The quick-change gearbox permits the operator to obtain the various pitches of threads without using loose gears. Stud gear to lead screw gear ratio = - = -ĭiagram showing use of total ratio in a compound change-gear setup. Selecting, for example, 18- and 36-tooth gears for the compound assembly, the setup would be as follows: A 16-tooth gear for the stud gear, 36- and 18-tooth gears for the compound assembly, and an 80-tooth gear for the lead screw (Figure 8-12).
Doubling these figures results in 16 teeth for the stud gear and 80 teeth for the lead screw gear, respectively. Therefore, the following procedure can be used:Ģ. When compound gearing is used, the ratio of the compound gears is usually 2 to 1, so that the threads are twice the number per inch as when simple gearing is used. Will provide a 10 to 1 ratio or speed reduction between the Tooth gears for the compound gear assembly, and place the 80-tooth gear on the lead screw (Figure 8-11). Use the 16-tooth gear as the stud gear, use the 64- and 32. Place these gears in the following order: 16, 64, 32, and 80. Multiply each term by a common number (8, for example), and obtain the required gears as follows:Ĥ.
Because this is a 10 to 1 ratio, the stud gear must make 10 revolutions to 1 revolution of the lead screw. The ratio between the stud gear and the lead screw gear is asĢ. Because a 16-tooth gear is usually the lowest number of teeth furnished in the set, multiply by 2 as follows:īecause a 160-tooth gear is entirely out of range of most equipment, and the required diameter would probably be too large to mesh with the stud gear, compound gearing is necessary so that smaller gears can be used.įor example, to determine the compound gears necessary to cut 80 threads per inch on a lathe having a lead screw with 8 threads per inch, the following procedure can be used:ġ. Multiply the number of teeth by a common number to obtain gears within the range of the change-gear set. It is impossible to use gears with 8 and 80 teeth to cut the required thread.
To cut 80 threads per inch, the spindle (or stud gear in an even-geared lathe) must make 80 revolutions to 8 revolutions of the lead To illustrate the necessity for compound gearing, suppose that it is desired to cut 80 threads per inch in a lathe having a lead screw with 8 threads per inch. As in simple gearing, the gear ratio must be determined between the stud gear and the lead screw gear. When the gears are arranged in a train, they are said to be compounded. The idler gear does not change the gear ratio. Therefore, an idler gear must be used to transmit the motion (Figure 8-10). Because these gears are impossible, multiply each by a common number to obtain the desired gears available in the change-gear set as follows:īecause change gears of various sizes are used, the distance between gears will vary. Thus, if gears of 12 teeth were available, they would cut the required thread. To cut 12 threads per inch, the spindle (or stud gear on an even-geared lathe) must make 12 revolutions to 8 revolutions of the lead screw. Simple gearing for cutting screw threads on a change-gear lathe.Ĭompound gearing for cutting screw threads on a change-gear lathe.įor example, to determine the size of the stud gear and the lead screw gear required to cut 12 threads per inch in a lathe having a lead screw with 8 threads per inch, use the following formula: This is expressed as the following formula: The ratio depends on the pitch of the lead screw and the number of teeth to be cut. The ratio between the number of teeth on the stud gear and the lead screw gear must be determined. This "compound gear assembly" is necessary when a large ratio between the stud gear and lead screw gear is required to cut extra-fine threads. As the two gears are keyed together, the assembly no longer functions as an idler, but as a first-stage ratio reduction between the stud gear and the lead screw gear (Figure 8-9).
In compound gearing, the idler is combined with another gear of different size. In simple gearing, an idler gear is used to transmit motion from the stud gear to the lead screw gear (Figure 8-8). If the stud gear revolves at any other speed, the lathe is an odd-geared lathe.Ĭhange gears are either simple or compound in form. On an even-geared lathe, the stud gear revolves at the same speed as the spindle gear (that is, the two gears are equal in size). On lathes equipped with tumbler reverse gears, it must first be determined whether the lathe is even-geared or odd-geared. In the absence of an index chart showing gear combinations for various threads, it is necessary to calculate the proper gears to use for cutting threads.
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