All tools for spherical or (as it is commonly termed) 'ball' turning operate on the same principle, which is that of a cutter-head, tool rest, or equivalent appliance, whose vertical axis is coincident with the axis of the ball or sphere to be turned, or, what amounts to the same thing, with the axis of the live-spindle of the lathe or machine, and which is capable of being rotated on its pivot or axis in such manner that a tool fixed in the cutter-head or tool rest, with its point or cutting edge exactly on the same level as the lathe centers, will, when set at any distance from the axis of the work, describe a true circle or arc around the same, whose radius, when measured from the axis of the work, will be the same at every point of its circumference, thereby producing (when the tool is adjusted to the cut, and the work is revolving in the ordinary way, and the cutter-head or tool rest is rotated on its axis [pivot] as described) a true sphere whose diameter measures the same at every point of its circumference.
In construction the rest consists of a base A, dove-tailed on the bottom to fit into or over the cross-slide of the lathe carriage, and bored at a to receive the pivot pin a of the tool rest B which is slotted at b for the tool post C; the tool D can be adjusted sufficiently for all practical purposes by means of the adjusting screw E and set screw E the body of the rest is drilled at c for the insertion of the lever F, by means of which the tool rest and tool are rotated (fed) around the work as it revolves in the lathe or machine.
On smaller lathes the base A is fitted directly on the lathe bed, which can also be done on the larger lathes when desired.
The tool rest B can be fitted with a slide for adjusting the tool in place of the slot b, if preferred.
For turning brass and other soft metal balls of small diameter the above is an excellent appliance ; but for iron or steel balls of any diameter a revolving sliding tool rest should be employed.
A base plate (not shown) is fitted onto the cross-slide ways of the lathe carriage, then upon this base plate is fitted the base A (of the tool rest B) which is circular in form, and is geared at C to engage with the worm D. The worm shaft F is journaled on the lathe carriage in the brackets a a', and on the lathe bed in the bracket a', and is actuated from the screw-cutting gears G. The method of operating is so clearly shown in the engraving that further explanation is thought to be unnecessary, except to touch upon the two most important points, viz.: First, the imperative necessity of having the vertical axis of the work and the pivot pin of the tool rest coincident, and, secondly, of having the point or cutting edge of the tool exactly level with the horizontal axes of the lathe spindle and work.
If there is any divergence from a true coincidence in either case, the ball would not be turned a true sphere, but would be oval to just twice the extent that the tool rest, or tool point, are out of a, true alignment.