This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
This entire range of threads can be made without stopping the lathe or removing a single gear. The feeds are four times the number of threads per inch. It will be noticed that the compounding generally adopted on this style of lathe is done away with, and that wherever there are coarse feeds or heavy threads the increase comes directly from the 4 to 1 gear on the stud D1 speeding up the feed mechanism of the feed box in the same proportion, so that it is placed under no additional strain.
Figures 175 and 176 illustrate the "rapid change gear attachment" of the Springfield Machine Tool Company's "Ideal" lathe. They make use of the design placed in the second class, that is, those which have their change-gears mounted upon studs or short shafts arranged in a circle.
The change-gears are mounted in a gear box, shown at the left-hand side of the engraving, Fig. 175, the intermediate and head-stock spindle gears being those ordinarily used. The cover of the gear box is rotated about a central stud, and the gears are carried on the inside of the cover, arranged in a circle concentric with the case, and this circle brings the change-gears opposite the end of the lead screw by revolving the cover of the case.
Fig. 175. - Rapid Change Gear Attachment built by the Springfield Machine Tool Company.
Referring to Fig. 176, the small clutch C moves a telescopic extension of the lead screw and enters it into a hole in the change-gear before the driving clutches between the change-gear and the extension come into contact. This device takes the bearing of the change-gear upon the extension for its support, and secures the change-gear to the lead screw as firmly as if fastened by a nut. In order to change the gear, the cover is revolved until the desired gear is opposite the center of the lead screw extension, when the small clutch is thrown. All of the eight change-gears are protected by the case except the top of the one which is in mesh with the intermediate gear.
To give a sufficient range of pitches, a set of three pairs of gears is provided in the head-stock to vary the speed of the intermediate gear. These are housed in the gear cases shown at the extreme end of the head-stock. These are clutched to their spindles by slipping them on until their clutches engage the spindles, which have clutches with their end sections reduced, as in the case of the lead screw shown at F in the sectional drawing, Fig. 176. These gears furnish ratios of from 1 to 1, 2 to 1, and 4 to 1. The last two may be reversed and five speeds may be given to the fixed pinion driving the intermediate gear. The intermediate gear revolves on a fixed stud on a quadrant to which the handle is attached, and is removed from contact by raising the quadrant.
Fig. 176. - Longitudinal Section of Rapid Change Gear Attachment built by the Springfield Machine Tool Company.
This lathe has a range of threads from 2 to 56 per inch, and a range of turning feeds from 8 to 224 turns per inch, and all the changes for any of these feeds or threads may be made while the lathe is running.
The objection to this device is the inherent weakness of the mechanism when the gears are arranged upon short studs or shafts set around a circle. These cased-in gears must of necessity be comparatively small and of narrow face. The teeth must, from the same conditions, be of fine pitch. Their support must be by comparatively small shafts. All these conditions render the mechanism structurally weak and less rigid than such a device should be to stand the strains of high-speed steel and modern cuts.
Fig. 177. - End Elevation of the Bradford Rapid Change Gear Device.
The Bradford Machine Tool Company build an ingenious rapid change gear device which is shown in the accompanying engravings, of which Fig. 177 is a front elevation, Fig. 178 an end elevation, and Figs. 179 and 180 are sectional views.
The method of transmitting motion from the head spindle to the change-gear mechanism will be readily understood by reference to Figs. 178 and 179. This mechanism is contained in a gear box located in front of the bed, as seen in these two figures. It contains at A a cone of eight gears carried on a shaft for driving the feed rod and screw, and a sliding gear B, which acts as a driver for the cone and may be dropped into mesh with any one of the eight gears forming that member. The shaft-driving gear B carries loosely at the outer end three gears C, any one of which may be connected by a sliding key to the shaft. The three gears are rotated by gears mounted freely at D, the intermediate E carried on a long stud on quadrant F being engaged with one of the three in the set D.
Fig. 178. - Front Elevation of the Bradford Rapid Change Gear Device.
Now, leaving the gears in the positions shown, it is obvious that by operating the plunger controlling the key in gears C, twenty-four rates of feed may be obtained, or eight for each gear in set C. Then, by the three positions for intermediate E, the number of feed change is increased to seventy-two.
At G will be noted a support for the driving gear which is formed solidly on its shaft. The bracket G is bolted to the quadrant and has the under part cut out to allow the intermediate gear to mesh in the pinion, and as the bracket turns with the quadrant it supports the pinion no matter which one of the three gears the intermediate may be in mesh with.