. The elements of railroad engineering . he drill withdrawn by reversing the movementof the feed-screw. A longer drill is then substituted andthe drilling continued. The cutting edges of the bits arenecessarily worn by the drilling and constant rotation in thehole so that the diameter of the bottom of each section ofhole is slightly less than that at the top; accordingly, at eachchange of drill, one is selected with a bit from \ to -^-^inch narrower than the one removed. In tunnel driving, the drills used in the heading areusually mounted on columns, similar to that shown in The colum
. The elements of railroad engineering . he drill withdrawn by reversing the movementof the feed-screw. A longer drill is then substituted andthe drilling continued. The cutting edges of the bits arenecessarily worn by the drilling and constant rotation in thehole so that the diameter of the bottom of each section ofhole is slightly less than that at the top; accordingly, at eachchange of drill, one is selected with a bit from \ to -^-^inch narrower than the one removed. In tunnel driving, the drills used in the heading areusually mounted on columns, similar to that shown in The column yJ is set in an upright position near theface of the heading, the top B of the column being forcedagainst the roof of the tunnel by the capstan screws Cwhich rest in special castings D on the floor of the is a common practice to place strong blocks of wood onthe head of the column and under the feet of the capstanscrews, which prevent the rock supports from becomingloosened by the continued jarring of the column, due to the. Fig. 417. 932 RAILROAD CONSTRUCTION. working of the drills. The arm E at right angles to thecolumn slides up or down the column by means of the collar F, and may be clampedin any position by the clamp drill is carried on this armand revolves about it as an axis,thus giving a wide range ofaction. Usually two drills aremounted on each column. Insinking shafts and driving tun-nels, as well as in mine work,compressed air is used instead ofsteam, which loses much of itspressure through use of compressed air greatlypromotes ventilation. Percus-sion drills are under a pressureof from 60 to 70 lb. per squareinch. In one hour one will drilla hole froni 2 to 2^ inches indiameter and from 4 to 10 feetin depth, depending upon thecharacter of the rock, the positionof the strata, and the size of themachine. The cost of drillingwill vary from 8 to 20 cents perFig. 418 lineal foot.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
