Another method of holding work is by the use of a mandrel. This is a piece of steel with a slight taper; the ends are flattened for the lathe dog, as shown in Fig. 109. It frequently happens that a piece with a hole in it is to be turned or finished over its outer surface. In this case a dog cannot be used, and it is troublesome to hold it in a chuck. Such a piece is shown in Fig. 110. This is a stuffing-box gland. If it were to be held by the jaws of a chuck, the face could not be reached at all, and only a portion of the edge B, whereas a dog clamped to it would offer even greater obstruction. The method of using the mandrel is to ream the gland out, so that it can be driven upon the mandrel. When this is done, the frictional resistance between the two will be sufficient to drive the piece. So held in place, it can be finished over its outer surface with but one setting in the lathe. All finishing possible may be done while it is in the chuck, leaving, in this case, only the face A and edge B to be finished while on the mandrel.
Fig. 108. Clamp Dog.
Fig. 109. Work Mandrel.
Fig. 110. Stuffing-Box Gland Held on Mandrel.
Should the gland be shaped, as shown in Fig. 1ll, it would be necessary to make a special mandrel to fit the bore. The cylindrical part A of the mandrel should be a driving fit, and the part B a loose fit.
Where a mandrel like that shown in Fig. 109 is frequently used, the constant driving of the work on and off will wear the mandrel to a smaller diameter, causing it to become useless. Again, solid mandrels are usually made of standard diameters, varying by sixteenths of an inch. It sometimes happens that a piece to be turned has a hole which will not fit any standard solid mandrel.
Fig. 111. Stuffing-Box Gland Requiring Special Mandrel.
To overcome these difficulties, an expanding mandrel shown in Fig. 112 is much used. This is really a chuck, so arranged that the grips can be forced out against the interior of the hole. When the work has been finished, the grips are again drawn in and the piece removed. Another form of expanding mandrel is shown in Fig. 113.
Fig. 112. Diagram Showing Use of Expanding Mandrel.
Fig. 113. Another Form of Expanding Mandrel.