The purpose of this paper is to discuss the relative merits of the large 3 1/8-in. machine and the small mining drill 2¼-in. tappet machine in driving development-headings; and although the data here presented were obtained from cross-cut headings alone, experience has shown that the results are equally true in drifting, raising and winzing.
Recently we drove two parallel cross-cuts through the same formation, using a 3 1/8-in. machine at the breast of one crosscut, and a 2¼-in. machine of the same make at the breast of the other. The results of this work afforded an ideal comparison, since in both cases the headings were advanced through rock practically of the same hardness and breaking-properties, the amount of sludging was equal, and there was no difference in the condition of the steel or the machines, the air-pressure or the experience of the operating crews.
Some operators in the Cripple Creek district contend that there is ground which cannot be handled with the small machine, the holes being too small to contain enough powder to “pull ” the ground, etc. The results obtained in working the property of the Portland Gold Mining Co., however, show that the ground worked by them does not fall in this class. During a period of two years there have been driven, with the small machine, 4 miles and 308 ft. of development-headings, through a diversity of ground, including Pike’s Peak granite (a coarsely porphyritic type of granite), highly indurated andesitic or phonolytic breccia, true massive andesite, trachytic phonolyte, tuffs, and along dikes of decomposed basalt and hard phonolyte. In every instance a satisfactory record was made.
The headings here described were driven through highly indurated andesitic breccia, having a hardness of from 5.2 to 7.2 and a sp. gr. of from 2.2 to 2.8. The action of the breccia under the drill was not materially different from that of ordinary red granite. The breccia was not as free-drilling as granite, and sludge accumulated very rapidly after a shallow depth of hole had been gained, but it broke better than granite.
Aside from the usual work of setting-up, drilling, and loading, the machine-men or helpers mucked back, cleaning the floor 3 or 4 ft. back from the breast of muck in order to position the column properly. If the “ lifters ” acted properly at the previous firing, the muck was fairly-well thrown back from the breast; but if either missed fire or were exploded before the other holes, considerable muck was left at the breast which required much additional labor. The usual time needed to muck back was 1.25 hr., but this varied considerably. Flat steel 48 by 96 by 3/8-in. sheets were used, from which to shovel the material. These sheets were placed in position 3 or 4 ft. back from the breast by the trammer, just before going off shift. The ground broke fine enough to require little or no sledging. A cubic foot of breccia in place will average 154 lb. in weight as compared with 90 lb. on the muck-pile, giving an average of 42 per cent, of void space. All the waste was trammed to the shaft, 800 ft. distant, and hoisted to the surface. No timber was used in either heading.
The following interesting summary of the results obtained by using both large and small machines has been prepared from the data given in Tables I. and II. Labor is the largest individual item. The wages of machine-men were $4 per shift, and the addition of the items given under the several heads of Table I. shows the total cost of labor performed in each heading. The cost of operating the machines per shift was $3.70 for the large machine and $1.85 for the small machine. These figures, which vary from month to month, include the cost of everything connected with the operation of the machines: engineer’s wages, blacksmith-expense, new steel repairs to the machines, cost and repair of air-lines, etc. The cost of labor, per foot driven, by the large machine was $3.45, and by the small machine $2.56.
The cost of explosives, a detailed report of which is given in Table II., shows that the 40-per cent, dynamite costs $0,127 per lb.; the fuse, $0.0035 per ft.; and the caps, $0,007 each. These figures, which include freight, unloading, wages of the powder-man, and one-third of the wages of the store-keeper,
represent the entire cost of the material, as laid down at the station for the machine-men. All fuse burned was in 7-ft. lengths. The number of feet of fuse burned and the number of caps used, per foot, are practically the same; but the cost of dynamite is $0.78 less per foot in the heading driven by the small machine than in that of the large machine.
The best record for a shift’s run made by the large machine, was 4.08 ft., as compared with 2.96 ft. for the small machine. In drilling these rounds it was found that the large machine had made 3109.56 cu. in. of hole, and the small machine 971.10 cu. in. Comparing these figures with the cubic feet of ground pulled, 1 cu. in. of hole drilled by the large machine broke 0.053 cu. ft. of breast, while the small machine gave 0.097 cu. ft. This comparison shows that too much work was done by the big machine on the breast for the amount of ground broken.
Figs. 1 and 2 show the number of holes drilled, the degrees of pitch from the horizontal, the depth drilled by the starters, seconds and thirds, and the order of firing. The cost of coal before the boilers was $4.40 per ton. Ordinary cross or square hits were used, and all the steel was sharpened by machine. At each sharpening the steel lost from 1/8 to 3/8 in. in length. The general tramming-cost includes repairs to tram-cars, tram-tracks and the greasing of the cars.
The cost of pipe and track is figured at $0.41 per ft., the 2-in. pipe costing, with connections, $0.10 per ft., the track, together with the spikes, plates and ties, costing $0.31 per ft. Lumber cost $20 per thousand feet.
Hoisting cost $0.243 per ton, which includes all accounts that can be charged to the maintenance of the hoisting-engines;—such as wages of the engineers, wipers, top-men and cagers, repairs, cost of steam, cables and repairs to shaft, The hoist used is a 500-h.p. Webster, Camp & Lane, first-motion hoist, size 20 by 48 in., having a capacity for a maximum depth of 2,500 ft., using 5 by 3/8 in. rope to hoist an unbalanced load of 8,000 lb. at an average speed of 1,500 ft. per min.
To supplies is charged the cost of picks and shovels. To general expense is charged the wages of foremen and shift bosses, assaying and surveying, pumping, lighting, including candles, office expense and general repairs on the surface.
The air was furnished by a 50-drill, cross-compound, Ingersoll-Sargent compressor, having steam-cylinders 24 by 44 by 48 in., and air-cylinders 22¼ by 38¼ by 48 in. The air-pressure at the receiver was 100 lb., and at the drills 85 lb. per sq. in.
The bore of the large machine cross-cut is 5.5 by 7.5 ft., that of the small machine is 4.5 by 7 ft. It is held that the increase of 1 ft. in width and 0.5 ft. in height of the large machine cross-cut over that of the small machine cross-cut does not facilitate mining-operations in any way.
The merits of the work done by the two machines may be briefly stated thus:—The use of the small machine saves 25 per cent, of the cost of labor necessary to operate a large machine foot for foot. The cost of operating a small machine is 50 per cent, less than that of operating a large machine, shift for shift. The general tramming-cost of the large machine cross-cut is lessened 20 per cent, by using a small machine. The cost of explosives per foot driven by the large machine can be reduced 37.7 per cent, by the use of the small machine. The cost of hoisting and general expense of the large machine cross-cut is lessened nearly 20 per cent, by using the small machine.
Greater speed, regardless of cost, can be obtained with the large machine, the small machine being from 10 to 20 per cent, slower. The cost of the large machine cross-cut was reduced 27 per cent, by using the small machine.