The plain or back center, five inch lathe head, fig. 97, is of cast iron, about nine inches long at the base, the under surface of which is flat, with a central tenon about an inch wide and deep, to fit the interval of the lathe bearers. The head is fixed upon the bearers, by a bolt screwing into the tenon with a washer beneath them. The two apertures in the front and rear of the lathe head, for the reception of the mandrel and back center, are precisely in one line; with which line, the under surface of the lathe head and the sides of the tenon are parallel. To ensure the correctness of these positions, the two apertures should be bored simultaneously, by a single cutter bar, carrying two cutters, after the method explained and shown by fig. 514, Vol. II.

The aperture in the front of the lathe head, is permanently filled with a hardened steel ring, called the collar, bored and turned out conically from behind and exactly fitting the front cone of the mandrel. The plain cylindrical aperture bored in the back of the lathe head, is fitted with a true steel cylinder, the front end of which is hardened and turned with a hollow center, to receive and support the pointed back end of the mandrel. The cylinder or back center is drawn forward, by means of the short portion of its front end which projects, this, is cut externally with a screw thread, screwing through a detached flat steel ring, attaching to the lathe head, and prevented turning by a pin. When sufficiently advanced, the back center is fixed by a screw from above, the end of which is prevented doing injury by an interposed piece of brass, bearing upon the plain portion of the cylinder. The back center for the lathe mandrel is sometimes formed as a screw, but this is far less efficient. The plain fitting of the cylinder, in fig. 97, being both independent of its means of advance and exactly true with the mandrel axis, secures the important quality, that the back center always advances in the axial line of the mandrel. Whence, the mandrel is never liable to jam or bind, from bearing more forcibly against one side of the collar than the other; while the wear between the hollow and pointed center and that between the mandrel and the collar, being impartially distributed around their respective circumferences, tends to the preservation of their forms and delays their deterioration.

Fig. 96.

Fig. 97.

The mandrel of fig. 97, is of steel, and fits the collar by the double cone fig. 82, previously described, the back end or point and the cones of the front end are hardened. The pulley by which the mandrel is driven, is of wood, iron, or brass, the metal pulleys being hollow, to avoid unnecessary weight and momentum.

The extreme end of the front cone of the mandrel, slightly projects through the face of the collar, and beyond it, is the nose or external screw to carry the chucks. The diameter of the nose being less than that of the projecting end of the mandrel, the latter has a small width of annular space, which is turned flat, to a true surface, and is known as the face of the mandrel. The face of the mandrel is necessary to the correct and continued fitting of the chucks; that is, that they may be removed as frequently as is required during the progress of the work, and when replaced, may again take up the same position, screwing up neither more nor less upon the nose; under which circumstances alone, the work will again run precisely true. This essential quality, is not ensured by the fitting between the external and internal screws of the nose and chuck, also highly necessary, but only by the intimate contact between the face of the mandrel and the face of the chuck; the two being also always screwed up together with moderate and uniform force.

This important subject is further developed with reference to tapping and making the screws in the chucks, by which they attach to the mandrel. But, it may be pointed out here, with respect to the terms employed, that the surfaces of chucks, are generally distinguished, as the front or the back; but, that that portion of the chuck which fits against the face of the mandrel, is always called the face of the chuck, although situated on its proper back.

A small vertical hole is pierced through the front of the lathe head and the upper side of the collar, to admit the oil required by the mandrel; and should be fitted with a cover or pin to exclude dust or turnings. The mandrel is usually supplied with a small quantity of oil, immediately before commencing turning, and also at intervals during the progress of the work; the oil used should be pure animal and not vegetable oil, the acids generally present in the latter being very injurious.

The mandrel when working revolves in a thin film of oil, which finds its way between it and the collar; but the revolution, tends both to dry up the oil, and also to force a portion to find its way out at the front of the collar, rendering a small additional supply frequently necessary. Should the mandrel be permitted to work, until the surfaces commence to dry, both rapidly heat, and if this be allowed to continue until they are absolutely without oil, they may adhere to and damage each other. The injury sustained, takes the form of a roughening of the surfaces, and in serious cases, the mandrel and collar are torn and disintegrated, blister-like excrescences appearing on the surfaces of both, and from this cause they sometimes become interlocked and fixed. All such injury in whatever degree is very difficult to recover, bad cases, requiring the reduction of the mandrel and the introduction of a new collar with consequent deterioration of their correct proportions. When the lathe has been for some time out of use, the mandrel may require to be taken out of its bearings, that it and the collar may be cleansed from the hardened oil collected around them. This may generally be effected by rubbing with a rag and fresh oil only, avoiding if possible, the use of any kind of grit.