To True A Grindstone. When a stone becomes untrue, or the outline of the face, which should be slightly convex, becomes concave, it may be corrected by using a piece of soft iron as a turning tool, the stone being run dry. The action of the tool may be explained as follows: the soft iron allows small particles of the stone to imbed themselves in its surface, from which position they act against the revolving stone, and the cutting is done by these imbedded particles and not by the iron. The latter is worn in the process, however, and, as its cutting surface becomes enlarged, it should be turned to bring a new angle or face into action. This operation is easily performed by using a piece of gas pipe (about 1") for a turning tool.

No. Truing Devices are now generally attached to power grindstones. They are of several forms, of which that shown by Fig. 133 may be taken as an example. The base of this attachment is secured to the grindstone frame as near the stone as may be convenient. A is a hardened steel screw which revolves freely on its bearings B. The frame in which B runs is pivoted at C, in such a way that by a movement of the hand-wheel D, B will move forward in the direction of the arrow. By adjusting the hand-wheel D, A is brought into contact with the face of the moving stone, and at once begins to revolve. The action of its thread would move it endwise, were it not prevented by its bearings. The effect of this angular advancement of the thread, which is not met by a corresponding lateral movement of the parts in contact, is a shearing cut across the face of the stone. When the screw becomes dull it may be softened and recut.