Fig. 2060.
In all grinding operations it is necessary that the stone should run true. This is sometimes accomplished by so mounting the stones in their frames that their perimeters touch at the back of each stone, one stone running slightly faster than the other. Or sometimes the work is placed between the two stones, as in [Fig. 2059], which represents a plan frequently used to grind circular saws; c in the figure represents the grinding-stone and a the saw. Long saws are mounted vertically as in [Fig. 2060], a representing a frame to which the upper end of the saw is attached and driven by a disk crank and connecting rod as shown, the two stones c e may, in this case, be of equal diameter.
Fig. 2061.
[Fig. 2061] represents a grindstone truing device (for tool-grinding stones) in which a series of serrated disks are employed in place on a threaded roll. The disks are fed to the stone by the hand wheel and screw, and are traversed back and forth across the stone face by means of the lever handle shown.
The fast running grindstones used for heavy and coarse grinding are trued by a process known as hacking. The high spots of the stone are marked by holding a piece of coal to the stone while it revolves slowly, and a tool similar to an adze is used to cut or chop indentations in the stone. The highest spots will be most plainly marked by the coal, and the hacking is spaced closer together in these places, the hacking marks crossing each other and varying in depth to suit, obviously being deepest where the marks are blackest. The hacking also sharpens the stone. To prevent the stone from wearing uneven across its face the file grinder mounts the stone in a very ingenious manner, causing it to traverse automatically, back and forth, while rotating.
