24 TUNNELING 



fully used for this purpose in Europe. The reason they have 

 not gained more favor among American tunnel builders is due to 

 some extent perhaps to prejudice, but chiefly to the great cost 

 of the machine as compared with percussion drills, and to the 

 expense of diamonds for repairs. Those who advocate these 

 machines for tunnel work point out, however, that under ordi- 

 nary usage the diamonds have a very long life, borings of 

 700 lin. ft. being recorded without repairs to the diamonds. 



The form of rotary drill used chiefly for prospecting pur- 

 poses is the diamond drill. This machine consists of a hollow 

 cylindrical bit having a cutting-edge of diamonds, which is 

 revolved at the rate of from two hundred to four hundred 

 revolutions per minute by suitable machineiy operated by steam 

 or compressed air. The diamonds are set in the cutting-edge of 

 the bit so as to project outward from its annular face and also 

 slightly inside and outside of its cylindrical sides (Fig. 14). 

 When the drill rod with the bit at- 

 tached is rotated and fed forward the 

 bit cuts an annular hole into the rock ; 

 the drillings being removed from the 

 hole by a constant stream of w r ater 

 which is forced down through the hol- 



FlG.14. Sketch of Diamond i j -it j j j-i 



Drill Bit. * ow " ru l r d an( l emerges, carrying the 



debris with it, up through the narrow 



space between the outside of the bit and the walls of the hole. 

 There are various makes of diamond drills, but they all operate 

 in essentially the same manner. 



The rotary drill principally employed in Europe in tunneling 

 is the Brandt. The cutting-edge of the Brandt drill consists of 

 hardened steel teeth. The bit is pressed against the rock by 

 hydraulic pressure, and usually makes from seven to eight revo- 

 lutions per minute. Some of the water when freed goes 

 through the hollow bit, keeping it cool, and cleaning the hole of 

 debris. A water pressure of from 300 to 450 Ibs. per square 

 inch is required to operate these drills. Rotary rock-drills 



