Rock Blasting / A Practical Treatise on the Means Employed in Blasting Rocks for Industrial Purposes

by George G. André

The tempering of steel, which is a phenomenon of a similar character to that of hardening, also claims careful consideration. When a bright surface of steel is subjected to heat, a series of colours is produced, which follow each other in a regular order as the temperature increases. This order is as follows: pale yellow, straw yellow, golden yellow, brown, brown and purple mingled, purple, light blue, full clear blue, and dark blue. Experience has shown that some one of these colours is more suitable than the rest for certain kinds of tools and certain conditions of working.

The selection of the proper colour constitutes a subject for the exercise of judgment and skill on the part of the smith. For rock drills, straw colour is generally the most suitable when the work is in very hard rock, and light blue when the rock is only of moderate hardness.

The processes of hardening and tempering drills are as follows: When the edge of the bit has been formed in the manner already described, from 3 to 4 inches of the end is heated to cherry redness, and dipped in cold water to a depth of about an inch to harden it. While in the water, the bit should be moved slightly up and down, for, were this neglected, the hardness would terminate abruptly, and the bit would be very liable to fracture along the line corresponding with the surface of the water. In cold weather, the water should be slightly warmed, by immersing a piece of hot iron in it, before dipping the steel. When a sufficient degree of hardness has been attained, the remainder of the hot portion is immersed until the heat is reduced sufficiently for tempering. At this stage it is withdrawn, and the colours carefully watched for. The heat which is left in the stock will pass down to the edge of the bit, and as the temperature increases in that part the colours will appear in regular succession upon the filed surface of the edge. When the proper hue appears, the whole drill is plunged into the water and left there till cold, when the tempering is complete. When the edge is curved or “bowed,” the colours will reach the corners sooner than the middle of the bit. This tendency must be checked by dipping the corners in the water, for otherwise the edge will not be of equal hardness throughout. As the colour can be best observed in the dark, it is a good plan to darken that portion of the smithy in which tempering is being carried on.

The degree of temper required depends upon the quality of the steel and the nature of the work to be performed. The larger the proportion of carbon present in the metal, the lower must be the temper. Also the state of the blunted edges, whether battered or fractured, will show what degree of hardness it is desirable to produce. From inattention to these matters, good steel is not unfrequently condemned as unsuitable.

To form the striking face, the end of the stock is heated to a dull red, and drawn out by a hammer to form a conical head. The extremity is then flattened to form a face from ¹⁄₂ inch to 1 inch in diameter. This head is then annealed to a degree that will combine considerable toughness with hardness. The constant blows to which the head is subjected tend to wear it down very rapidly. There is great difference in the lasting qualities of steel in this respect; some drills will wear away more quickly at the striking than at the bit end.

A smith will, with the assistance of a striker, sharpen and temper about thirty single-hand drills of medium size in an hour, or twenty double-hand drills of medium size in the same time. Of course, much will depend on the degree of bluntness in the cutting edge; but assuming the drills to be sent up only moderately blunted, this may be taken as a fair average of the work of two men.

It will be evident from the foregoing remarks, that to enable a drill to stand properly it must be made of good material, be skilfully tempered in the smithy, and provided with a cutting edge having an angle and a shape suited to the character of the rock in which it is used. To these conditions, may be added another, namely, proper handling; for if the drill be carelessly turned in the hole so as to bring all the work upon a portion only of the cutting edge, or unskilfully struck by the sledge, fracture or blunting will speedily result. Improper handling often destroys the edge in the first five minutes of using.

Drills, as before remarked, are used in sets of different lengths. The sets may be intended for use by one man or by two. In the former case, the sets are described as “single-hand” sets, and they contain a hammer for striking the drills; in the latter case, the sets are spoken of as “double-handed,” and they contain a sledge instead of a hammer for striking. It may appear at first sight that there is a waste of power in employing two men, or, as it is termed, the double set, for that two men cannot bore twice as fast as one. This rate of speed can, however, be obtained, and is due less to the greater effectiveness of the stroke than to the fact that two men can, by repeatedly changing places with each other, keep up almost without intermission a succession of blows for an indefinite length of time; whereas, with the single set, the man is continually obliged to cease for rest.

Hammers.

—To deliver the blow upon a rock drill, hammers and sledges are used. The distinction between a hammer and a sledge is founded on dimensions only: the hammer being intended for use in one hand, is made comparatively light and is furnished with a short handle, while the sledge, being intended for use in both hands, is furnished with a much longer handle and is made heavier. The striking face of the blasting sledge should be flat, to enable the striker to deliver a direct blow with certainty upon the head of the drill; and to facilitate the directing of the blow, as well as to increase its effect, the mass of metal composing the head should be concentrated within a short length. To cause the sledge to fly off from the head of the drill in the case of a false blow being struck, and thereby to prevent it from striking the hand of the man who holds the drill, the edges of the striking face should be chamfered or bevelled down till the diameter is reduced by nearly one-half. This requirement is, however, but seldom provided for.