It is sometimes necessary to cut a screw for which there are no gears which make a direct connection, in which case the simple gearing shown in Fig. 164 cannot be used. This necessitates the compounding of the gears on the intermediate spindle as shown in the set-up, Fig. 165. The stud is represented by A and the screw by B. Suppose, with a lead-screw having three threads to the inch, it is desired to cut a screw having thirteen threads to the inch. This makes the ratio of teeth on the spindle gear to those on the screw as 3 to 13. The work can be done with spindle gears having 15, 30, or 45 teeth, with screw gears having 65, 130, and 195 teeth, respectively. If it is found that there are no gears having 15, 45, 130, or 195 teeth on hand, compounding must be resorted to. To determine the gears to be used, it must be remembered that the product of the numbers of teeth of the driving gears must be to the product of the numbers of teeth of the driven gears, as the number of threads per inch on the lead-screw is to the number to be cut In this case it is as 3 to 13. Multiply each of these figures by any convenient multiple. In the example in hand, let the multiple be 200. Then,

Fig. 166. Change-Gear Set-Up.

Select from the factors thus obtained two sets, each of which, when multiplied together, will give products equal to the number of teeth that are on hand.

Thus, in the numerator, we may take 3X2X5, and 2X2X5, giving 30 and 20 as gears that are to be used as the spindle and intermediate drivers, respectively.

For the denominator, take 13X5, and 2X2X5X2, or 65 and 40, for the driven gears of the intermediate stud and the screw, respectively. Placing these in position as in Fig. 166, we have

Gear A with 30 teeth Gear C with 65 teeth Gear D with 20 teeth Gear B with 40 teeth

Example For Practice

It is desired to cut a screw with 11 threads to the inch on a lathe having a lead-screw with a pitch of 1/4 inch. The gears available have 30, 40, 45, 50, 55, 60, 65, 70, 80, 90, and 100 teeth, respectively. What ones are to be used, and where?

The preceding examples may be taken as applying to either right- or left-hand threads. The change or direction in the travel of the carriage is obtained by shifting the handle at its right center, Fig. 165, thus reversing the rotation of the lead-screw.

The following description of the method of cutting a V-thread will suffice to illustrate the cutting of any form, with the slight changes which are necessary in the other forms because of the shape of the tool employed:

First set the cutting point so that a line at right angles to the lathe axis bisects the tool angle, and so that the tool is exactly at the height of the center.

The relation between the rotary motion of the work and the axial traverse of the tool, determines the pitch of the thread being cut; and the mechanism connecting the work and the tool must be of a positive character.

Owing to the lost motion of backlash in the mechanism connecting the tool and the work, the tool cannot be returned to the starting point for a new and deeper cut by simply reversing the lathe. The tool must first be withdrawn, the lathe reversed, the tool returned to the starting point, and then advanced for the new cut.

To place the tool for the new cut with accuracy, a stop or graduated device is provided.

When the work is removed from the lathe for testing, care should be taken in replacing, to get the tail of the dog in the same slot in the faceplate that was used to cut the original thread; this can be done by marking or otherwise indicating the slot.