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.

Threads per inch calculator