Fig. 4.

Fig. 5.

Fig. 6.

The stock is octagonal in section; it is made in lengths varying from 20 inches to 42 inches. The shorter the stock the more effectively does it transmit the force of the blow, and therefore it is made as short as possible. For this reason, several lengths are employed in boring a shot-hole, the shortest being used at the commencement of the hole, a longer one to continue the depth, and a still longer one, sometimes, to complete it. To ensure the longer drills working freely in the hole, the width of the bit should be very slightly reduced in each length. It has already been remarked that the diameter of the stock is less than the width of the bit; this difference may be greater in coal drills than in rock or “stone” drills; a common difference in the latter is 3⁄8 of an inch for the longer. The following proportions may be taken as the average adopted:—

Width
of the
Bit.		Diameter
of the
Stock.
1 inch 5⁄8inch
11⁄8„ 3⁄4„
11⁄4„ 7⁄8„
11⁄2„1 „
13⁄4„11⁄8„
2 inches13⁄8„
21⁄4„11⁄2„
21⁄2„15⁄8„

The striking face of the drill should be flat. The diameter of the face is less than that of the stock in all but the smallest sizes, the difference being made by drawing in the striking end. The amount of reduction is greater for the largest diameters; that of the striking face being rarely more than one-eighth of an inch.

The making and re-sharpening of rock drills constitute an extremely important part of the labour of the mine smith. The frequent use of the drill, and its rapid wear, necessitate a daily amount of work of no trifling proportions, and the judgment and skill required in proper tempering render some degree of intelligence in the workman indispensable; indeed, so much depends upon the smith whose duty it is to repair the miners’ tools, that no pains should be spared to obtain a man capable of fulfilling that duty in the most efficient manner possible.