and also gives the principal dimensions of a roller bearing for a 1¾-inch spindle. A is a hardened steel bushing, which is driven into the machine steel spindle. The parts B are the hardened steel rollers which travel in opposite directions. These rollers have a small amount of friction, and this is distributed over a large area. A spindle revolving at 2,300 revolutions per minute will not cause these rollers to rotate very rapidly, while a ball bearing with balls traveling in a channel 1½ inch or 2 inches in diameter would be traveling at the same speed as the driving spindle. They also wear out rapidly as the end strain is very great, it being necessary to force the center against the metal with considerable pressure to keep it from slipping. C is the hardened pin upon which the rollers revolve, and D is the hardened spindle on which the various back-centers are screwed. The collar E should either be flattened for a wrench, or a ⁵/₁₆-inch hole, in which a wire can be inserted, should be drilled through the spindle, so that it can be kept from rotating when screwing on the back-centers. Some spinners prefer the spindle loose, so that it can be withdrawn when changing the centers, while others prefer one with considerable lateral motion, but not enough to permit of withdrawal. By inserting a screw-point in the recess F, the center has considerable lateral motion, but not enough to allow it to be withdrawn. This recess is useful in that it helps to distribute the oil. All parts should be hardened and drawn to a light straw color; they should also be ground or lapped to a true fit after hardening. Back-centers of this construction have been in use for over three years in one establishment, and it has not been necessary to replace a single part.
Fig. 35. Detailed View of a Spinning-lathe Tailstock
Fig. 36. Sectional View showing the Back-center and its Double Roller Bearing
Tools Used in Metal Spinning
Fig. 37 shows an attachment which is used to roll any bead or form. This tool, when in use, is inserted in the tailstock spindle in place of the regular center. It is adjustable for any diameter. The roll illustrated is for making a sharp turn, but rounds and other forms are used. The shell being spun by this tool should be held on a hollow chuck. The roll is set at a point where the metal is to be turned over, and by its use the curve may be governed and made uniform with less skill than when the work is done by “air spinning.” In addition, the spinning may be done in less time. This attachment, for some shapes, makes the use of sectional chucks unnecessary.
Fig. 37. Attachment used for Rolling Sharp Turns and Beads
Fig. 38 shows several spinning tools, the heads of which were turned in the lathe instead of being forged. This method of making spinning tools is believed to be original. The spinners prefer them to the tools which are forged in one piece, because the heads which are screwed to the shanks are made of the best quality of steel, such as the high-speed or self-hardening steel. The shapes are also better and the surfaces more true. The heads of these tools are all threaded with standard ¼-inch, ⅜-inch and ½-inch pipe taps, according to the size. Obviously, a spinner can have as many different shaped heads as may be required of each of the sizes given, and only one handle. The tapering threads in these heads insure that they will always screw on the shanks tightly no matter how often they may be replaced. The ¼-inch size takes a ½-inch cold rolled holder; the ⅜-inch, a ⅝-inch holder, and the ½-inch, a ¾-inch holder. These will be found large enough for the heaviest work. The egg-shaped tool A is a good form for roughing or breaking down, as it has plenty of clearance on the heel, and a blunt point that will not tear the metal. This tool is shown in four sizes. The ball or spherical tool B is a good one to use on curves and large sweeps. The tool C is elliptic, and is slightly different from A, as it has a blunter point. One of these heads is shown at D screwed onto a reducer by which it is held in the lathe chuck while being turned. These heads or points can also be turned while on the handle by using a steady rest.