughtto bear upon the cone on the back of the drill by means of a single Fig. 126.—Double Screw Column with Arm. The price of a 4|-in. steel column, from 6 ft. to 8 ft. long, with armand clamp complete for mounting, 2| in. to 3^ in. diameter drills, is
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