There are many pieces of lathe work which are pierced with a cylindrical hole and which can be mounted on a bar suspended between lathe centers. Fig. 177 shows an arrangement of this kind. The bar B is called a mandrel. It has a very slight taper and must fit the work very closely, in fact it is driven into the work by means of a copper maul. Such an arrangement enables both sides of the work to be readily machined. When the work is finished, the mandrel is driven out.

Mandrels are so frequently used in the machine shop, and they must fit the work so neatly that several forms of expanding mandrels have been devised so that one mandrel may suffice for holes varying within a range of about 5 per cent in diameter. Fig. 178 shows a useful type of expanding mandrel which may easily be made in the shop. It consists of a sleeve or bushing B and a mandrel M. The mandrel is usually about 8 or 10 inches long, centered for suspending between the lathe centers, threaded at one end for a nut, and tapered for a distance of 3 or 4 inches from the threaded end so thai the largest diameter of the tapered part is about 5 per cent more than the smallest diameter. The bushing is reamed inside to the taper of the mandrel, is turned cylindrical outside, has a number of slits cut along its outer surface nearly through the metal, and has one slit cut entirely through. The bushing is slipped, large end first, over the threaded end of the mandrel and is pushed along by aid of the nut until the taper expands it out firmly against the work if is intended to fit.

Fig. 178. - Expanding Mandrel.