Fig. 34. (A) Radial Facing.
(B) Boring Pulley Held in Chuck
Application of Chucks.—As an example of chuck work, we shall assume that the sides of disk D, [Fig. 34], are to be turned flat and parallel with each other and that an independent chuck is to be used. First the chuck is screwed onto the lathe spindle after removing the faceplate. The chuck jaws are then moved out or in, as the case may be, far enough to receive the disk and each jaw is set about the same distance from the center by the aid of concentric circles on the face of the chuck. The jaws are then tightened while the disk is held back against them to bring the rough inner surface in a vertical plane. If the work is quite heavy, it can be held against the chuck, before the jaws are tightened, by inserting a piece of wood between it and the tailstock center; the latter is then run out far enough to force the work back. The outside or periphery of the disk should run nearly true and it may be necessary to move the jaws in on one side and out on the other to bring the disk to a central position. To test its location, the lathe is run at a moderate speed and a piece of chalk is held near the outer surface. If the latter runs out, the “high” side will be marked by the chalk, and this mark can be used as a guide in adjusting the jaws. It should be remembered that the jaws are moved only one-half the amount that the work runs out.
Fig. 35. Tools Ground so that Top Slopes away from Working Part of Cutting Edge
A round-nosed tool t of the shape shown can be used for radial facing or turning operations of the kind illustrated. This tool is similar to the form used when turning between centers, the principal difference being in the direction of the top slope. The radial facing tool should be ground to slope downward toward a (see [Fig. 35]) whereas the regular turning tool slopes toward b, the inclination in each case being away from that part of the cutting edge which does the work. The cutting edge should be the same height as the lathe centers, and the cut is taken by feeding the tool from the outside in to the center. The cut is started by hand and then the power feed is engaged, except for small surfaces. The first cut should, if possible, be deep enough to get beneath the scale, especially if turning cast iron, as a tool which just grazes the hard outer surface will be dulled in a comparatively short time.
If it were simply necessary to turn a true flat surface and the thickness of the disk were immaterial, two cuts would be sufficient, unless the surface were very uneven, the first or roughing cut being followed by a light finishing cut. For a finishing cut, the same tool could be used, but if there were a number of disks to be faced, a square-nosed tool F, [Fig. 35], could probably be used to better advantage. This type has a broad flat cutting edge that is set parallel with the rough-turned surface and this broad edge enables a coarse feed to be taken, thus reducing the time required for the finishing cut. If a coarse feed were taken with the round tool, the turned surface would have spiral grooves in it, whereas with the broad cutting edge, a smooth surface is obtained even though the feed is coarse. The amount of feed per revolution of the work, however, should always be less than the width w of the cutting edge. Very often broad tools cannot be used for finishing cuts, especially when turning steel, because their greater contact causes chattering and results in a rough surface. An old and worn lathe is more liable to chatter than one that is heavy and well-built, and as the diameter of the work also makes a difference, a broad tool cannot always be used for finishing, even though, theoretically, it would be preferable. After one side of the disk is finished, it is reversed in the chuck, the finished surface being placed against the jaws. The remaining rough side is then turned, care being taken when starting the first cut to caliper the width of the disk at several points to make sure that the two sides are parallel.
Example of Boring.—Another example of chuck work is shown at B, [Fig. 34]. In this case a cast-iron pulley is to have a true hole h bored through the hub. (The finishing of internal cylindrical surfaces in a lathe is referred to as boring rather than turning.) The casting should be set true by the rim instead of by the rough-cored hole in the hub; this can be done by the use of chalk as previously explained. Even though a universal type of chuck were used, the jaws of which, as will be recalled, are self-centering, it might be necessary to turn the pulley relative to the chuck as a casting sometimes runs out because of rough spots or lumps which happen to come beneath one or more of the jaws.