Fig. 1. Measuring Number of Threads per Inch—Setting Thread Tool
Fig. 2. Plan and Elevations of Engine Lathe
Selecting the Change Gears for Thread Cutting.—Suppose a V-thread is to be cut on the end of the bolt B, Fig. 2, having a diameter of 11/4 inch and seven threads per inch of length, as shown at A in [Fig. 1], which is the standard number of threads per inch for that diameter. First the change gears to use are found on plate I which is shown enlarged in [Fig. 3]. This plate has three columns: The first contains different numbers of threads to the inch, the second the size gear to place on the “spindle” or “stud” at a ([Fig. 2]) for different threads, and the third the size of gear c for the lead-screw. As the thread selected as an example has 7 threads per inch, gear a should have 48 teeth, this being the number given in the second column opposite figure 7 in the first. By referring to the last column, we find that the lead-screw gear should have 84 teeth. These gears are selected from an assortment provided with the lathe and they are placed on the spindle and lead-screw, respectively.
Intermediate gear b does not need to be changed as it is simply an “idler” for connecting gears a and c. Gear b is mounted on a swinging yoke Y so that it can be adjusted to mesh properly with different gear combinations; after this adjustment is made, the lathe is geared for cutting 7 threads to the inch. (The change gears of many modern lathes are so arranged that different combinations are obtained by simply shifting a lever. A lathe having this quick-change gear mechanism is described in the latter part of this chapter.) The work B is placed between the centers just as it would be for turning, with the end to be threaded turned to a diameter of 11/4 inch, which is the outside diameter of the thread.