. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . lare at the bottom (a) about 15 to 30%. Forhard rock the curve of the edge should be somewhat flatter andfor soft rock somewhat more curved than shown, Fig. 66, (a).Sometimes the angle of the two faces is varied from that given,Fig. 66, (6) and occasionally the edge is purposely blunted soas to give a crushing rather than a cutting effect. The drillswill require sharpening for each 6 to 18 inches depth of hole, and will require ^ iiew edge tQ be worked every 2 to 4 days. 186 RA
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . lare at the bottom (a) about 15 to 30%. Forhard rock the curve of the edge should be somewhat flatter andfor soft rock somewhat more curved than shown, Fig. 66, (a).Sometimes the angle of the two faces is varied from that given,Fig. 66, (6) and occasionally the edge is purposely blunted soas to give a crushing rather than a cutting effect. The drillswill require sharpening for each 6 to 18 inches depth of hole, and will require ^ iiew edge tQ be worked every 2 to 4 days. 186 RAILROAD CONSTRUCTION. 151. For drilling vertical holes the churn-drill is the most econom-ical. The drill-bar is of iron, about 6 to 8 feet long, IJ indiameter, weighs about 25 to 30 lbs., and is shod with a pieceof steel welded on. The bar is lifted a few inches between eachblow, turned partially around, and allowed to fall, the impactdoing the work. From 5 to 15 feet of holes, depending on thecharacter of the rock, is a fair days work—10 hours. In verysoft rocks even more than this may be done. This method is.
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