212 MIKING INDUSTRY. 



lifted to make a partial revolution about its vertical axis, which rotary mo- 

 tion being continued during the free fall of the stamp, produces a grinding 

 effect between the shoe and die upon the substance to be crushed. Not 

 only is the eifective duty of the fitamp at each blow increased in this way, but 

 the shoe wears down much more evenly than when it falls without such 

 rotary motion. 



Guides.— The stamp is held vertically in its movement by guides, 

 between which the stem passes. These were formerly made of iron, but 

 such have been almost entirely replaced by wooden ones in Nevada and Cal- 

 ifornia. One set of guides is placed below the tappet, about a foot above the 

 top of the mortar; the other set is placed near the top of the stem, so that 

 six inches or a foot of the latter may project above the guides. They are 

 supported by the cross-timbers, or ties, G, G' , which form a part of the battery 

 frame, connecting the two uprights or posts. They are usually made of pine, 

 though hard wood is preferred, and are from 10 to 16 inches wide. One 

 part of the guide is made in a single piece for the whole battery, bolted to 

 the cross-timber; the other part may be in one piece like the first, or cut into 

 as many pieces as there are stamps in the battery, as in Fig. 5, Plate XX, 

 which are then secured to the corresponding part by bolts. In each part are 

 cut semi-circular recesses, which, when the two parts are put together so 

 that the recesses correspond, the holes or stemways for the reception of the 

 stamp-stems are formed. When the guides are so worn by friction as to 

 permit too much motion of the stems, they may be dressed down on their 

 adjacent faces, by which means the recesses are reduced to nearly the proper 

 dimensions. 



Cams. — The cam is a curved arm fixed to a shaft, which is so placed in 

 front of the battery that, by the revolution of the shaft, the cam is brought 

 into contact with the tappet of the stamp-stem, causing the latter to rise to a 

 height determined by the length of the cam, and to fall at the moment of i;s 

 release from such contact. 



In Nevada the cams are made of tough cast iron, and are usually '' double 

 armed," that is, having two arms attached to one central hub. Figs. 6 and 

 7, on Plate XX, show the form of cams most generally in use ; in Fig. 7, a 

 is the hub, ft, h are the arms, c is the face, and d, a strengthening rib. 



f 



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