Fig. 32. Grinder for Truing Lathe Centers
Some lathes are equipped with hardened centers in both the head-and tailstock and others have only one hardened center which is in the tailstock. The object in having a soft or unhardened headstock center is to permit its being trued by turning, but as a soft center is quite easily bruised and requires truing oftener than one that is hard, it is better to have both centers hardened. Special grinders are used for truing these hardened centers. One type that is very simple and easily applied to a lathe is shown in [Fig. 32]. This grinder is held in the lathe toolpost and is driven by a wheel A that is held in contact with the cone-pulley. The emery wheel B is moved to a position for grinding by adjusting the carriage and cross-slide, and it is traversed across the conical surface of the center by handle C. As the grinding proceeds, the wheel is fed inward slightly by manipulating the cross-slide.
This grinder is set to the proper angle by placing the two centered ends D and D1 between the lathe centers, which should be aligned as for straight turning. The grinding spindle will then be 30 degrees from the axis of the lathe spindle. The grinder should be carefully clamped in the toolpost so that it will remain as located by the centered ends. After the tailstock center is withdrawn, the emery wheel is adjusted for grinding. As the wheel spindle is 30 degrees from the axis of the lathe spindle, the lathe center is not only ground true but to an included angle of 60 degrees, which is the standard angle for lathe centers. There are many other styles of center grinders on the market, some of which are driven by a small belt from the cone-pulley and others by electric motors which are connected with ordinary lighting circuits. The tailstock center is ground by inserting it in the spindle in place of the headstock center. Before a center is replaced in its spindle, the hole should be perfectly clean as even a small particle of dirt may affect the alignment. The center in the headstock is usually referred to as the “live center” because it turns around when the lathe is in use, and the center in the tailstock as the “dead center,” because it remains stationary.
Fig. 33. (A) Lathe Chuck.
(B) Faceplate Jaw
Universal, Independent and Combination Chucks.—Many parts that are turned in the lathe are so shaped that they cannot be held between the lathe centers like shafts and other similar pieces and it is often necessary to hold them in a chuck A, [Fig. 33], which is screwed onto the lathe spindle instead of the faceplate. The work is gripped by the jaws J which can be moved in or out to accommodate various diameters. There are three classes of chucks ordinarily used on the lathe, known as the independent, universal and combination types. The independent chuck is so named because each jaw can be adjusted in or out independently of the others by turning the jaw screws S with a wrench. The jaws of the universal chuck all move together and keep the same distance from the center, and they can be adjusted by turning any one of the screws S, whereas with the independent type the chuck wrench must be applied to each jaw screw. The combination chuck, as the name implies, may be changed to operate either as an independent or universal type. The advantage of the universal chuck is that round and other parts of a uniform shape are located in a central position for turning without any adjustment. The independent type is, however, preferable in some respects as it is usually stronger and adapted for holding odd-shaped pieces because each jaw can be set to any required position.
