Rules for Calculating Cutting Speeds.—The number of revolutions required to give any desired cutting speed can be found by multiplying the cutting speed, in feet per minute, by 12 and dividing the product by the circumference of the work in inches. Expressing this as a formula we have
| C × 12 | ||
| R | = | ——— |
| πd |
in which
| R | = | revolutions per minute; |
| C | = | the cutting speed in feet per minute; |
| π | = | 3.1416; |
| d | = | the diameter in inches. |
For example if a cutting speed of 60 feet per minute is wanted and the diameter of the work is 5 inches, the required speed would be found as follows:
| 60 × 12 | ||||
| R | = | ————— | = | 46 revolutions per minute. |
| 3.1416 × 5 |
If the diameter is simply multiplied by 3 and the fractional part is omitted, the calculation can easily be made, and the result will be close enough for practical purposes. In case the cutting speed, for a given number of revolutions and diameter, is wanted, the following formula can be used:
| Rπd | ||
| C | = | —— |
| 12 |
Machinists who operate lathes do not know, ordinarily, what cutting speeds, in feet per minute, are used for different classes of work, but are guided entirely by past experience.