'446 



BORING 



184 



of the stroke, the exhaust valve is opened for the steam to escape, allowing the 

 piston-rod and carrying pulley to fall freely with the boring tool, which falls with its 

 full weight to tho bottom of the borehole. The exhaust port is 6 inches above the 

 bottom of the cylinder, while the steam port is situated at the bottom ; and there is 

 thus always an elastic cushion of steam retained in the cylinder of that thickness 

 for the piston to fall upon, preventing tho piston from striking the bottom of the 

 cylinder. Tho steam and exhaust valves are worked with a self-acting motion by 

 tappets, which are actuated by tho movement of the piston-rod ; and a rapid 

 succession of blows is thus given by the boring tool on the bottom of the borehole. 

 As it is necessary that motion should bo given to the piston before the valves can 

 be acted upon, a small jet of steam is allowed to be constantly blowing into the 

 bottom of the cylinder ; this causes the piston to move slowly at first, so as to take up 

 the slack of the rope and allow it to receive the weight of the 

 boring head gradually and without a jerk. An arm attached to tho 

 piston-rod then comes in contact with a tappet which opens tho 

 steam-valve, and the piston rises quickly to the top of the stroke ; 

 another tappet worked by the same arm then shuts off the steam, 

 and the exhaust valve is opened by a corresponding arrangement 

 on the opposite side of the piston-rod. By shifting these tappets 

 the length of stroke of the piston can be varied from 1 to 8 feet 

 in the large machine, according to the material to be bored through ; 

 and the height of fall of the boring head at the bottom of tho 

 borehole is double the length of stroke of the piston. The fall of 

 the boring head and piston can also be regulated by a weighted 

 valve on the exhaust pipe, checking the escape of the steam, so as 

 to cause the descent to take place slowly or quickly as may be 

 desired. 



The boring head ~Bfig. 184 consists of a wrought-iron bar about 

 4 inches diameter and 8 feet long, to the bottom of which a cast- 

 iron cylindrical block c is secured. This block has numerous 

 square holes through it, into which the chisels or cutters are 

 inserted with taper shanks so as to be very firm when working, 

 but to be readily taken out for repairing and sharpening. A little 

 above the block o another cylindrical casting E is fixed upon tho 

 bar B, which acts simply as a guide to keep the bar perpendicular. 

 Higher still is fixed a second guide F. To effect tho rotation of 

 the boring tool, two cast-iron collars G and H are cottered fast to 

 the top of the bar B, and placed about 12 inches apart. The upper 

 face of the lower collar G is formed with deep ratchet-teeth of 

 about two inches pitch, and the under face of the top collar H is 

 formed with similar ratchet-teeth set exactly in line with those on 

 the lower collar. Between these collars and sliding freely on the 

 neck of the boring bar B is a deep bush -T, which is also farmed 

 of corresponding ratchet-teeth on both its upper and lower faces ; 

 but the teeth on the upper face are set half a tooth in advance of 

 those on the lower face, so that the perpendicular side of each 

 tooth on the upper face of the bush is directly above the centre of 

 the inclined side of a tooth on the lower face. To this bush is 

 attached the wrought-iron bow x by which the whole boring bar 

 is suspended with a hook and shackle from the end of the flat rope. 

 The rotary motion of the bar is obtained as follows: when the 

 boring tool falls and strikes tho blow, tho lifting bush J, which 

 during the lifting has been engaged with the ratchet-teeth of the 

 top collar H, falls upon those of the bottom collar o, and thereby 

 receives a twist backwards through the space of half a tooth ; and 

 on commencing to lift again, the bush rising up against the 

 ratchet-teeth of tho top collar H receives a further twist backwards 

 through half a tooth. The flat rope is thus twisted backwards 

 to the extent of one tooth of the ratchet ; and during the lifting of 

 the tool it untwists itself again, thereby rotating tho boring tool forwards through 

 that extent of twist between each successive blow of the tool. The amount of tho 

 rotation may be varied by making the ratchet-teeth of coarser or finer pitch. Tho 

 motion is entirely self-acting, and the rotary movement of the boring tool is ensured 

 with mechanical accuracy. This simple and most effective action taking place at 

 every blow of the tool produces a constant change in the position of the cutters, 

 thus increasing their effect in breaking the rock. 



The Shell-Runp, for raising the material broken up by the boring head, is shown 



