Supporting Outer End of Chucked Work.—[Fig. 45] shows how the tailstock center is sometimes used for supporting the outer end of a long casting, the opposite end of which is held in a chuck. This particular casting is to be turned and bored to make a lining for the cylinder of a locomotive in order to reduce the diameter of the cylinder which has been considerably enlarged by re-boring a number of times. These bushings are rough-turned on the outside while the outer end is supported by the cross-shaped piece or “spider” which forms a center-bearing for the tailstock. This spider has set screws in the flanged ends of the arms, which are tightened against the inner surface of the casting and are adjusted one way or the other in order to locate it in a concentric position. After roughing the outside, the inside is bored to the finish size; then centered disks, which fit into the bore, are placed in the ends of the bushing and the latter is finish-turned. The object in rough turning the outside prior to boring is to avoid the distortion which might occur if this hard outer surface were removed last.
Boring Large Castings in the Lathe.—An ordinary engine lathe is sometimes used for boring engine or pump cylinders, linings, etc., which are too large to be held in the chuck or on a faceplate, and must be attached to the lathe carriage. As a rule, work of this class is done in a special boring machine (see [“Horizontal Boring Machines”]), but if such a machine is not available, it may be necessary to use a lathe. There are two general methods of boring.
Fig. 46. Boring a Cylinder Lining in an Ordinary Engine Lathe
[Fig. 46] shows how the lining illustrated in [Fig. 45] is bored in a large engine lathe. The casting is held in special fixtures which are attached to the lathe carriage, and the boring-bar is rotated by the lathe spindle. The tool-head of this boring-bar carries two tools located 180 degrees apart and it is fed along the bar by a star-feed mechanism shown attached to the bar and the tailstock spindle. Each time the bar revolves, the star wheel strikes a stationary pin and turns the feed-screw which, as the illustration shows, extends along a groove cut in one side of the bar. This feed-screw passes through a nut attached to the tool-head so that the latter is slowly fed through the bore. When using a bar of this type, the carriage, of course, remains stationary.
Cylindrical parts attached to the carriage can also be bored by using a plain solid bar mounted between the centers. The bar must be provided with a cutter for small holes or a tool-head for larger diameters (preferably holding two or more tools) and the boring is done by feeding the carriage along the bed by using the regular power feed of the lathe. A symmetrically shaped casting like a bushing or lining is often held upon wooden blocks bolted across the carriage. These are first cut away to form a circular seat of the required radius, by using the boring-bar and a special tool having a thin curved edge. The casting is then clamped upon these blocks by the use of straps and bolts, and if the curved seats were cut to the correct radius, the work will be located concentric with the boring-bar. When using a boring-bar of this type, the bar must be long enough to allow the part being bored to feed from one side of the cutter-head to the other, the cutter-head being approximately in a central location.
