Fig. 789.
The advantage of this form is that the cone at g can be easily turned or ground to keep it true, and the gripping pieces a may be fastened in their mortises by means of the screws shown at h in the end view, and thus kept true. It is obvious that for long work there may be gripping pieces at each end of the mandrel, as in [Fig. 789], and the work will be held true whether its bore be parallel, stepped, or taper, a valuable feature not usually found in expanding mandrels.
Fig. 790.
Fig. 791.
When a mandrel is used upon work having its bore threaded the mandrel also must be threaded, and must abut against a radial face, as at a, in [Fig. 790], because otherwise the pressure of the cut would hold the work still while the mandrel revolved, thus causing the work to traverse along the mandrel. If the thread of the mandrel be made so tight a fit that it will drive the work by friction it will require considerable force to remove the work from the mandrel, so much so, in fact, that finished pieces would be much damaged in the operation. It is better therefore to have the work such a fit that it can be just screwed home against the radial face of the mandrel under heavy hand pressure (if the work be not too heavy for this, in which case a clamp may be employed). Small work, as nuts, &c., are turned on a mandrel of this kind, which has a stem, and fits into the cone or live spindle in the same manner as the live centre, which will drive work up to about 1 inch in diameter without fear of slipping. Threaded mandrels that are in frequent use soon become a loose fit to the work by reason of the thread wear, with the result that if the face of the work is not true with the thread, it meets the mandrel shoulder, as in [Fig. 791], and as the nut cants over, one side as t in the figure, is turned too thick. When the nut is reversed on the mandrel, the turned face will screw up fair against the mandrel shoulder, and the faces of the nut, though true one with the other, are not square with the axis of the thread, and will not therefore bed fair when placed in position upon the work.