If the bore of the work is coned, and of too great a cone to permit the mandrel to be driven, and drive the work by friction, the cone mandrel shown in [Fig. 781] may be used. m is the mandrel in one piece with the collar c. The work w is held between two cones a, a, which slide a close fit upon the mandrel, and grip the work by screwing up the nut n, there being a thread upon the mandrel, as at s, to receive the nut. It is obvious, however, that work having a parallel bore may also be held by the cone mandrel, as shown in [Fig. 782].
To obviate the necessity of having the large number of mandrels that would be necessary so as to have on hand a mandrel of any size that might happen to be required, mandrels with provision for expanding or contracting the diameter of the parts used to hold the work are made.
Fig. 783.
Thus in [Fig. 783] is shown Le Count’s expanding mandrel, in which g h is the body of the mandrel, turned parallel along a certain distance, to fit the bore of the sleeve a, which is a close-sliding fit on this parallel part of e.
From the end h of the mandrel there extends towards the end g four dovetail grooves, which receive four keys b. The heads of these four keys are enclosed and fit into an annular groove provided in the head c of the sleeve a, so that moving the sleeve a along the mandrel causes the four keys to slide simultaneously in their respective grooves.
Now these grooves, while concentric at any one point in their transverse section to the axis of the mandrel, are taper to that axis, so that sliding the sleeve a along the parallel part of the mandrel increases or decreases (according to the direction in which a is moved) the diameter of the keys.
If the sleeve be moved towards the end g, the keys while sliding in their taper grooves recede from the axis of the mandrel, while if moved towards h they approach the axis of the mandrel, or what is the same thing, if the sleeve be held stationary and the body of the mandrel be moved, the keys open or close in diameter in the same manner; hence all that is necessary is to insert the mandrel in the bore of the work, and drive the end g, when the keys will expand radially and grip the work bore.
The keys, it will be observed, are stepped on their diametral or work-gripping surfaces, which is done to increase the capacity of the tool, since each step will expand to the amount equal to the whole movement of the keys in their grooves or slots.