The last four numbers indicate the gears which should be used. The upper two having 40 and 30 teeth are the driving gears and the lower two having 80 and 60 teeth are the driven gears. The driving gears are gear a on the spindle stud and gear c on the intermediate stud, meshing with the lead-screw gear, and the driven gears are gears b and d. It makes no difference which of the driving gears is placed on the spindle stud, or which of the driven is placed on the lead-screw.
Fractional Threads.—Sometimes the lead of a thread is given as a fraction of an inch instead of stating the number of threads per inch. For example, a thread may be required to be cut, having 3/8-inch lead. The expression “3/8-inch lead” should first be transformed to “number of threads per inch.” The number of threads per inch (the thread being single) equals:
| 1 | 3 | 8 | ||||
| ——— | = | 1 ÷ | — | = | — | = 22/3 |
| 3 | 8 | 3 | ||||
| — | ||||||
| 8 |
To find the change gears to cut 22/3 threads per inch in a lathe having a screw constant of 8 and change gears varying from 24 to 100 teeth, increasing by 4, proceed as follows:
| 8 | 2 × 4 | (2 × 36) × (4 × 24) | 72 × 96 | |||
| — | = | ——— | = | ————————— | = | ———— |
| 22/3 | 1 × 22/3 | (1 × 36) × (22/3 × 24) | 36 × 64 |
As another illustration, suppose we are to cut 13/4 thread per inch on a lathe having a screw constant of 8, and that the gears have 24, 28, 32, 36, 40 teeth, etc., increasing by four up to one hundred. Following the rule:
| 8 | 2 × 4 | (2 × 36) × (4 × 16) | 72 × 64 | |||
| — | = | ——— | = | ————————— | = | ———— |
| 13/4 | 1 × 13/4 | (1 × 36) × (13/4 × 16) | 36 × 28 |
The gears having 72 and 64 teeth are the driving gears, and those with 36 and 28 teeth are the driven gears.