Mining and Scientific Press Volume 115 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 Excerpt: ...10-ft. post is used to support a bottom heading, and from five to eight holes are drilled per round. The holes vary in depth from 10 to 12 ft., from 75 to 150 tons of ore being broken per round. After the bottom heading has been advanced from 10 to 25 ft., holes are drilled in the overhanging ore, which is blasted down cheaply. A comparison between these two methods is shown in Fig. 1. Two types of air-drills are used, the 'piston' and the 'hammer.' In sheet-ground, where the dust is an important consideration, the hammer type of water-drill has been used with success, showing a drilling efficiency 25 to 40% more than the piston-machine. Drills are operated with air at a pressure of from 85 to 100 lb. High-carbon hollow drill-steel is used with hammerdrills. Holes are started with a 2f-in. bit and finished with lj-in. It is necessary to change the size of gauge in each two feet drilled, and it is sometimes necessary, in exceedingly hard ground, to use several steels to drill the two feet. Usually the drill-steel is re-sharpened in an underground blacksmith-shop. Mechanical sharpeners are preferred. Table 1 shows the cost of drilling in a mine where the faces are from 10 to 16 ft. high, for four months of 1917: Table 1 Tonnage broken 136,272 Cost per Amount ton of ore Machine-men $9,630.40 $0.0706 Machine-helpers 7,940.45 0.0583 Drill-repairs 3,505.14 0.0257 Drill-steel 2,627.29 0.0193 Sharpening drill-steel 3,194.86 0.0234 Oil for machines 207.78 0.0015 Compressed air 6,168.63 0.0453 Air-hose and fittings 703.43 0.0052 Total $33,977.98 $0.2493 Blasting. After drilling, the holes are chambered and prepared for powder. Both ammonia and gelatin dynamite (30 to 40%) are used, in sticks of from 1 to li in. diameter. No. 8 caps have been found to give the best re...