4f>8 



BORING 



momentum of the tool at 30 Ibs. pressure per square Inch 150 Ibs. In this machine 

 the movements are automatic, and completely free from striking gear. For drawings, 

 see Specification No. 1,734, A. D. 1873. 



In machine- or hand-boring it is of great importance to employ suitable steel, 

 properly sharpened. 



For tough slate of variable hardness the cast steel should be of the very best quality. 

 For hard brittle siliceous rock, steel of slightly inferior brand will afford good 

 working results. The pointing of steel is a matter of great moment, overheating, 

 or a white heat, must be avoided. The borer point should be formed with a 

 hammer, well beaten from red to a black heat ; a file, or grinding instrument, ought 

 never to be used. The temper should be established at a dull red heat, the point, if 

 dipped in oil or grease, is said to be toughened, as well as hardened. The hardest 

 temper is indicated by a light straw colour, the softest by blue. The best working 



198 



temper is considered by some smiths to be just past straw, scarcely red purple ; the 

 tool at this colour to be instantly cooled in water. 



The proper cutting angle of borers is a subject on which there is a great diversity 

 of opinion, which probably arises from the circumstance that rocks differ oven in one 

 and the same mine, and will differ more widely in mines far distant from each other, 

 and also from the fact that smiths do not readily change their practice. But it is 

 self-evident that for soft argillaceous stone the point may be comparatively thin and 

 flat, and that for flinty tough rock it should be well supported by /lowness of angle. 

 In machine-drilling, borer points do much more service than in hand-boring. In the 

 former this is due to the uniform conditions of the blow. In the latter the blow is 

 frequently an upset one, its effect being expended mostly on one side of the point. 

 In hand-boring the chisel is almost invariably employed ; but with boring machines the 

 crown," and Z or 'set' point is used ; the crown for starting a hole, and both the 

 crown and Z for boring in cavernous ground. The Cornish miner is in favour of 

 striking the borer both lightly and quickly. A single handed mallet or hammer 

 weighs 4-5 Ibs., a double handed one from 6-8 Ibs. Each man gives about 20 blows 

 per minute; the size of steel employed is from | to l|th inch diameter, and 

 when sharpened will present points fully one quarter of an inch wider : that is, 



