Fig. 25 shows a back-knee frame with the countershaft on the battery sills. The battery posts are usually of 12" x 24" timbers with their lower ends framed into the 12" x 12" sills. They are FIG. 24 bound together by cross timbers, of which there are four sets, as shown, the upper two being used as supports for the guide timbers, while the two lower ones act as binders for the battery blocks. The frames are strongly braced for both tension and compression. 26. The Guides.-The stamps are directed in their fall by two sets of guides bolted to the cross timbers of the battery frame. The lower set is generally about 18 inches above the top of the mortar, and the upper These guides are usually made entirely of wood, Fig. 26, though combination guides of set about 7 feet higher. wood and iron shown in Figs. 27 and 28 present some advantages in the way of quick repairs and economy of timber, ast scrap timber can be used for guide blocks, and the iron frame is practically imperishable. They also may have the friction along instead of across the grain, which increases the life of the block. Iron guides have also been tried, but are not as satisfactory as wooden and combination guides. Wooden guides for an ordinary five-stamp battery are usually made of 4" x 12" hardwood plank, and consist of two timbers bolted face to face, FIG. 25 with grooves on the inner faces for the passage of the stamp stems. The grooves are not cut hemispherical, but are left slightly shallow, with thin strips of wood placed between the two timbers, so that as the grooves wear deeper lost FIG. 26 FIG. 27 motion may be taken up by removal of the strips and by then drawing the timbers towards each other. The wear may be still further taken up by planing off the inner faces of the blocks. The guide blocks fit in between the battery posts and are held firmly against the back of the cross timber by bolts passing through all three timbers. In case repairs are needed, the back block can be removed by simply taking off the nuts and drawing it off. Guides should be kept well lubricated with a paste of graphite and linseed oil, care being taken that none of the lubricant gets into the mortar, as it would interfere seriously with the recovery of the gold. 27. Sectional guides of both wood and iron are also made, as shown in Fig. 28. This construction permits of their adjustment, so that lost motion caused by the stamp stem may be taken up as desired. In Fig. 28 (a) the guide blocks are held in place by two clamping plates c and d, which in turn are held against the girth a by the bolts e and nuts e. The back plates c are dipped rearwardly to fit snugly into the girth beam, so that there will be no lost motion. When it is desired to change a stamp guide or take up lost motion, the plate c is loosened and the block b is either taken out or moved forwards with shimmers. If the guide is badly worn, it is discarded and a new one put in its place; if only slightly worn, it is shimmered to fit snugly. 28. Single-Discharge Mortar.-In Fig. 29 there are two styles of mortars shown in cross-section. In shape they resemble cast-iron troughs. The ore is fed into a slot f at the back, and the crushed ore finds its exit through the screen in the front of the mortar, on to an apron plate or into collecting troughs called launders. The left-hand d FIG. 29 mortar is arranged for inside amalgamation, both at the front and back, while the right-hand mortar in the figure is arranged for amalgamation on the screen side only. The chief difference between them is in the feeding arrangement. The back amalgamating plate, being put in a recess, is protected from the falling rock as it is fed into the mortar. There are many styles of mortars, some high and some low discharge, some with straight-screen fronts and others leaning outwards, as shown. Each style has an advocate, N. M. III.-7 because that style may be particularly adapted to the work in hand; but the best mortars are built to meet the circumstances of high and low speed, inside and outside amalgamation, high and low drop, also convenience, and all these particulars cannot be embodied in one mortar. 29. Double-Discharge Mortars.—In Fig. 30 (a) and (b) two double-discharge mortars are shown. Such mortars are used where dry crushing is practiced—that is, no water enters the mortar to assist in washing the ore through the screen. For wet crushing they might increase the product where the ore was coarse, but it would be done at the expense of screens. 30. Feeding Ore to the Battery.-To obtain the maximum crushing duty of a battery, feeding should be carefully regulated and kept just short of the point where the striking of metal on metal is noticeable. If it is allowed to get below this, breakage is apt to follow the uncushioned fall of the stamp upon the die, and even if no break occurs, energy will be wasted to no useful purpose. On the other hand, if too much ore is kept on the dies, the cushioning is so great as to lessen the crushing efficiency of the stamp, |