liquid at 39 degrees below zero. according to the grade required. Mercury becomes with the proportion used in mixing, which is varied numerous workings of the metal. With steel it varies lead cast in a mold, the highest for wire drawn after tested. In the case of lead, the lowest strength is for due to the different forms and qualities of the metal NOTE. The wide variations in tensile strength are

Chapter VI.

Rules for Selecting Change Gears for
Screw Cutting.

(For explanation of principle on which these rules are founded, see Chapter XVII., page 98.)

First look over the lathe and see how it is made; see if the change gears go directly on the spindle and the leading screw. If not, ascertain how many revolutions the stud which takes the first change gear makes, to one revolution of the spindle, or how many revolutions the stud that takes the last change gear makes to one revolution of the leading screw, or both.

Multiply the number of threads per inch in the leading screw by the number or numbers found, as above, and assume the result to be the number of threads per inch in the leading screw.

Examples: Number of revolutions of spindle to one revolution of stud that takes the first change gear=2. Number

of threads per inch in leading screw=4. Then 4×2=8, the assumed number of threads per inch in the leading screw, or

Number of revolutions of stud that takes the last change gear, to one revolution of leading screw=2. Number of threads per inch in leading screw-4. Then 4×2=8, the assumed number of threads per inch in the leading screw, or

Number of revolutions of spindle to one revolution of stud that takes the first change gear=2, and number of revolutions of stud that takes the last change gear to one revolution of leading screw =2. Then 4x2x2=16, the assumea number of threads per inch in leading

screw.

Then take the number of threads per inch desired to be cut, and the assumed number of threads per inch of the leading screw, and divide the larger by the smaller to get the ratio.

This ratio must exist between the gears to be used, therefore:

Select any gear and multiply the number of teeth by this ratio to find the

number of teeth in the other gear. The ratio will usually come in the form of a mixed number.

Example: Assumed number of threads per inch in leading screw=16.

Number of threads per inch desired to be cut 14. Then ratio=1=14. [Reduce all fractions to lowest terms.]

In order to select two gears, one of which has 14 times as many teeth as the other, it is evident that the other gear must have some number of teeth that can be divided by 7.

Therefore, if there is no change gear with the lathe, with a number of teeth which can be divided by 7, the number of threads per inch desired cannot be cut on that lathe.

When it is known that the number of threads can be cut on the lathe, it is usually easier to proceed thus:

Ascertain the number by which the teeth of the change gears increase, and multiply the assumed number of threads in leading screw and the number of threads desired to be cut, both by this number.

Example: Number by which teeth of change gears increase=4.

Assumed number of threads in leading screw=8.

If these two gears are not in the set, multiply both by any number that will give gears that are in the set.

When the thread desired to be cut is coarser than the assumed number of threads of leading screw, put the larger gear on the spindle (or stud connected with the spindle) and when the thread desired to be cut is finer than the assumed number of threads of leading screw, put the smaller gear on the spindle.

When fractional threads are desired to be cut, multiply both the fractional number and the assumed number of threads of leading screw, by some number that will raise the fractional number to a whole number. Example: