Construction of the Tailstock and Back-center
Fig. 35 shows a spinning-lathe tailstock, which has been changed from the hand-wheel-and-screw type to one having a lever and a roller bearing. The spindle A which is withdrawn from the lever and turned one-quarter of a revolution to give a better view of the rollers, is made from 1¾-inch cold rolled steel. The rollers against which the center bears do not project beyond the spindle, so that the latter can be withdrawn through the tailstock. This eliminates the excessive overhang caused by ball bearings and other centers. When the center projects too far, the tailstock cannot be set close to the work owing to the necessity of withdrawing the center when removing the spun part. The application of this principle to a spinning lathe is original and the type of center illustrated was used only after all other kinds had failed, including all the types of ball bearings and revolving pins. The best forms of ball bearing centers do not last over a year, if in constant use, and they will not always revolve on small work. Two other spindles are shown in this engraving, which were taken from other lathes in order to show different views of the parts. The cylindrical pieces B are the hardened friction rollers which belong in the slot of the spindle F, and C is the hardened pin upon which they revolve. The hardened center D has a threaded end on which the back-centers E of different lengths and shapes are screwed. The friction rollers should always be in a vertical position, and care should be taken to have them exactly central with the spindle.
[[See Transcriber’s Note at the end of this book].]
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