The less the amount of taper the more firmly the centre is held, but the more difficult it becomes to remove the centre when necessary.
Fig. 560.
The principal methods of removing live centres are shown in [Fig. 560], in which is shown at b a square part to receive a wrench, it being found that if not less than about 1⁄2-inch taper per foot of length be given to the live spindle socket, then revolving the centre with a wrench will cause it to release itself, enabling it to be removed by hand. Another method employed on small lathes is to drill a hole through the live spindle to receive a taper pin p, the live centre end being shown at c.
Another and excellent plan for large lathes, is to thread the centre and provide it with a nut m, which on being screwed against the end face of the live spindle will release the centre. The objection to the use of the pin p is that it is apt to become mislaid, and it is not advisable to use a hammer about the parts of the lathe, especially in such an awkward place as between the journal bearing and the cone, which is where the pin hole requires to be located. The square section is, therefore, the best method for small lathes, and the nut for large ones.
In cases where the live spindle is made hollow a bar may be passed through from the rear end to remove the centre; this also enables rods of iron to be passed through the spindle, leaving the end projecting through the chuck for any length necessary for the work to be turned out of its exposed end.
The dead centre may be extracted from the tail spindle by a pin and hole as in [Fig. 560], or, what is better, by contact with the end of the tail screw as described when referring to the tail stock of the S. W. Putnam lathe.
The cone pulley should be perfectly balanced, otherwise at high speeds the lathe will shake or tremble from the unbalanced centrifugal motion, and the tremors will be produced to some extent on the work. The steps of the cone should be amply wide, so that it may have sufficient power, without overstraining the belt, to drive the heaviest cut the lathe is supposed to take without the aid of the back gear.
In some cases, as in spinning lathes, the order of the steps is reversed, the smallest step of the cone being nearest to the live centre, the object being to have the largest step on the left, and therefore more out of the way.