If one end only of the work requires to be turned and it can be completely finished without moving the work driver, the work will be true (assuming the live spindle to run true in its bearings and to fit the same). It will also run true if the work be taken from the lathe and replaced without moving the driver or carrier, providing that the driver be so placed as to receive the driving pressure at the same end as it did when the work was driven; and it is therefore desirable, on this account alone, to always so place the work in the lathe that the driver is driven by its tail end, and not from the screw or screw head. But if the work be turned end for end it will not run true, because the work centre at the unturned end of the work will not be true or central to the turned part of the work.
It is obvious then that lathe centres should run true. But this will not be the case unless the holes into which they fit in the lathe are axially true one with the other and with the lathe spindles. If these holes are true, and the centres are turned true and properly cleaned before insertion, the centres may be put into their places without any adjustment of position. Otherwise, however, a centre punch mark is made on the radial or end face of the live spindle, and another is made on the live centre, so that both for turning up and for subsequent use the centre will run true when these centre punch marks are exactly opposite to each other.
Fig. 1156.
The best way to true lathe centres is with an emery-wheel. In some lathes there are special fixtures for emery grinding, while in others an attachment to go in the tool post is used. [Fig. 1156] shows such an attachment.
In the figure a is a frame to be fastened in the slide rest tool post at the stem a′. It affords journal bearing to the hand wheel b, to the shaft of which is attached the gear-wheel c, which drives a pinion d, on a shaft carrying the emery-wheel e, the operation being obviously to rotate wheel b, and drive the emery-wheel e, through the medium of the multiplying gear-wheels c, d.
The emery-wheel is fed to its depth of cut on the lathe centre p, by the cross feed screw of the lathe, and is traversed by pulling or pushing the knob f, the construction of this part of the device being as follows: g and h are two bushes, a sliding fit in the arms of frame a, but having on top flat places i and j, against which touch the ends of the two set-screws k, l, to prevent them from rotating. The emery-wheel and gear pinion d are fast together, and a pin passes through and holds g and h together. Hence the knob f pushes or pulls, as the case may be, the bushes through the bearings g, h, in the frame a, the pinion and emery-wheel traversing with them. Hence pinion d is traversed to and fro by hand, and it is to admit of this traverse that it requires its great length. The stem a is at such an angle that, if it be placed true with the line of cross feed, the lathe centre will be ground to the proper angle.
