BALL TURNING.—One of the best methods of turning balls of the softer materials, such as wood, bone, or ivory, is shown in [Figs. 1248] and [1249], in which are shown a blank piece of material and a tubular saw, each revolving in the direction denoted by the respective arrows. The saw is fed into the work and performs the job, cutting the ball completely off. In this case the saw requires to be revolved quicker than the work—indeed, as quickly as the nature of the material will permit, the revolving of the work serving to help the feed. Of course, the teeth of such a saw require very accurate sharpening if smooth work is to be produced, but the process is so quickly performed that it will pay to do whatever smoothing and polishing may be required at a separate operation. This method of ball cutting undoubtedly gave rise to the idea of using a single tooth, as in [Fig. 1250]. But when a single tooth is employed the work must revolve at the proper cutting speed, while the tooth simply advances to the feed. If the work was cut from a cylindrical blank the cutter would require to be advanced toward the work axis to put on a cut and then revolved to carry that cut over the work, when another cut may be put on, and so on until the work is completed. The diameter of ball that can be cut by one cutter is here obviously confined to that of the bore of the cutter, since it is the inside edge of the cutter that does the finishing.
This naturally suggests the employment of a single-pointed and removable tool, such as in [Fig. 1251], which can be set to turn the required diameter of ball, and readily resharpened. To preserve the tool for the finishing cut several of such tools and holders may be carried in a revolving head provided to the lathe or machine, as the case may be. In any event, however, a single-pointed tool will not give the smoothness and polish of the ball cutter shown in [Fig. 1252], which produces a surface like a mirror. It consists of a hardened steel tube c, whose bore is ground cylindrically true after it has been hardened. The ball b is driven in a chuck composed of equal parts of tin and lead, and the cutter is forced to the ball by hand. The ball requires to revolve at a quick speed (say 100 feet per minute for composition brass), while the cutter is slowly revolved.
A simple attachment for ball turning in an ordinary lathe is shown in [Fig. 1253]. It consists of a base a, carrying a plate b, which is pivoted in a; has worm-wheel teeth provided upon its circumference and a slideway at s, upon which slides a tool rest r, operated by the feed-screw handle h. The cut is put on by operating h, and the feed carried around by means of the screw at w. The base plate a may be made suitable to bolt on the tool rest, or clamped on in place of the tool, as the circumstances may permit; or in some cases it might be provided with a stem to fit in place of the dead centre. For boring the seats for balls or other curved internal surfaces the device shown in [Fig. 1254] may be used. It consists of a stem or socket s, fitting to the dead spindle in place of the dead centre, and upon which is pivoted a wheel w, carrying a tool t. r is a rack-bar that may be held in the lathe tool post and fed in to revolve wheel w and feed the tool to its cut. At p is a pin to maintain the rack in gear with the wheel. Obviously, a set-screw may be placed to bear against the end of the tool to move it endwise and put on the cut. An equivalent device is shown in [Fig. 1255], in which the tool is pivoted direct into the stem and moved by a bar b, held in the tool post. The cut is here put on by operating the tail spindle, a plan that may also be used in the device shown in [Fig. 1254]. The pins p upon the bar are for moving or feeding the tool to its cut. It is obvious that in all these cases the point of the tool must be out of true vertically with the axis of the work.
Fig. 1256.
Fig. 1257.
In turning metal balls by hand it is best to cast them with a stem at each end, as in [Fig. 1257].