For cooling the tool while performing heavy duty, a solution of sal soda is preferable to water, as it prevents rusting of the work and machinery. Its office is simply to keep the tool cool. If a tool becomes overheated, the edge begins to turn over and it becomes dull.

Referring to Fig. 116, it will be seen that the chip, as it is being removed, presses down on the top face of the tool. This pressure increases with the depth of cut and the feed. The resulting friction would soon cause a high temperature in the tool if it were not reduced by the lubricant. The lubricant cools the tool by absorbing a portion of the heat, and lessens the amount of heat developed by reducing the friction between the tool and the chip. Clean, pure water is the only lubricant which can be used on cast iron; but the rapid rusting which follows its use makes it undesirable, and as a result cast iron is usually turned dry. Brass is also usually turned dry. Prime quality lard oil is sometimes used for cooling the tool; but the greater cost prevents its extended use, unless some means are provided for collecting, separating, and filtering it.