Fig. 797.
Fig. 798.
Fig. 799.
Mandrels may be employed to turn work, requiring its outside diameter to be eccentric to the bore, by the following means:—In [Fig. 796], let the centre c represent the centre of the mandrel, and d a centre provided in each end of the mandrel, distant from c to one half the amount the work is required to be eccentric. The mandrel must be placed with the centres d receiving the lathe centres. In this operation great care must be taken that a radial line drawn on each end of the mandrel, and passing through the centre of the centres d, shall exactly meet and coincide with the line l drawn parallel to the axis of the mandrel. If this be not the case the work will be less eccentric at one end than at the other. As it is a somewhat difficult matter to test this and ascertain if the mandrel has become out of true from use, it is an excellent plan to turn such a mandrel down at each end, as shown in [Fig. 797], and draw on it the lines l, l, which correspond to the line l l in [Fig. 796]. If then a steel point be put in the lathe rest and fed in to the work, so that revolving the latter just causes the tool point to touch the lines l at each end, or if the tool point makes long lines as at a, a, the two lines l, l, should intersect the lines a, a at the centre of their respective lengths. The lines l l should be marked as fine as possible, but deep enough to remain permanently, so that the truth of the eccentricity of the mandrel may be tested at any time. An equivalent device is employed in turning the journals of crank shafts, as is shown in [Figs. 798] and [799], in which d, d are two pieces fitted on the ends of the crank shaft, being equal in thickness to the crank throw, as shown at a, b in the figure, so that when d, d lie in the same plane as the crank cheeks (as when all will lie level on a plate, as in the figure) the centres c will be in line with the journal in the crank throw. Pieces d are broadened at one end to counterbalance the weight of the crank, which will produce more true work than counterbalancing by means of weights bolted to the face plate of the lathe, as is sometimes done, causing the crank throw to be turned oval instead of round. In the case of a double crank, however, the centre pieces cannot be widened to counterbalance, because what would counterbalance when the centres a in [Fig. 799] were used, would throw the crank more out of balance when centres b were used for the throw b. In this case, therefore, the centre pieces are provided with seats for the bars e, e, which may be bolted on to carry the counterbalancing weights, the bars being changed on the centre pieces when the centres are changed. The bars, for example, are shown in their position when the centres a are being used to turn up the journal a, the necessary amount of weight for counterbalancing being bolted on them with a set-screw through the weight.
The centres are steel plugs screwed tightly into the pieces d, and are hardened after being properly centre-drilled and countersunk.