Drill And Blast

DRILL AND BLAST
Underground excavation dates right back to the Cave Men where they excavated for there homes and developed underground flint mines. This has advanced through history with man’s need for protection and hiding, aswell as mining metal and precious stones. From picks made of horn and flint techniques moved onto metal picks. Then came ‘fire setting’, where fires heated up the rockwhich was then dowsed in cold water, the artificial expansion and contraction causing the rock to shatter. Then with invention of gunpowder, the necessity of boring holes arrived and the first inception of “drill&blast” was born. 1 HISTORY OF THE ROCK DRILL ‘Beating’ the bore or drilling by hand uses the same method of penetration as all rock drills. When a chisel bit hits a rock surface the inducedstress causes a shatter zone around the bit, if the bit is then rotated through 360 degrees and hit after each few degrees of rotation a circle of crushed rock will result. When this is continued, a round hole is formed.

shaft or stock was octagonal in section, and since it was found that a shorter stock transmitted a better blow several lengths were used in drilling one hole, each new lengthhaving a slightly reduced bit size, (as in all rock drills). The striking face was flat and had a similar cross section to the stock.

Figure 2 Beating the bore in a Cornish tin mine

Figure 1 Rock failure mechanism for a percussive drill bit

The Drill or ‘Jumper’ as it was called, consisted of a bit or chisel edge which was usually flat since it cut more freely than the curved edge and wastherefore used on deep holes. The curved edge was much stronger at the corners and hence more suitable for hard rock. The
David Lees - History of Drill and Blast -1 -

Figure 3 Examples of the drills and sledges ( after G.G. Andre 1887 )

The drill or ‘jumper’ was hit by a sledge or hammer. The sledges were carefully designed as well, those used for drilling blast holes had a flat face todeliver a direct blow with a chamferring of the head to ensure that the sledge would fly off when a false blow was struck, and hence miss a man’s hands holding the drill. A hammer was smaller than a sledge, about 3lbs with a 10 inch handle to be used with one hand. The sledge was between 5lbs. and l0lbs with a handle designed for two hands between 20 incites and 30 inches long.

Figure 4 Holman’sRock Drill

In 1870 Ingersoll and Rand rock drills were produced using compressed air. These drills were of American origin and were a great achievement since they were designed with very few moving parts. The design consisted of two tappet levers, therefore the vibration of the shock is much less and wear on the parts is less also. Then to ensure durability, the valve gear consists in makingthe spindles separate from the valves and tappet levers and in the case of the piston striking the cover an elastic cushion is provided. Mokean designed a rock drill for the St. Gothard Tunnel with a very ingenious mechanism for automatic feed, but this feature was not possible to include in the lighter machines for ordinary mining and quarrying. The Sach’s rock drill, designed by the German CarlSachs had less durable moving parts but, with its screw feed as compared to the conventional winch handle or a wheel as used firstly on the Dubois/Francois drill, it was much favoured on the Continent.

The Schram rock drill, a Swedish and German design, was the start in an era of simplification. It had just four moving parts; a working piston which drove the borer; a slide valve; a slide rod anda small piston which drove the working piston and all these were worked directly by the motor fluid. Mr. Schram was a mining engineer and like his compatriots realised an automatic feed was impractical and therefore did not include it in his design. The advantages of this design were quite obvious, the piston is perfectly free, the full fluid pressure is kept during the whole stroke, the...